patterns and impacts of fish bycatch in a scallop dredge fishery
TRANSCRIPT
Patterns and impacts of fish bycatch in a scallop dredge fishery
HELEN R. CRAVENa, ANDREW R. BRANDb and BRYCE D. STEWARTa,*aEnvironment Department, University of York, Heslington, York UKbSchool of Biological Sciences, University of Liverpool, Liverpool UK
ABSTRACT
1. Dredging for marine bivalves can cause considerable damage to benthic invertebrates and habitats.However, it is largely unknown how dredging affects fish communities. In this study patterns and impacts offish bycatch in scallop dredges around the Isle of Man, in the north Irish Sea, were investigated by analysingdata from fisheries-independent surveys conducted between 1992 and 2005.
2. Almost all (97.6%) tows of the survey gear generated fish bycatch, with a total of approximately 50 speciesrecorded. Cuckoo ray (Leucoraja naevus) and monkfish (Lophius piscatorius) dominated the bycatch, accountingfor 46.82% of the total. Three other species of particular commercial or ecological interest; lemon sole(Microstomus kitt), plaice (Pleuronectes platessa) and lesser spotted dogfish (Scyliorhinus caniculus), were alsoabundant. Most of the cuckoo ray, monkfish and plaice captured were juveniles, whereas lesser spotted dogfishand lemon sole were a mixture of juveniles and adults.
3. In general, rates of fish bycatch appeared low, but this may be at least partly because background fishdensities around the Isle of Man were also low. There was considerable spatial, temporal, and species-specificvariation in fish bycatch, and the finer meshed dredges traditionally used to catch queen scallops (Aequipectenopercularis), caught significantly more fish than the great scallop (Pecten maximus) dredges.
4. The density of lesser spotted dogfish bycatch increased significantly over the 14 years of the study whereas thedensity of monkfish decreased significantly. These patterns appear to reflect differences in the susceptibility of thetwo species to capture and damage by scallop dredging, and/or have been caused by regional trends in stock levels.An assessment of the impact of the local great scallop dredge fishery indicated that it may be catching substantialnumbers of monkfish.
5. Given the recent expansion of scallop dredging around the UK, such effects should be factored intoecosystem-based management plans.Copyright # 2012 John Wiley & Sons, Ltd.
Received 22 November 2011; Revised 10 August 2012; Accepted 18 August 2012
KEY WORDS: ocean; ecosystem approach; sustainability; fish; invertebrates; fishing; dredging
INTRODUCTION
It is now widely acknowledged that an ecosystem-basedapproach to managing fisheries is necessary toaccount for all of the impacts that differentfishing methods may have on the environment
(Pikitch et al., 2004; Howarth et al., 2011). Fishingfor scallops with the toothed Newhaven dredgescommonly used around the United Kingdom(UK) has been considered one of the mostdamaging of all fishing gears to non-target benthiccommunities and habitats (Kaiser et al., 2006).
*Correspondence to: B. D. Stewart, Environment Department, University of York, Heslington, York, YO10 5DD, England.E-mail: [email protected]
Copyright # 2012 John Wiley & Sons, Ltd.
AQUATIC CONSERVATION: MARINE AND FRESHWATER ECOSYSTEMS
Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
Published online 5 November 2012 in Wiley Online Library(wileyonlinelibrary.com). DOI: 10.1002/aqc.2289
Given the rapid recent expansion of the UK scallopfishery (Shephard et al., 2010) this is of particularconcern to fisheries managers and conservationscientists. Scallop dredging affects benthic fauna,flora and habitats by causing changes in overallbiomass, species composition and size structureof demersal communities in the ecosystem(Eleftheriou and Robertson, 1992; Bianchi et al.,2000). The ways in which dredges may directly affectthe sea bed include scraping (or ploughing), sedimentre-suspension, physical destruction of bedforms,and the removal or scattering of non-target benthicorganisms (Collie et al., 1997). Delayed effects onbenthic community structure may also be seen,either from the post-fishing mortality of organismsdue to being hit by dredges, or from becominginjured during capture and release back overboard(Jones, 1992). Removal of large surface dwellingorganisms and homogenization of sedimentcharacteristics will reduce spatial heterogeneityin benthic communities, which may significantlyalter the density of megafaunal species in anecosystem, thereby affecting recruitment in apopulation (Thrush et al., 1995; Jenkins et al., 2001).
The response of organisms to fishing depends onthe life-history characteristics of species, trophicinteractions among species, and changes that haveoccurred to the physical habitat (Bianchi et al.,2000). An impact often associated with bottomfisheries is a reduction in larger bodied speciesand a relative increase in smaller species, thelatter probably as a result of competitiverelease (Bianchi et al., 2000; Dulvy et al., 2000).Although most studies examining the effects ofscallop dredging have concentrated on benthicinvertebrates and habitats (Kaiser et al., 2006),alteration to marine food webs through changes inthe abundance and size distribution of demersalfish populations could also have importantconsequences for benthic ecosystems. All rays andsome sharks (e.g. dogfish) are important benthicpredators, although their wider ecological role israrely studied (Thrush and Dayton, 2002). Benthicfish also form an integral part of the natural system;therefore exploitation and mortality throughbycatch are likely to have important ramificationsnot just on the functioning of the marine ecosystemsbut on the fishing industry, its management andsustainability (Thrush et al., 1995).
Scallop dredges can cause alterations to theecosystem through the removal of species asbycatch and by causing an increase in robust,
scavenging organisms (Veale et al., 2000a). Oneof the most consistent observations fromexperimental studies is that epibenthic anddemersal scavengers are attracted to areas recentlydisturbed by trawls and dredges (Thrush et al.,1998; Jenkins et al., 2004). The presence ofscavengers exploiting dead, damaged or exposedorganisms, however, is likely to be a very transientphenomenon, and such short-term events are mostlikely only apparent in intensive short-livedstudies. A very high intensity and frequency ofdisturbance would be needed before such patternswould emerge from broad-scale studies (Thrushet al., 1998). This has been seen in the IrishSea where a study by Bradshaw et al. (2002)found that mobile, robust, and scavenginginvertebrate taxa had increased in abundanceover a 60 year time period while slow-moving orsessile, fragile taxa had decreased.
Nevertheless, the long-term effects of bottomfisheries on benthic ecosystems are still debated(Philippart, 1998; Gray et al., 2006). This isbecause a major challenge when evaluating theimpacts of bottom fisheries is that mostexperimental work is short term and consistentlong-term records are scarce, with most databeing available only after the commencement ofintensive fishing (Philippart, 1998; Collie et al.,2000). In particular, very little research has beenundertaken on the long-term, direct impacts ofscallop dredging on benthic fish communities andthe implications of this for the wider ecosystem.
This study examined the impact of scallopdredging on fish communities around the Isle ofMan in the north Irish Sea using bycatch datafrom fisheries-independent surveys conductedbetween 1992 and 2005. These surveys aimed tosimulate commercial fishing practices by deployingthe two types of scallop dredge utilized by thelocal fleet during that time period. These dredgeshave previously been documented to retain fishbycatch (Hill et al., 1996; Kaiser et al., 1996;Veale et al., 2000b; Enever et al., 2007), but onlyfrom observations made over a single survey and/orin very limited detail. In contrast, this present studywill document temporal, spatial, and demographictrends in fish bycatch over a 14-year period andassess impact by utilizing fishing effort data fromthe local scallop fishing fleet. Commercial fisheriesaround the Isle of Man are dominated by dredgingand trawling for scallops (Beukers-Stewart andBeukers-Stewart, 2009), with almost no targeted
FISH BYCATCH IN A SCALLOP FISHERY 153
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
fishery for finfish for several decades (Brand et al.,1991; FAO, 2008). Any bycatch in scallop fisherieswould therefore represent the main source of fishingmortality for demersal fish populations in thearea. Furthermore, results from the study willbe used to evaluate the recovery, or otherwise, ofseveral fish stocks that are known to have beenoverfished around the Isle of Man in the past.
METHODS
Study area and fisheries
The Isle of Man (58�080N, 4�270W) is situated in thenorth Irish Sea. The waters within the Isle of Man’s12 mile territorial limits are shallow, particularly tothe east of the island, where they are mostly lessthan 30m deep. To the south and west they slopegradually down to around 80–90m at the 12 milelimit. There are extensive shallow sandy banks offthe north-east coast. Nearshore sediments arepredominately fine sand, while offshore sedimentscan be divided into four clear categories; coarsesands and gravel, fine sand, muddy sand and mud(Barne et al., 1996).
Scallop dredging around the Isle of Man startedin 1937 for the great scallop (Pecten maximus)and 1967 for the queen scallop (Aequipectenopercularis) (Brand et al., 1991). There is also along history of other fisheries including herring,Dublin Bay prawn, cod, plaice, whiting, dogfish,monkfish, Dover sole, crabs, lobsters, squid, andwhelks (Bradshaw et al., 2002). Most of thesefisheries, especially those for finfish, have nowcollapsed (Brand et al., 1991; FAO, 2008) andcurrently the largest fishery in the Isle of Man isfor great scallops and to a lesser extent the queenscallop. Any fish still landed in the Isle of Man aremostly taken as bycatch in the scallop fisheries(Tim Croft, Island Seafare, pers. comm.). The twoscallop species now account for approximately85% of the value of all seafood landed on theisland (Brand, 2006). The commercial fishery forgreat scallops was estimated to be worth £1.75mto the Isle of Man in 2005 and the fishery forqueen scallops £450 000 in 2004 (Beukers-Stewartand Beukers-Stewart, 2009).
The scallop fisheries peaked in the early 1980swhen the local fleet reached a maximum of 70 boats(Brand et al., 2005). Local fleet size is now less thanhalf that of the 1980s, however, increasing numbersof boats from Scotland, England, Wales, Northern
Ireland and the Republic of Ireland have cometo exploit the Isle of Man fishing grounds, withapproximately 70 vessels seen in offshore areasin 2008 (Beukers-Stewart and Beukers-Stewart,2009).
The number of fishing grounds around the Isle ofMan have increased since the introduction of theNewhaven type, spring-toothed dredges in 1972(reviewed in Chapman et al., 1977) which haveallowed stonier grounds to be exploited for bothgreat and queen scallops (Bradshaw et al., 2002).The spring-toothed dredges used in the Isle ofMan, now mainly for great scallops, are designedto rake the sea bed to a depth of up to 10 cm andhave belly rings along the underside of the net bagthat drag along the sea bed (Bradshaw et al.,2002). The dredges used for queen and greatscallops differ somewhat in that queen scallopdredges have 10 teeth of 60mm length comparedwith 9 teeth of 110mm on great scallop dredges,and belly rings of 55mm internal diametercompared with 80mm on great scallop dredges.Although queen scallops are now largely targetedwith otter trawls instead of dredges (Hinz et al.,2009), the design of dredge used in the presentsurveys was commonly used in the commercialqueen scallop fishery during the period of thisstudy, particularly throughout the 1990s (Vauseet al., 2007).
The great scallop fishery around the Isle of Manis regulated in a number of ways including aminimum legal landing size of 110mm shell length,an annual closed season from June–October,and various restrictions on fishing gear, area,and times (Beukers-Stewart et al., 2003, 2005).Fishing for queen scallops mainly takes placeduring the summer when the great scallop fisheryis closed (Brand et al., 1991) and until recently wasregulated only by market demand (Vause et al.,2007). However, since 2010 a byelaw has been putin place to regulate the queen scallop fisherythrough a total ban on dredging with toothed bars,a fully closed season between 1 April and 31 May,various restrictions on fishing gear and times, and aminimum landing size (Sea-Fisheries (QueenScallop Fishing) Bye-Laws, 2010).
Fisheries-independent surveys
Scallop fishing sites surround the island from inshoreto beyond the 12mile limit, with the majority of greatscallop fishing grounds seen to the west and south ofthe island (Figure 1). Fisheries-independent surveys
H. R. CRAVEN ET AL.154
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
were conducted between 1992 and 2005 on the mainfishing grounds for the great scallop. This analysiswill concentrate on eight study sites to the west,south and east of the Isle of Man (Figure 1; studysites in bold) for which there are the mostcomprehensive datasets.
These eight fishing grounds were surveyed forgreat and queen scallop size, age, and abundance,twice a year whenever possible. The surveys weredesigned to replicate local commercial practicesand therefore generate comparable catches andbycatch. Surveys took place just before the start ofeach great scallop fishing season (October) and justafter the season was closed (June). The RV‘Roagan’, a 20m converted beam trawler wasoriginally used to conduct the surveys, althoughfrom October 2002 several commercial fishingboats of similar size and design were charteredinstead to do the work. Each ground was surveyedusing tows approximately 2 nautical miles (nm)(3.70 km) in length using spring-toothed Newhaventype dredges, with three or four replicate towsconducted. Each tow was at an average speed of2.5 knots (4.63 kmh-1) thus lasting 45–50min. Toensure that the same area was targeted duringeach survey, as far as possible, tow length andlocations were recorded using a differential globalpositioning system (DGPS) linked with Microplotsoftware (Sea Information Systems, Aberdeen).Both standard scallop dredges and queen scallop
dredges were fished, with a gang of four of eachtype situated on either side of the research vessel(for full details see Beukers-Stewart et al., 2003).All dredges were 0.76m in width, making a fishedstrip of 3.04m on either side of the vessel.
All fish caught were identified (to species wherepossible), and measured (total length) to thenearest mm. Tow length data were combined withthe dimensions of the dredge gear to calculate thearea of sea bed swept by the dredges on each tow.A standardized value for the tow area of 11500m2
was used for each set of four dredges if noproblems occurred with the tow. For shorter tows,the area covered was calculated in proportion tothis standard area for a normal tow, i.e. if a towwas only 1 nm rather than 2 nm the area coveredwas calculated to be 5750m2. Any data fromnon-functional dredges, or where bycatch was notrecorded, was removed from the analysis to avoidbias in the data.
Data analysis
Relative densities of fish bycatch for each surveydate and fishing ground dredged were calculatedfrom these data and expressed as the meannumber of fish per 1000m2 of dredged area, usingeach tow as a replicate. These densities werecalculated for total fish bycatch and five speciesthat were both abundant and of particular
Figure 1. Map of the Irish Sea showing the main fishing grounds for great scallop (shaded areas). Grounds analysed in this study are labelled in bold.(Source: Beukers-Stewart et al., 2003).
FISH BYCATCH IN A SCALLOP FISHERY 155
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
commercial or ecological interest; the cuckoo ray(Leucoraja naevus), monkfish (Lophius piscatorius),lemon sole (Microstomus kitt), lesser spotted dogfish(Scyliorhinus caniculus), and plaice (Pleuronectesplatessa). The data were separated into relativedensities of fish caught by queen and scallopdredges each year from 1992–2005 and for eachfishing season. The relative density of fish capturedwas also calculated for the start of the great scallopseason (October survey) and compared with that atthe end of the season (the following June survey).
Statistical analysis
Temporal and spatial trends in fish bycatch
Initially, data were combined across all eightgrounds to give relative densities of all fishcaptured each year around the Isle of Man from1992–2005. This was also done separately forcuckoo ray, monkfish, lemon sole, lesser spotteddogfish, and plaice. Regression analysis wasperformed on the data to investigate any temporaltrends in the relative density of fish caught aroundthe Isle of Man over the duration of the study.The density of fish caught on the different groundswas also compared, but it was not possible toconduct statistical analysis due to at least somemissing survey dates on almost all grounds.
Effect of dredge type
Relative density data were tested for normalityusing a Kolomogorov–Smirnov test and werefound to not differ significantly from a normaldistribution. Therefore variation in the relativedensity of all fish caught by great and queenscallop dredges was analysed for each of the eightfishing grounds using a one-way ANOVA. Onlygrounds sampled in both June and October ofeach year were included in this analysis to ensureany differences found were due to dredge type andnot from within year variations.
The Kruskal–Wallis method was used to analysethe relative densities of the five main species caughtby scallop and queen dredges at each of the eightfishing grounds as these data were not normallydistributed.
Effect of fishing season
The effect of the great scallop fishing season onthe density of total fish bycatch was analysedusing two-way ANOVA by comparing bycatchlevels before the fishing season commencing in
October and after its completion the followingJune. This analysis was also done separately formonkfish as the temporal analysis (see above)suggested the scallop fishery may be having asignificant impact on this species. Data were testedfor normality using a Kolomogorov–Smirnov test.Not all data were normally distributed; however,two-way ANOVAs have still been used in thisanalysis. ANOVA have been shown not to besensitive to moderate deviations from normalityand as the design of the study was balanced theANOVA was considered robust to any departuresfrom homogeneity of variance and normality(Underwood, 1997; McDonald, 2009). The twofixed factors used in the two-way ANOVA werethe month of the survey (October or June) and thefishing season (where a season for great scallopsruns from October to the following June). Onlygrounds sampled at both the beginning (October)and end (June) of the season could be included inthis analysis.
Size structure of bycatch
Kolomogorov–Smirnov tests for two independentsamples were carried out to establish whether therewas a significant difference in the size frequencydistribution of fish caught by queen and greatscallop dredges. This was done for all bycatch andfor the five main species across the eight grounds.
The mean lengths of all fish captured by queenand great scallop dredges were calculated alongwith the specific mean length for cuckoo ray,monkfish, lemon sole, lesser spotted dogfish, andplaice. The data were tested for normality using aKolomogorov–Smirnov test and subsequently, asthe data were normally distributed, t-tests werecarried out to determine if there was a differencein the mean length of fish caught by the two typesof dredge.
Impact of the local fishing fleet
The significance of these rates of fish bycatch wasexamined by multiplying the density of bycatch inthese surveys by the fishing effort expended by theIsle of Man registered (local) great scallop fishingfleet over the course of the study. Fishing effortwas obtained from a voluntary logbook schemerun by the Port Erin Marine Laboratory between1981 and 2005 (Beukers-Stewart et al., 2003).Fishermen taking part in this scheme recorded thenumber of hours fished each day and the width ofdredges used. Using a standard fishing speed of
H. R. CRAVEN ET AL.156
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
2.5 knots (Beukers-Stewart et al., 2003) these datacould be multiplied to give the total area of seabed swept by the dredges on each boat, each day.On average approximately 30% of the local fleet,which ranged in size from 53 to 26 boats duringthe study period, took part in the scheme. Wetherefore multiplied the recorded fishing effortlevels by the relevant inverse proportion toestimate the total fishing effort expended by alllocal boats. The total number of fish (all species)captured by the local fleet for each year of thestudy were then calculated. A more in-depthanalysis was also conducted for monkfish whereby,in addition to assessing numbers, the lengthmeasurements from the surveys and the knownlength–weight relationship (Fishbase, 2010) wereused to estimate the biomass of monkfish capturedeach year and this was compared with knowncommercial landings of monkfish.
RESULTS
Overall, 3440 tows of the survey gear were carriedout on the eight fishing grounds between 1992 and2005. Of the 3440 tows, 3357 (97.6%) generatedfish bycatch, with 4697 individuals (1.37 per tow)belonging to approximately 50 species recorded(Table 1). Two types of fish (blennies and 12skate/ray individuals) were identified only tofamily and pipefish only to genus. Most tows,3072 (89.3%) caught only one or two fish; with sixfish being the most recorded from a single tow.The two most abundant species; cuckoo ray andmonkfish, made up almost half (46.82%) of totalfish bycatch (Table 1). The next most abundantspecies, the common dragonet (Callionymus lyra)accounted for a further 12.1% of the bycatch. Twoflatfish species of particular commercial interest;lemon sole and plaice, and the ecologicallyimportant scavenger, lesser spotted dogfish, werealso in the top eight most abundant species andmade up a further 19.7% of the bycatch (Table 1).The only ETP (endangered, threatened orprotected) species recorded were two haddock(Melanogrammus aeglefinus) and one cod (Gadusmorhua), which are (somewhat controversially)classified as vulnerable by the IUCN (www.iucnredlist.org). Figure 2 shows the catch from asingle scallop dredge, demonstrating the typicalrelative proportions of target scallop species(Pecten maximus and Aequipecten opercularis) toinvertebrate and fish bycatch.
Spatial variation
Overall, the density of demersal fish bycatch aroundthe Isle of Man appeared low, with an average of0.297 (� 0.063 SE) fish per 1000m2 towed. TheEast Douglas ground had the highest density ofbycatch, with an average of 0.655 (� 0.067 SE)fish per 1000m2 towed across the 14 years. Laxeyalso showed consistently higher densities (average0.468� 0.052 SE fish per 1000m2) with significantpeaks in 1999 and 2004. Bradda Offshore showedlevels of bycatch close to the average, as didChickens, but had large fluctuations betweenyears. The remaining grounds; Bradda Inshore,15 Miles South, Peel and Targets had similar butlower than average densities; 0.169 (� 0.021 SE),0.146 (� 0.018 SE), 0.193 (� 0.019 SE) and 0.182(� 0.040 SE) fish per 1000m2 towed, respectively,and showed more consistent numbers of fishcaught throughout the duration of the surveys.
The site at which the highest density of each ofthe five species was observed varied. Cuckoo rayand plaice were consistent with the overall trendfor bycatch, showing higher densities at EastDouglas, 0.428 (� 0.071 SE) and 0.014 (� 0.009SE) and Laxey, 0.236 (� 0.037 SE) and 0.036(� 0.009 SE), respectively. Densities of monkfishand lesser spotted dogfish were relatively consistentacross all eight fishing grounds. In contrast to theoverall trend for bycatch, lemon sole had a lowdensity at Laxey (0.006� 0.002 SE) but a higherdensity at Chickens (0.050� 0.009 SE).
Temporal variation
Relative densities of all fish captured around the Isle ofMan remained fairly stable from 1992 to 2005,generally oscillating between 0.2 and 0.4 per 1000m2
towed (Figure 3). There was no significant trendover time (df=1, F=0.169, P=0.711, R2=0.014),however, a larger than average density of fish (almost0.5 per 1000m2) was captured in 2004 (Figure 3).
The most abundant fish captured as bycatch,cuckoo ray, also showed no significant trend overthe course of the surveys (df = 1, F= 0.892,P=0.364, R2= 0.069). Fluctuations in densityfollowed similar patterns to all bycatch, however,there was no large spike in 2004 (Figure 3).
A significant downward trend in the relativedensity of monkfish caught since the surveys startedwas seen (df = 1, F=24.837, P< 0.001, R2=0.674).The highest density of fish captured was in 1995(0.127 per 1000m2 towed) (Figure 3). After this
FISH BYCATCH IN A SCALLOP FISHERY 157
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
densities decreased and reached their lowest level in2000 (0.022 fish per 1000m2 towed), before risingslightly. But at the end of the survey in 2005 thelowest figure had almost been reached again.
In contrast, the relative density of lesser spotteddogfish captured increased over the duration of thesurveys (df = 1, F= 14.184, P=0.003, R2 = 0.542).This increase was consistent throughout the surveyperiod, with the trend appearing to become morepronounced in the last few years of the surveyfrom 2003–2005 (Figure 3).
Effects of dredge type
Catchability of demersal fish was significantlygreater with the queen scallop dredge than withthe great scallop dredge on all grounds whentested with one-way ANOVA (Table 2, Figure 4).However, only two of the most abundant bycatchspecies showed a significant difference between thetwo dredges when analysed with a Kruskal–Wallistest, and this was identified only at certain sites.Lemon sole showed the greatest difference between
Table 1. List of fish species and total numbers captured by queen and scallop dredges during the surveys on the eight main fishing grounds from 1992–2005
Species Total catch
Common name Latin name No. % Cum. %
Cuckoo ray Leucoraja naevus 1276 27.17 27.17Monkfish (Anglerfish) Lophius piscatorius 923 19.65 46.82Common dragonet Callionymus lyra 570 12.14 58.96Lemon sole Microstomus kitt 464 9.88 68.83Lesser spotted dogfish Scyliorhinus caniculus 330 7.03 75.86Dab Limanda limanda 141 3.00 78.86Thickback sole Microchirus variegatus 134 2.85 81.72Plaice Pleuronectes platessa 132 2.81 84.53Blenny Blenniidae 103 2.19 86.72Spotted ray Raja montagui 77 1.64 88.36Red gurnard Aspitrigla cuculus 69 1.47 89.83Topknot Zeugopterus punctatus 57 1.21 91.04Thornback ray Raja clavata 55 1.17 92.21Spotted dragonet Callionymus maculatus 46 0.98 93.19Common (Dover) sole Solea solea 39 0.83 94.02Poor cod Trisopterus minutus 38 0.81 94.83Norwegian topknot Phrynorhombus norvegicus 32 0.68 95.51Two spot clingfish Diplecogaster bimaculata 25 0.53 96.04Pipefish Syngnathus spp. 21 0.45 96.49Lesser sand eel Ammodytes tobianus 20 0.43 96.92Grey gurnard Eutrigla gurnardus 16 0.34 97.26Pogge Agonus cataphractus 16 0.34 97.60Bib (Pouting) Trisopterus luscus 14 0.30 97.90Skate / Ray Rajidae 12 0.26 98.15Brill Scophthalmus rhombus 11 0.23 98.39Common goby Pomatoschistus microps 11 0.23 98.62Megrim Lepidorhombus whiffiagonis 8 0.17 98.79Hake Merluccius merluccius 7 0.15 98.94Solenette Budlossidium luteum 7 0.15 99.09Witch Glyptocephalus cynoglossus 7 0.15 99.24Tub gurnard Trigla lucerna 5 0.11 99.34Atlantic cod Gadus morhua 4 0.09 99.43Cornish sucker Lepadogaster lepadogaster 2 0.04 99.47Haddock Melanogrammus aeglefinus 2 0.04 99.51John dory Zeus faber 2 0.04 99.56Ling Molva molva 2 0.04 99.60Long-rough dab Hippoglossoides platessoides 2 0.04 99.64Norway pout Trisopterus esmarki 2 0.04 99.68Reticulated dragonet Callionymus reticulatus 2 0.04 99.73Scald fish Arnoglossus laterna 2 0.04 99.77Turbot Psetta maxima 2 0.04 99.81Two-spot goby Gobiusculus flavescens 1 0.02 99.83Ballan wrasse Labrus bergylta 1 0.02 99.85Blonde ray Raja brackyura 1 0.02 99.88Bull huss Scyliorhinus stellaris 1 0.02 99.90Crystal goby Crystallogobius linearis 1 0.02 99.92Bullhead Taurulus bubalis 1 0.02 99.94Shanny Lipophrys pholis 1 0.02 99.96Lesser weever Echiichthys vipera 1 0.02 99.98Whiting Merlangius merlangus 1 0.02 100.00Total 4697
H. R. CRAVEN ET AL.158
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
the relative density caught by queen and greatscallop dredges with differences seen at five of thegrounds; Bradda Inshore (P=0.007), BraddaOffshore (P=0.002), 15 Miles South (P=0.025),Peel (P=0.019) and Targets (P=0.010). Lesserspotted dogfish showed a significant difference atfour of the grounds; Bradda Inshore (P< 0.001),Chickens (P =0.014), East Douglas (P=0.019)and Laxey (P=0.002). Cuckoo ray and monkfishshowed a significant difference at only one siteeach, East Douglas (P=0.037) and 15 Miles South(P=0.032), respectively. No significant differencein relative density of fish caught by queen andgreat scallop dredges was recorded for plaice.
Effect of fishing season
All bycatch
The relative density of fish captured in both theOctober and June surveys fluctuated significantlybetween seasons, from as low as no fish beingcaptured at Bradda Inshore and Bradda Offshore in
June 2002 to as high as 1.22 fish per 1000m2 at EastDouglas in October 1995 (Figure 5). At all grounds,apart from Targets, there was a significantdifference in the relative density of fish caughtbetween seasons (i.e. across years) (Table 3). Incontrast, the timings of the surveys (October orJune) only had a significant effect on the density offish captured at two grounds, 15 Miles South andTargets (Table 3). More fish were captured at 15Miles South in the June surveys, at the end of thegreat scallop fishing season, than in October, thebeginning of the fishing season. At Targets, aninteraction between month and season was alsoshown to be significant indicating that there wasfluctuation between years as to whether October orJune surveys captured larger densities of fishper 1000m2 towed. Interactions between monthand season were also seen at Bradda Offshore,East Douglas, and Laxey (Figure 5; Table 3).
At Chickens, Laxey, and Peel the relative densityof fish captured increased on average over thecourse of the surveys, with the biggest increaseseen at Chickens (Figure 5). Although, at thisground, a significant drop in fish densitiescaptured was seen for the last season of thesurvey, 2004/2005. In contrast, density of fishcaptured at East Douglas and Bradda Inshorefell, with the most substantial decrease seen atEast Douglas (Figure 5). Bradda Offshore andTargets showed wide fluctuations in relativedensities caught whereas 15 Miles South showedfairly stable levels of bycatch recorded, highlightedby the fact that no significant difference was seenbetween seasons when tested with 2-way ANOVA(Table 3).
Monkfish
The relative density of monkfish caught in dredgesdiffered significantly between seasons at allgrounds (Table 4). The capture rates for monkfishin both October and June have fallen sincethe surveys started in 1992 (Figure 6). Timings ofthe surveys only had a significant effect attwo grounds, 15 Miles South and Targets, with15 Miles South showing a greater relative densityof monkfish captured at the end of the season inJune. At Targets a significant interaction betweenseason and month was also seen, showing thatseason had an effect on whether the highestdensity of monkfish was captured in October orJune. Interaction between season and month wasalso shown for Peel. Similar levels of monkfish
Figure 2. Typical catch from a single scallop dredge. Great scallops(Pecten maximus) can be seen in the bottom right hand corner andqueen scallops (Aequipecten opercularis) towards the top right handside. Various other species of invertebrate bycatch are also present,with a single monkfish (Lophius piscatorius) in the centre of the
picture (photo: BD Beukers-Stewart).
FISH BYCATCH IN A SCALLOP FISHERY 159
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
were caught across all grounds, apart from slightlylower than average numbers at 15 Miles South.
Size structure of bycatch
Size distribution
Abroad size range of fish were captured, ranging from22mm up to 909mm, however, the most abundantsizes were between 160mm and 300mm (Figure 7).The length distribution of all bycatch caught wassignificantly lower in queen than great scallop
dredges when tested with Kolomogorov–Smirnov(D= 0.239, P< 0.001). This was also seen for thefour most abundant species caught; cuckoo ray(D= 0.243, P< 0.001), monkfish (D= 0.126,P=0.001), lemon sole (D= 0.300, P< 0.001) andlesser spotted dogfish (D= 0.200, P= 0.026).However, the length distribution of plaice caughtdid not significantly differ between the two dredgetypes (D= 0.173, P=0.29).
For cuckoo ray, monkfish, lemon sole and plaicethe majority of fish caught were juveniles (Figure 7)for both great and queen scallop dredges. Lesserspotted dogfish bycatch was more of a mixture ofjuveniles and adult fish.
Mean sizes
The mean length of all fish caught by great scallopdredges was 323.53mm (� 3.21 SE), significantlygreater than the mean length of 263.56mm(� 2.33 SE) caught with queen scallop dredges(t = –14.857, df = 4695, P< 0.001).
The mean length of cuckoo rays (t= –8.864,df= 1274, P <0.001), monkfish (t= –3.215, df= 921,
Figure 3. Temporal variation in the relative density (mean number per 1000m2 towed� SE) of all fish and five key species caught by great scallop andqueen scallop dredges during surveys around the Isle of Man from 1992 to 2005 (all fishing grounds and dredge types combined). Note the different
density scales used for all fish, cuckoo ray, and monkfish.
Table 2. Results of one-way ANOVA examining the variation inrelative densities of demersal fish captured by queen dredges andscallop dredges. P-values in bold indicate a significant result
Ground df SS MS F ratio P-value
Bradda Inshore 1 0.093 0.093 11.623 0.002Bradda Offshore 1 0.355 0.355 18.075 <0.001Chickens 1 0.154 0.154 24.314 <0.001East Douglas 1 1.101 1.101 26.521 <0.00115 Miles South 1 0.060 0.060 13.338 0.003Laxey 1 0.293 0.293 7.410 0.013Peel 1 0.156 0.156 15.679 0.001Targets 1 0.069 0.069 25.236 0.001
H. R. CRAVEN ET AL.160
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
P=0.001), lemon sole (t= –3.184, df = 462,P< 0.001), and lesser spotted dogfish (t = –1.864,df= 328, P=0.044) were all significantly larger inscallop dredges. No significant difference was seenin the length of plaice captured by queen and greatscallop dredges (t = –0.985, df= 130, P=0.326)(Figure 8).
Impact of the local fishing fleet
The total area estimated to have been fished forgreat scallops by the local fishing fleet each seasonfluctuated between highs of above 800 km2 in themid-1990s to a low of approximately 380 km2 in
2000/2001 (Figure 9). It is important to note thatthese figures may represent fishing the same areamore than once. There was no apparent temporaltrend in the data. Estimates of the total number offish caught as bycatch by these boats largelyfollowed these trends in the fishing effort data,with a peak of approximately 230 000 fish in1994/1995 and a low of approximately 55 000 fishin 2000/2001 (Figure 9).
The estimated catch of monkfish by the local greatscallop dredge fleet peaked at approximately70 tonnes (88 000 individuals) in the 1994/1995season and reached a low of approximately 5 tonnes inboth 2001/2002 and 2004/2005 (<12 000 individuals)
Figure 4. Temporal and spatial variation in the relative density (mean number per 1000m2� SE) of all fish captured by great scallop and queen scallopdredges during surveys on eight fishing grounds around the Isle of Man from 1992 to 2005. Only grounds surveyed in June and October of each year
were included. Note the different density scale used at East Douglas and Laxey.
FISH BYCATCH IN A SCALLOP FISHERY 161
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
(Figure 10). Overall there was a general declinein estimated catches of monkfish throughout thestudy period, which reflected the patterns seen bothin these surveys (Figure 3) and the commerciallandings into the Isle of Man (Figure 10).
DISCUSSION
In other scallop dredge fisheries around the world,such as those operating off the east coast of NorthAmerica, fish bycatch rates are high enough to beof concern to managers (McIntyre et al., 2006),raising the possibility of a similar scenario in theUK. The Newhaven dredges examined in the
present study consistently caught certain species ofdemersal fish, including a high proportion ofjuveniles. Overall, rates of fish bycatch around theIsle of Man appeared to be low, especiallycompared with number of the target species (greatscallops) retained in the surveys over the sameperiod: a ratio of 201.95 scallops (all sizes), or 102.96scallops (legal size), per fish (Beukers-Stewart et al.,2003; unpubl. data). It is likely that low rates of fishbycatch were at least partly because background fishdensities around the Isle of Man were alsolow (Bradshaw et al., 2002; FAO, 2008). This isconsidered to be the main reason for the lack ofany recent targeted fisheries for fin-fish (pers.
Figure 5. Annual and seasonal variation in the relative density (mean number per 1000m2 towed� SE) of all fish captured by great scallop and queenscallop dredges during surveys on eight fishing grounds around the Isle of Man from 1992 to 2005 (dredge types combined). Results from Octobersurveys each year are plotted in line with results from June surveys the following year to coincide with the great scallop fishing season. Only
grounds surveyed at the beginning and end of a fishing season were included. Note the different density scale used at East Douglas and Laxey.
H. R. CRAVEN ET AL.162
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
comm., TimCroft, Island Seafare). Unfortunately, nostudies have aimed to directly survey fish densitiesaround the Isle of Man. However, in the absence ofa dedicated dataset, the present study providesinformation on the relative densities of certain fishspecies and suggests that scallop dredging may havehad a negative effect on monkfish, which is likely tohave added to the impacts of the past commercialfishery for this species.
Spatial trends
Relative densities of total fish bycatch variedconsiderably between the eight fishing groundssurveyed, with higher densities observed atEast Douglas and Laxey, which are to the east ofthe Isle of Man. In contrast, sites to the west andsouth generally showed lower densities for totalfish bycatch. Sediment type on either side of theisland varies, with sandy substrata to the east andmore gravelly sediments to the west and south(Barne et al., 1996). Capture rates may thereforehave varied according to the known effect ofsediment type on the efficiency of scallop dredges(Beukers-Stewart et al., 2001; Jenkins et al., 2001).
Alternatively/in addition, the specific habitat(sediment) preferences of different fish species mayhave affected their distribution and abundance (Hinzet al., 2006). A third possibility is that fishingdisturbance, either before or during this study,affected patterns of fish abundance. Since logbookrecords began in 1982, scallop fishing effort hasconsistently been much higher on west coast groundssuch as Bradda Inshore, Chickens, and Peel than onany others, particularly Laxey, where fish catcheswere highest (Beukers-Stewart et al., 2003; unpubl.data). This may appear to suggest fish densities werelower on the most intensively fished grounds.However, the trend was not consistent, for example,15 Miles South was the second least fished area buthad the lowest level of fish by-catch. Patterns ofspatial variation in fish abundance are probably dueto a combination of factors. To determine therelative contribution of each will require furtherinvestigation.
Temporal trends
Rates of total fish bycatch remained relativelystable throughout the duration of the study, with
Table 3. Results of two-way ANOVA examining the relative density of total fish bycatch in different seasons (where each season runs from Octoberuntil the following June) and different months (i.e. June or October). All viable comparisons between 1992 and 2005 were included. P-values in boldindicate a significant result
Factor df SS MS F-ratio P-value
Bradda Inshore Season 11 0.873 0.079 3.070 0.001Month 1 0.091 0.091 3.527 0.063Season by month 11 0.351 0.032 1.235 0.271Error 3.205 124 0.026
Bradda Offshore Season 11 1.565 0.142 2.618 0.005Month 1 0.044 0.044 0.816 0.368Season by month 11 1.128 0.103 1.887 0.047Error 6.628 122 0.054
Chickens Season 11 2.008 0.183 5.101 < 0.001Month 1 0.029 0.029 0.813 0.369Season by month 11 0.438 0.040 1.114 0.354Error 5.725 160 0.036
East Douglas Season 7 3.668 0.524 3.187 0.004Month 1 0.493 0.493 2.999 0.086Season by month 7 3.014 0.431 2.619 0.016Error 16.769 102 0.164
15 Miles South Season 6 0.251 0.042 2.304 0.040Month 1 0.299 0.299 16.444 < 0.001Season by month 6 0.112 0.019 1.029 0.411Error 1.745 96 0.018
Laxey Season 8 2.653 0.332 4.367 <0.001Month 1 0.000 0.000 0.004 0.948Season by month 8 2.085 0.261 3.433 0.002Error 7.213 95 0.076
Peel Season 8 0.823 0.103 2.845 0.006Month 1 0.135 0.135 3.719 0.056Season by month 8 0.217 0.27 0.751 0.647Error 4.412 122 0.36
Targets Season 5 0.713 0.143 1.874 0.108Month 1 0.323 0.323 4.245 0.043Season by month 5 1.024 0.205 2.694 0.027Error 5.931 78 0.76
FISH BYCATCH IN A SCALLOP FISHERY 163
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
only moderate deviations from an average ofapproximately 0.3 fish per 1000m2 towed. Analysisof the most common individual species also revealedthat no significant trend in densities of cuckoo ray,lemon sole, and plaice captured occurred over thecourse of the surveys. This suggests that fishingpressure and/or environmental conditions duringthis period were having either a low or constanteffect on local populations of these species. Morewidespread surveys in the Irish Sea (Ellis et al.,2005) also found the abundance of cuckoo ray to berelatively stable between 1992 and 2003. In contrast,Dulvy et al. (2000) found cuckoo ray abundance tohave increased considerably between surveysconducted in the Irish Sea from 1959–1965 and
those done from 1988–1997. This was hypothesizedto be due to small species such as the cuckoo rayhaving undergone competitive release from thedecrease in abundance of larger species of rays.
In order to assess the significance of bycatch inany fishery it is necessary to know how susceptiblethe species is to capture, what proportionis retained for commercial purposes, and thesurvival rates of those individuals that arediscarded or otherwise affected. Responses tothe effects of fishing also depend on the life-historycharacteristics of the species in question (Bianchiet al., 2000). Productive and robust species oflimited commercial value may therefore be littleaffected by fishing disturbance. In support ofthis argument, densities of lesser spotted dogfishactually increased over the course of these surveys.Lesser spotted dogfish are commonly caughtin both scallop dredges (this study) and trawls(Duncan, 2009; Hinz et al., 2009) and havemoderately slow growth and reproductive rates, butdue to their limited commercial value are generallydiscarded (Ivory et al., 2005). Fortunately, theyhave remarkably high post-discard survival rates ofup to 98% (Revill et al., 2005; Rodríquez-Cabelloet al., 2005), which may be why they appearlittle affected by high levels of fishing disturbance.Furthermore, lesser spotted dogfish are oftenobserved preying on dead or damaged individualsleft behind in the dredge tracks (Eleftheriou andRobertson, 1992; Bianchi et al., 2000; Jenkinset al., 2001, 2004), showing that as a scavengerspecies, they may actually be benefiting from thedredge fishery. Similar trends were observed forinvertebrates in the Irish Sea by Bradshaw et al.(2002) who found that robust, mobile scavengingtaxa had increased in abundance over a 60 yeartime period due to long-term disturbances fromscallop dredging.
In contrast to dogfish, the density of monkfishcaptured in these surveys decreased significantlybetween 1992 and 2005. Monkfish have a highcommercial value (Seafish, 2010) and appeardisproportionately susceptible to capture indredges; most of those landed in the Isle of Man arethought to have come from bycatch in the greatscallop dredge fishery or queen scallop trawl fisheryrather than from a targeted fishery (Duncan, 2009;Andrews et al., 2011). Monkfish generally grow andreproduce slowly and are therefore inherentlyvulnerable to overexploitation (Laurenson et al.,2005). Furthermore, personal observations made
Table 4. Results of two-way ANOVA examining the relative density ofmonkfish captured by dredges in different seasons (where each seasonruns from October until the following June) and different months (i.e.June or October). All viable comparisons between 1992 and 2005were included. P-values in bold indicate a significant result
Factor df SS MS F-ratio P-value
Bradda InshoreSeason 11 0.268 0.024 4.585 <0.001Month 1 0.009 0.009 1.626 0.205Season by month 11 0.032 0.003 0.554 0.863Error 0.660 124 0.005
Bradda OffshoreSeason 11 0.345 0.031 4.444 <0.001Month 1 0.017 0.017 2.406 0.123Season by month 11 0.115 0.010 1.479 0.148Error 0.860 122 0.007
ChickensSeason 11 0.128 0.012 1.990 0.033Month 1 0.000 0.000 0.024 0.878Season by month 11 0.027 0.002 0.417 0.947Error 0.932 160 0.006
East DouglasSeason 7 0.167 0.024 3.382 0.003Month 1 0.000 0.000 0.041 0.839Season by month 7 0.080 0.011 1.614 0.140Error 0.721 102 0.007
15 Miles SouthSeason 6 0.063 0.010 2.589 0.023Month 1 0.026 0.026 6.534 0.012Season by month 6 0.021 0.003 0.847 0.537Error 0.388 96 0.004
LaxeySeason 8 0.190 0.024 7.397 <0.001Month 1 0.004 0.004 1.165 0.283Season by month 8 0.032 0.004 1.233 0.288Error 0.305 95 0.003
PeelSeason 8 0.124 0.015 2.822 0.007Month 1 0.008 0.008 1.535 0.218Season by month 8 0.184 0.023 4.196 <0.001Error 0.668 122 0.005
TargetsSeason 5 0.281 0.056 9.406 <0.001Month 1 0.048 0.048 8.119 0.003Season by month 5 0.117 0.023 3.938 0.006Error 0.465 78 0.006
H. R. CRAVEN ET AL.164
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
over 8 years of conducting these surveys indicatedthat monkfish routinely suffer serious damagewhen captured in scallop dredges. Post-discardsurvival of monkfish is therefore likely to be verylow. The trends in the data correspond withcommercial landings of monkfish into the Isle ofMan and from the Irish Sea in general, whichfell dramatically over the same time period asthis study (Figure 11; FAO, 2008; ICES, 2010).There are no data on commercial fishing effortfor monkfish in this area, but the landingsfigures indicate the fishery has been negligible since1999/2000. The data suggest that the stock has
stayed low since then. Recovery of monkfishpopulations may be hindered by continueddisturbance from the scallop fishery (see below).
It should be noted when observing these overalltrends in fish bycatch that there are some potentialbiases in the data. Not all grounds were coveredin every survey (Figure 4), therefore whengrounds on the east of the island, which hadhigher than average densities, were not surveyed,this may have resulted in an estimate of lowerthan average density across the Isle of Man.Likewise, in some cases the reverse may havebeen true. However, the length of our dataset
Figure 6. Annual and seasonal variation in the relative density (mean number per 1000m2 towed� SE) of monkfish (Lophius piscatorius) caught bygreat scallop and queen scallop dredges during surveys on eight fishing grounds around the Isle of Man from 1992 to 2005 (dredge typescombined). Results from October surveys each year are plotted in line with results from June surveys the following year to coincide with the greatscallop fishing season. Only grounds surveyed at the beginning and end of a fishing season were included. Note the different density scale used at
15 Miles South and Targets.
FISH BYCATCH IN A SCALLOP FISHERY 165
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
indicates it is likely to be robust to theseoccasional deviations and that the overall trendsobserved were real.
Effect of dredge type
Queen scallop dredges captured significantlymore fish bycatch overall than great scallopdredges, probably because they were designedfor a smaller more mobile species. However,when considered on a species by species basisonly lemon sole and lesser spotted dogfishshowed any significant difference in levelscaught between the two gears and this was onlyseen at certain fishing grounds. Queen scallopdredges are no longer used in the Isle of Man,having largely been replaced by otter trawls(Andrews et al., 2011). Unfortunately fishbycatch is even higher in otter trawls than itwas in dredges (Duncan, 2009), although theyappear to retain somewhat different species (seebelow).
Figure 7. Size frequency distribution of bycatch caught around the Isle of Man for all species and for the five most abundant species captured. Thegreen line represents the average length of maturity for each species (Fishbase, 2010). Note the different scales.
Figure 8. Mean length (� SE) of the five key species captured by greatscallop and queen scallop dredges during surveys around the Isle of
Man from 1992 to 2005.
H. R. CRAVEN ET AL.166
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
Effect of fishing season
A significant effect of survey timing (June or October)on fish bycatch levels was observed on someoccasions, however, there was little consistency oroverall trend in these patterns. This is perhaps notsurprising, as during our study most of the fishspecies studied were being subject to year rounddisturbance – by great scallop dredging in the winterand by queen scallop fishing (both dredging andtrawling) in the summer. Furthermore, seasonal(water temperature) effects on fish behaviour andintra-annual growth of individuals is likely to haveinfluenced their catchability in complex ways(Arreguin-Sanchez, 1996). Clear trends in thedensity of fish bycatch therefore only becameapparent over the full duration of the study.
Size structure of bycatch
Great scallop dredges captured significantly largerfish than the queen scallop dredges, with a mean
length of 324mm caught by great scallop dredgescompared with 264mm in the queen scallopdredges. This difference was less pronounced whenthe main species were considered individually, butwas still significant, except for plaice (possibly dueto low sample size).
Bycatch of lesser spotted dogfish was a mixtureof both adults and juveniles, but bycatch of allother main species predominately consisted ofjuveniles. For example, looking solely at greatscallop dredges, which are now the main type usedaround the Isle of Man, the mean length ofmonkfish captured was only 343mm. This is farbelow a length at which this species matures(730mm for females and 490mm for males)(Afonso-Dias and Hislop, 1996).
Impact of the local fishing fleet
Estimates of total fish bycatch by the Isle of Mangreat scallop fleet indicated that in some years asmany as 230 000 individuals may have beencaught. To put this into context, Enever et al.(2007) estimated that as many as 3.3 million fishmay be caught annually by the entire scallopdredge fleet (vessels≥ 10m registered length only)operating in the English Channel, WesternApproaches, Celtic and Irish seas (ICES subareaVII). This level of bycatch, however, is dwarfed bythe amount of bycatch from beam and ottertrawlers operating in the same area, which wasestimated at 110.1 million fish and cephalopodsper year (Enever et al., 2007). Otter trawls are nowbeing increasingly used in the Isle of Man fisheryfor queen scallops, and these certainly havesubstantially higher total levels of fish bycatchthan dredges (Duncan, 2009; Hinz et al., 2009).However, the trawl fishery appears to affect asomewhat different component of the fishcommunity, with a much higher proportion ofgadoid bycatch, in particular whiting, Merlangiusmerlangus (Duncan, 2009). In comparison, thisin-depth analysis of the scallop dredge fisherysuggests it is capturing a disproportionate amountof strictly benthic species; in particular monkfish,which were almost never caught in otter trawls(Duncan, 2009).
The evidence from this study suggests that incombination with the commercial fishery, scallopdredging has had a significant impact on monkfishstocks around the Isle of Man. Both these surveysand the commercial landings data demonstrated asignificant decline in monkfish abundance over the
Figure 10. Estimates of total weight (primary axis) and numbers(secondary axis) of monkfish (Lophius piscatorius) caught by the Isleof Man great scallop fleet during each fishing season (November toMay inclusive). For comparison the total weight of commerciallandings of monkfish into the Isle of Man over the same time period
is also shown.
Figure 9. Temporal variation in estimates of total area fished by the Isleof Man great scallop fleet (primary axis) and total number of fishcaught (secondary axis) during each fishing season (November to
May inclusive) from 1992 to 2005.
FISH BYCATCH IN A SCALLOP FISHERY 167
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
course of the study period. The estimates of totalmonkfish catch by the local great scallop dredgefleet also exceeded the known commercial landingsinto the Isle of Man. Furthermore, these estimatesdo not include the catch from the previouslyactive queen scallop dredge fleet, or that taken byvisiting vessels, which generally far outnumber thelocal fleet (Brand, 2006; Beukers-Stewart andBeukers-Stewart, 2009). When these considerationsare combined with the fact that most of themonkfish caught by the dredges were juveniles, itappears highly likely that the scallop fishery hashindered recruitment into the adult stock. Themore widespread decline of monkfish throughoutthe Irish Sea (ICES, 2010) may also havecontributed to the reduction in numbers around theIsle of Man, as mixing between sub-populations isthought to occur (Laurenson et al., 2005).Environmental factors are unlikely to beresponsible for this regional decline, as in all otherareas around the UK monkfish stocks have beenincreasing in recent years (Seafish, 2010). Instead, itappears that various fishing pressures (includingscallop dredging) have combined to reduce stocksize throughout the Irish Sea.
The 14year duration of this present study hasrevealed a number of interesting trends, however,this is still a relatively short-time scale comparedwith the length of time that scallop dredging andother fisheries have been taking place around the Isleof Man (70+ years). A baseline of information fromnon-dredged areas is also lacking. Thereforededucing the original composition of fish faunabefore dredging is difficult. It has been noted byprevious studies that the effects of fishing are mostsevere in the early phases of a fishery (Kaiser et al.,1996; Jennings and Kaiser, 1998; Hall-Spencer andMoore, 2000), therefore it is highly likely thatscallop dredging and other fisheries around the Isleof Man had already altered the composition ofdemersal fish before these bycatch datawere recorded.
To fully gauge the impact of scallop dredging onbenthic fish even longer-term and larger-scaledatasets need to be collected. These need to be ata species level as trends for specific species wherelower numbers are seen could be masked bylooking at data overall (Dulvy et al., 2000).
CONCLUSIONS
The potential effect of scallop dredging on benthicfish communities has received little attention to
date. The present study indicates that althoughbycatch rates in the surveys originally appeared low,when they are scaled up to represent commercialfleet activity they may represent substantial amountsof certain species. Furthermore, scallop dredgingwill not only be affecting benthic fish via directremoval as bycatch, but also through damage toessential fish habitat (Rosenberg et al., 2000) andindirect mortality caused when individuals are hit,but not retained, by the passing dredges. Levels ofindirect mortality in some invertebrate species havebeen estimated to cause even greater levels ofmortality than bycatch (Eleftheriou and Robertson,1992; Jenkins et al., 2001).
Benthic fish form an integral part of the naturalecosystem, therefore increased mortality byremoval or damage from scallop dredges couldhave important ramifications not just on anypotential fishery for them but on the function of theecosystem (Thrush et al., 1995). For species wherepopulations appear to have reached low levels, e.g.monkfish around the Isle of Man, the removal ofjuveniles as bycatch in the scallop dredges could behindering the recovery or even viability of thesepopulations. The impact of scallop dredging onbenthic fish may increasingly become a problem inthe future as there is currently little control of effortin the UK fishery for scallops and demand for theseshellfish appears to be continuously expanding(Shephard et al., 2010). A more rigorous, ecosystembased management system, which addresses all ofthe impacts of the UK scallop fishery, is sorelyneeded. Paramount to this is continued and morewidespread monitoring of all bycatch, includingfin-fish, in scallop fisheries. If areas are identifiedwhere high levels of fish are being captured,particularly juveniles and/or vulnerable species,then this issue could be managed with real timeclosures (Diamond and Beukers-Stewart, 2011) andadaptation of fishing gear (McIntyre et al., 2006).Complex and biogenic habitats such as maerl,which are known to be particularly importantnursery and feeding areas for fish and other species(Kamenos et al., 2004), are particularly susceptibleto damage from scallop dredging (Hall-Spencer andMoore, 2000) and should be afforded full protection.
ACKNOWLEDGEMENTS
This research was largely funded by the Departmentof Environment, Food and Agriculture of the Isle ofMan Government. Supplementary funding was
H. R. CRAVEN ET AL.168
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
provided by the Marine Biological Association ofthe United Kingdom. Many thanks to the skippersand crews of the RV Roagan, FV Spaven Mor, FVde Bounty and FV Heather Maid, who madecollecting these data possible, and to the countlesspeople who helped with the surveys over the years,especially Graham Hughes, Belinda Vause,Matthew Mosley and Jon Kenny. Will Rowlandsalso put in a sterling effort by re-entering all ofthese data from the original datasheets in 2006.Thanks must also go to Katie Brooks and LeighHowarth for their valuable comments on drafts ofthe manuscript, Steve Rocliffe and Reinhard Kypefor helping with the figures, Alex Senechal forhelping process the fishing effort data, and Tim Croftfor providing information on Isle of Man fishlandings. Comments from Dr John Baxter,Dr David Donnan and an anonymous reviewerundoubtedly improved our original submission.
REFERENCES
Afonso-Dias IP, Hislop JRG. 1996. The reproduction ofanglerfish Lophius piscatorius Linnaeus from the north-westcoast of Scotland. Journal of Fish Biology 49: 18–39.
Andrews JW, Brand AR, Holt TJ. 2011. MSC assessmentreport for Isle of Man queen scallop trawl and dredgefishery. Client: Isle of Man Government. Version 4:Final report. http://www.msc.org/track-a-fishery/certified/north-east-atlantic/Isle-of-Man-queen-scallop/assessment-downloads[13 September 2011].
Arreguin-Sanchez F. 1996. Catchability: a key parameter forfish stock assessment. Reviews in Fish Biology and Fisheries6: 221–242.
Barne JH, Robson CF, Kaznowska SS, Doody JP, DavidsonNC (eds). 1996. Coasts and Seas of the United Kingdom.Region 13 Northern Irish Sea: Colwyn Bay to Stranraer,including the Isle of Man. Joint Nature ConservationCommittee: Peterborough, UK.
Beukers-Stewart BD, Beukers-Stewart JS. 2009. Principles forthe management of inshore scallop fisheries around theUnited Kingdom. Report to Natural England, CountrysideCouncil for Wales and Scottish Natural Heritage:University of York, United Kingdom.
Beukers-Stewart BD, Jenkins SR, Brand AR. 2001. Theefficiency and selectivity of spring-toothed scallop dredges:a comparison of direct and indirect methods of assessment.Journal of Shellfish Research 20: 121–126.
Beukers-Stewart BD, Mosley MWJ, Brand AR. 2003.Population dynamics and predictions in the Isle of Manfishery for the great scallop (Pecten maximus, L.). ICESJournal of Marine Science 60: 224–242.
Beukers-Stewart BD, Vause BJ, Mosley MWJ, Rossetti HL,Brand AR. 2005. Benefits of closed area protection for apopulation of scallops. Marine Ecology Progress Series 298:189–204.
Bianchi, G, Gislason H, Graham K, Hill L, Jin X, KorantengK, Manickchand-Heileman S, Paya I, Sainsbury K,Sanchez F, Zwanenburgm K. 2000. Impact of fishing onsize composition and diversity of demersal fishcommunities. ICES Journal of Marine Science 57: 558–571.
BradshawC,Veale LO, BrandAR. 2002. The role of scallop-dredgedisturbance in long-term changes in Irish Sea benthiccommunities: a re-analysis of an historical dataset. Journal ofSea Research 47: 161–184.
Brand AR. 2006. The European Scallop Fisheries for Pectenmaximus, Aequipecten opercularis and Mimachlamys varia.In Scallops: Biology, Ecology and Aquaculture. Shumway S,Parsons GJ (eds). Elsevier: Amsterdam; 991–1058.
Brand AR, Allison EH, Murphy EJ. 1991. North Irish Seascallop fisheries: a review of changes. In An InternationalCompendium of Scallop Biology and Culture. Shumway SE,Sandifer PA (eds). World Aquaculture Society: BatonRouge; 204–218.
Brand, AR, Beukers-Stewart BD, Vause BJ, Mosley MWJ.2005. Shellfish research report to the Isle of ManGovernment, Department of Agriculture, Fisheries andForestry. University of Liverpool, United Kingdom.
Chapman CJ, Mason J, Kinnear JAM. 1977. Divingobservations on the efficiency of dredges used in theScottish fishery for the scallop, Pecten maximus (L.).Scottish Fisheries Research Report, Aberdeen, UnitedKingdom.
Collie JS, Escanero GA, Valentine PC. 1997. Effects of bottomfishing on the benthic megafauna of Georges Bank. MarineEcology Progress Series 155: 159–172.
Collie JS, Hall SJ, Kaiser MJ, Poiner IR. 2000. A quantitativeanalysis of fishing impacts on shelf-sea benthos. Journal ofAnimal Ecology 69: 785–798.
Diamond B, Beukers-Stewart BD. 2011. Fisheries discards inthe North Sea: waste of resources or a necessary evil?Reviews in Fisheries Science 19: 231–245.
Dulvy NK, Metcalfe JD, Glanville J, Pawson MG, Reynolds J.2000. Fishery stability, local extinctions, and shifts incommunity structure in skates. Conservation Biology 14:283–293.
Duncan P. 2009. An assessment of bycatch in the Isle of Manqueen scallop trawl fishery. Report for the Isle of ManGovernment, Department of Agriculture, Fisheries and Forestry.http://www.gov.im/lib/docs/daff/anassessmentofbycatchintheisle.pdf [19 September 2011]
Eleftheriou A, Robertson MR. 1992. The effects ofexperimental scallop dredging on the fauna and physicalenvironment of a shallow sandy community. NetherlandsJournal of Sea Research 30: 289–299.
Ellis JR, Dulvy NK, Jennings S, Parker-Humphreys M, RogersSI. 2005. Assessing the status of demersal elasmobranchs inUK waters: a review. Journal of the Marine BiologicalAssociation of the United Kingdom 85: 1025–1047.
Enever R, Revill A, Grant A. 2007. Discarding in the EnglishChannel, Western Approaches, Celtic and Irish seas (ICESsubarea VII). Fisheries Research 86: 143–152.
FAO. 2008. FISHSTAT Plus: universal software for the fisherystatistical time series, version 2.3 2000. http://www.fao.org/fishery/statistics/software/fishstat/en [19 September 2011]
Fishbase. 2010. Lophius piscatorius Linnaeus, 1758, Anglerhttp://www.fishbase.org/search.php [18 June 2010]
Gray JS, Dayton P, Thrush S, Kaiser MJ. 2006. On effects oftrawling, benthos and sampling design. Marine PollutionBulletin 52: 840–843.
Hall-Spencer JM, Moore PG. 2000. Scallop dredging hasprofound effects, long terms impacts on maerl habitats.ICES Journal of Marine Science 57: 1407–1415.
Hill AS, Brand AR, Wilson UAW, Veale LO, Hawkins SJ.1996. Estimation of by-catch composition and numbersof by-catch animals killed annually on Manx scallopfishing grounds. In: Aquatic Predators and their Prey,Greenstreet SPR, Tasker ML (eds). Blackwell Scientific:Oxford; 111–115.
FISH BYCATCH IN A SCALLOP FISHERY 169
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)
Hinz H, Bergmann M, Shucksmith R, Kaiser MJ, Rogers SI.2006. Habitat association of plaice, sole, and lemon sole inthe English Channel. ICES Journal of Marine Science 63:912–927.
Hinz H, Murray LG, Kaiser MJ. 2009. Efficiency andenvironmental impacts of three different Queen scallopfishing gears. Fisheries and Conservation report No. 8,Bangor University, Bangor, United Kingdom.
Howarth LM, Wood HL, Turner AP, Beukers-Stewart BD.2011. Complex habitat boosts scallop recruitment in a fullyprotected marine reserve. Marine Biology 158: 1767–1780.
ICES. 2010. Catch statistics for ICES subarea VIIa. http://www.ices.dk/fish/CATChSTATISTICS.asp [18 June 2010]
Ivory P, Jeal F, Nolan CP. 2005. Age determination, growthand reproduction in the lesser-spotted dogfish, Scyliorhinuscanicula (L.). Journal of Northwest Atlantic Fishery Science35: 89–106.
Jenkins SR, Beukers-Stewart BD, Brand AR. 2001. Impact ofscallop dredging on benthic megafauna: a comparison ofdamage levels in capture and non-captured organisms.Marine Ecology Progress Series 215: 297–301.
Jenkins SR, Mullen C, Brand AR. 2004. Predator andscavenger aggregation to discarded by-catch from dredgefisheries: importance of damage level. Journal of SeaResearch 51: 69–76.
Jennings S, Kaiser MJ. 1998. The effects of fishing on marineecosystems. Advances in Marine Biology 34: 201–351.
Jones JB. 1992. Environmental impact of trawling on the seabed: a review. NZ Journal of Marine and FreshwaterResearch 26: 59–67.
Kaiser MJ, Hill AS, Ramsay K, Spencer BE, Brand AR, VealeLO, Prudden K, Rees EIS, Munday BW, Ball B, Hawkins SJ.1996. Benthic disturbance by fishing gear in the Irish Sea: acomparison of beam trawling and scallop dredging. AquaticConservation: Marine and Freshwater Ecosystems 6: 269–285.
Kaiser MJ, Clarke KR, Hinz H, Austen MCV, Somerfield PJ,Karakassis I. 2006. Global analysis of response andrecovery of benthic biota to fishing. Marine EcologyProgress Series 311: 1–14.
Kamenos NA, Moore PG, Hall-Spencer JM. 2004. Small-scaledistribution of juvenile gadoids in shallow inshore waters;what role does maerl play? ICES Journal of Marine Science61: 422–429.
Laurenson CH, Johnson A, Priede IG. 2005. Movements andgrowth of monkfish Lophius piscatorius tagged at theShetland Islands, northeastern Atlantic. Fisheries Research71: 185–195.
McDonald JH. 2009. Handbook of Biological Statistics, 2ndedn. Sparky House Publishing: Baltimore, ML.
McIntyre TM, Cunningham R, Robert G, Branton R. 2006.Gear trial experiment to reduce groundfish bycatch in theoffshore scallop fishery on Georges Bank – an introduction.Canadian Manuscript Report of Fisheries and AquaticSciences 2745:1.
Philippart CJM. 1998. Long-term impact of bottom fisheries onseveral by-catch species of demersal fish and benthicinvertebrates in the south-eastern North Sea. ICES Journalof Marine Science 55: 342–352.
Pikitch EK, Santora C, Babcock EA, Bakun A, Bonfil R,Conover DO, Dayton P, Doukakis P, Fluharty D,Heneman B, et al. 2004. Ecosystem-based fisherymanagement. Science 305: 346–347.
Revill A, Dulvy NK, Holst R. 2005. The survival of discardedlesser-spotted dogfish (Scyliorhinus canicula) in the westernEnglish Channel beam trawl fishery. Fisheries Research 71:121–124.
Rodríquez-Cabello C, Fernández A, Olaso I, Sánchez F. 2005.Survival of small-spotted catshark (Scyliorhinus canicula)discarded by trawlers in the Cantabrian Sea. Journal of theMarine Biological Association of the United Kingdom 85:1145–1150.
Rosenberg A, Bigford TE, Leathery S, Hill RL, Bickers K.2000. Ecosystem approaches to fishery managementthrough essential fish habitat. Bulletin of Marine Science 66:535–542.
Seafish. 2010. Responsible sourcing guide: monkfish. Version 4May 2010. http://www.seafish.org/media/sustainability/responsible-sourcing-guides [14 September 2011]
Sea-Fisheries (Queen Scallop Fishing) Bye-Laws. 2010.Statutory Document No, 668/10. http://www.gov.im/lib/docs/daff/Fisheries/queeniebyelawfinal07aug2010.pdf [14May 2012]
Shephard S, Beukers-Stewart B, Hiddink JG, Brand AR,Kaiser MJ. 2010. Strengthening recruitment of exploitedscallops Pecten maximus with ocean warming. MarineBiology 157: 91–97.
Thrush SF, Dayton PK. 2002. Disturbance to marine benthichabitats by trawling and dredging: implications formarine biodiversity. Annual Review of Ecological Systems33: 449–473.
Thrush SF, Hewitt JE, Cummings VJ, Dayton PK. 1995. Theimpact of habitat disturbance by scallop dredging onmarine benthic communities: what can be predicted fromthe results of experiments? Marine Ecology Progress Series129: 141–150.
Thrush SF, Hewitt JE, Cummings VJ, Dayton PK, Cryer M,Turner SJ, Funnell GA, Budd RG, Milburn CJ, WilkinsonMR. 1998. Disturbance of the marine benthic habitat bycommercial fishing: impacts at the scale of the fishery.Ecological Applications 8: 866–879.
Underwood AJ. 1997. Experiments in Ecology: their LogicalDesign and Interpretation using Analysis of Variance.Cambridge University Press: Cambridge.
Vause BJ, Beukers-Stewart BD, Brand AR. 2007. Fluctuationsand forecasts in the fishery for queen scallops (Aequipectenopercularis) around the Isle of Man. ICES Journal ofMarine Science 64: 1124–1135.
Veale L, Hill AS, Brand AR. 2000a. An in situ study ofpredator aggregations on scallop (Pecten maximus (L.))dredge discards using a static time-lapse camera system.Journal of Experimental Marine Biology and Ecology 255:111–129.
Veale LO, Hill AS, Hawkins SJ, Brand AR. 2000b. Effects oflong-term physical disturbance by commercial scallopfishing on subtidal epifaunal assemblages and habitats.Marine Biology 137: 325–337.
H. R. CRAVEN ET AL.170
Copyright # 2012 John Wiley & Sons, Ltd. Aquatic Conserv: Mar. Freshw. Ecosyst. 23: 152–170 (2013)