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Journal of Sea Research 50 (2003) 129–137

Flatfish discarding practices in bivalve dredge fishing

off the south coast of Portugal (Algarve)

J. Palma*, C. Reis, J.P. Andrade

CCMar, Faculdade de Ciencias do Mar e do Ambiente, Campus de Gambelas, Universidade do Algarve, 8000-810 Faro, Portugal

Received 15 October 2002; accepted 28 April 2003

Abstract

An evaluation of flatfish discards from bivalve dredge fishing on the south coast of Portugal (Algarve) was undertaken

through fishing surveys carried out on board a professional dredge fleet. Data collection of the dredge fishery was carried out

between November 2000 and February 2002. The data were compared with information gathered from abundance analysis

surveys, and analysed independently of the target bivalve species (Chamelea gallina, Donnax trunculus, and Spisula solida).

Flatfish and non-flatfish species accounted for 20.1% and 79.9%, respectively, of the by-catch species of the bivalve dredge

fishery. Significant differences have been calculated between length composition of flatfish species in the by-catch and in the

abundance surveys. This indicates an optimised performance of the bivalve dredge, which catches a small amount of undersized

flatfish as by-catch.

D 2003 Elsevier B.V. All rights reserved.

Keywords: Bivalve fishery; Dredges; Flatfish; By-catch; Portugal

1. Introduction studied by some authors (Bergman and Hup, 1992;

Fishing gear used to catch demersal fish and shell-

fish often disturbs both the seabed and the organisms

living within or on it (Collie et al., 2000), since the way

in which trawling is conducted affects the survival of

all the animals caught in the trawl (Wassenberg and

Hill, 1989). The passage of a bivalve dredge, as with

any other type of trawl across the seabed, leads to

direct and/or indirect mortality of both commercial and

non-commercial species. Indirect fishing mortality for

beam, bottom and otter trawl has been extensively

1385-1101/$ - see front matter D 2003 Elsevier B.V. All rights reserved.

doi:10.1016/S1385-1101(03)00065-0

* Corresponding author.

E-mail address: jpalma@ualg.pt (J. Palma).

Fonds, 1994; De Groot and Lindeboom, 1994; Kaiser

et al., 1994; Kaiser and Spencer, 1995; Engel and

Kvitek, 1998; Piet et al., 2000; Bergmann and Moore,

2001a,b; Bergmann et al., 2001).

Although bivalve dredging gear is specifically

designed to catch bivalves, it also catches some

(undersized) fish and benthic invertebrates. These

animals are rapidly sorted on board and specimens

that are undersized or of no commercial interest are

returned to the sea. Thus, the main issue is to ascertain

the chances of survival of the discarded fish, since

some of them may be permanently damaged by the

gear and will not survive after they had been returned

to the sea (Bergman and Van Santbrink, 2000).

Although some research has focussed on the effect

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137130

of the bivalve dredge on benthic invertebrates, more

recently attention has shifted to the impact on the fish

community, because some of the species caught

constitute very important resources. Economically,

flatfish species are the most important fish family

captured as by-catch of the bivalve dredge fishery on

the south coast of Portugal (Algarve).

According to Portuguese regulations, during the

bivalve dredge activity it is not permitted to maintain

on board, or to land, any type of aquatic organism

other than the bivalves. However, it is known that the

larger individuals of some fish species are kept on

board for the fishermen’s own consumption (due to

factors such as high market price and gastronomic

interest). It is important to know whether the number

of fish captured by bivalve dredging is substantial and

should therefore be considered in management and

conservation studies.

In Portugal, bivalve dredge fishing is operated by

small vessels (the so-called local fleet) and larger

vessels (the coastal fleet). Small vessels work single

dredges, while larger vessels work simultaneously

with two dredges. The design of the currently used

dredges is based in a prototype developed by IPIMAR

(Portuguese Institute for Sea Research) (Fig. 1). It has

an iron grid reservoir connected with the mouth for

catch retention, instead of the traditional net bag. The

major improvement of this collector is that less debris

and undersized bivalves and fish are hauled on to the

deck. The use of this modified gear has been regulated

by law since November 2000. In order to endorse a

sustainable fishery, input controls were imposed, such

as limitations in the fishing gear (dredges) and vessel

characteristics (size and engine power). Closures were

Fig. 1. Design of the dredge currently used in the bivalve dre

also imposed, and hydraulic dredging was forbidden.

Output controls set limits on the minimum catch size

of bivalves and restrict the daily catch quota per boat.

Presently, the dredge fleet catches a large variety of

species, the most important being the donax clam

(Donax trunculus), the striped venus (Chamelea gal-

lina), the white clam (Spisula solida), and the razor

clam (Pharus legumen). According to official data, a

total of 8000 t was caught between 1995 and 1999 on

the south coast of Portugal (Algarve), with an average

yield of 1600 t y� 1. Although with slight fluctuations,

the yearly production has been kept stable since

1999–2000.

The by-catch of this bivalve fishery, especially that

of the discarded fish species, has not been quantified.

Gaspar and Monteiro (1998) studied the mortality due

to fishing operation and/or on board handling, but

only bivalve species were considered.

This study intends to evaluate the impact caused

by the bivalve dredging on the fish communities on

the south coast of Portugal (Algarve), with special

emphasis on flatfish. Within this group, some atten-

tion was also paid to the capture of juveniles. A

comparison of the fishing yield with abundance

information gathered in those same fishing grounds

was also performed.

2. Material and methods

2.1. Data collection

Data collection of the dredge fishery was under-

taken between 27 November 2000 and 28 February

dge fishery along the south coast of Portugal (Algarve).

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137 131

2002 and was based on information gathered directly

from the fishing surveys made on board commercial

dredging vessels on the south coast of Portugal

(Algarve) (Fig. 2). Surveys were performed on board

two fishing vessels (FV) (FV1, 11 m, 95 hp; FV2, 9

m, 91 hp) in the area between Armac�ao de Pera (37j04VN 008j 24VW) and Vila Real de Santo Antonio

(37j 12VN 007j 25VW). Dredges had a mouth height

of 50 cm, tooth spacing of 1.5 cm, tooth length of 10

cm, and 12 mm distance between the bars of the iron

grid (Fig. 2).

Operational parameters such as towing speed and

duration were the same as during normal commercial

fleet operations, and were the same amongst all boats.

Tows were usually performed with the same speed

(3F 0.5 knots) and duration (21F 3 min). Tows were

performed at similar depths (3.4F 0.7m). The duration

of sorting time for all the material hauled on deck did

not exceed 10 min. Physical damage due to handling

and compression by the weight of the catch was also

moderate since the total catch in each tow (bivalve,

debris, fish, cephalopods and invertebrates, when

occurring) did not exceed 50 kg (J. Palma, pers. obs.).

Fig. 2. Map of sampling area.

Individuals in the by-catch were identified at the

species-level, weighed (to the nearest 0.1 g) and

measured (to the nearest lower 0.1 cm). Whenever

it was impossible to weigh the individuals on board

the commercial dredge vessels, the weight/length

relationships suggested by Dorel (1985), Goncalves

et al. (1997), and Andrade et al. (2000) were

applied.

Short-term mortality (mortality following trawling)

was assessed after discards had been separated from

the bivalves and debris. The percentage of each

species in the total by-catch was calculated as well

as the number of fish under the minimum legal catch

size (% undersized).

Abundance analysis was performed in the same

area as where the commercial fleet operates. Surveys

were performed during the summer of 2001, using

two different fishing gears, the beach seine and the

beam trawl. The beach seine was operated during low

tide in the bivalve fishing grounds. A 60-m-long net

(with 15 mm mesh size at the cod-end) was dragged to

land from a distance of 70 m off the shoreline. The

beam trawl was operated during high tide in the same

fishing grounds as where the bivalve dredge fleet

operates. This gear had a 5 m mouth opening (with

the same mesh size as the beach seine) and was

trawled through a determined distance (interpolated

from a GPS device) at a constant speed of 3 knots.

Biological information on the specimens caught in

this experiment was collected as described above for

the survey data. Differences of the by-catch total

numbers of individuals, numbers of species, and

density of selected species (number of individuals

per 100 m2) were examined. To avoid a seasonal

intrusion in the statistical analysis, only the survey

data collected during summer were compared with the

abundance data.

2.2. Data analysis

One-way ANOVAs (Zar, 1984) were used to

compare the seasonal differences in the length sizes

among the flatfish species. The same method was

used to analyse the differences in the length sizes

of the specimens collected from the dredge surveys

and abundance surveys. Only species with more

than ten specimens collected were considered for

statistical analysis. Significant differences were con-

Table 1

Number, length (averageF stand. dev.), percentage of undersized specimens (according to the present legislation of the species) and abundance

of fish collected during the dredge and abundance surveys

Family Species Dredge surveys Abundance surveys

N Av. Length

F sd (cm)

%

undersized

%

in catch

N Av. Length

F sd (cm)

%

undersized

Abundance

(njind m� 2)

Bothidae Arnoglossus thori – – – – 242 9.2F 2.66 * 0.93

Bothus podas – – – – 155 8.63F 3.14 * 0.60

Scophthalmidae Psetta maxima 13 26.7F 7.1 76.92 0.45 1 32.1 0 0.00

Scophthalmus rhombus 105 23.1F 4.4 94.24 3.64 23 15.47F 4.85 100 0.09

Soleidae Dicologoglossa cuneata 71 18.9F 3.5 19.71 2.46 8 12.31F 3.05 75 0.03

Microchirus boscanion 1 6.6 * 0.03 21 5.89F 0.83 * 0.08

Solea lascaris 234 23.8F 3.1 64.95 8.11 97 12.37F 4.73 98.97 0.37

Solea senegalensis 56 29.8F 8 7.14 1.94 1 26.5 0 0.00

Solea vulgaris 2 21.6F 0.8 100 0.07 – – – –

Synaptura lusitanica 48 31.5F 4.6 * 1.66 – – – –

Ammodytidae Hyperoplus lanceolatus 130 14F 5.7 * 4.50 8 9.11F 2.99 * 0.03

Atherinidae Atherina presbyter 2 9F 0.14 * 0.07 1788 10.35F 1.26 * 6.88

Balistidae Balistes carolinensis 2 29.1F1.6 * 0.07 – – – –

Batrachoididae Halobatrachus didactylus 205 16.3F 8.36 * 7.10 – – – –

Blennidae Parablennius pilicornis 1 7.8 * 0.03 – – – –

Callionymidae Callionymus lyra 8 19.5F 6.1 * 0.28 1 9.6 * 0.00

Callionymus marmuratus 1 20.5 * 0.03 22 7.95F 1.68 * 0.08

Carangidae Trachurus trachurus – – – – 263 8.11F1.4 100 1.01

Trachynotus ovatus – – – – 2 10F 0.71 * 0.01

Clupeidae Sardina pilchardus – – – – 340 6.26F 1.88 98.17 1.31

Gadidae Merluccius merluccius 2 12.7F 0.42 * 0.07 – – – –

Gobiidae Pomatochistus spp. – – – – 39 4.99F 0.68 * 0.15

Labridae Labrus spp. 1 9.5 * 0.03 – – – –

Symphodus bailloni – – – – 5 4.26F 1.54 * 0.02

Symphodus cinereus – – – – 1 9 * 0.00

Moronidae Dicentrarchus labrax 1 14.5 * 0.03 – – – –

Mugilidae Lisa aurata 1 25.6 * 0.03 – – – –

Mullidae Mullus surmuletus – – – – 37 8.31F1.05 100 0.14

Nemichthyidae Ophisurus serpens 1 83.3 * 0.03 – – – –

Rajidae Raja clavata 15 21.9F 9.1 * 0.52 – – – –

Raja undulata 42 48.9F 7.5 * 1.46 11 25.58F 12.65 * 0.04

Sparidae Diplodus annularis 6 13.8F 2.3 66.67 0.21 – – – –

Diplodus bellottii 28 14.8F 3.46 35.71 0.97 – – – –

Diplodus sargus 5 13.2F 2.8 100 0.17 – – – –

Diplodus vulgaris 1 18.7 0 0.03 25 4.53F 1.86 100 0.10

Lithognathus mormyrus 189 14.6F 3.7 79.90 6.55 62 8.23F 2.06 98.39 0.24

Pagellus erythrinus – – – – 1 15.2 0 0.00

Pagrus auriga – – – – 1 7.7 100 0.00

Pagrus pagrus – – – – 1 4.6 100 0.00

Sparus aurata – – – – 2 18.1F 0.28 100 0.01

Spondyliosoma cantharus 4 14.1F 7 100 0.14 24 6.88F 0.85 100 0.09

Syngnathidae Hippocampus hippocampus – – – – 7 8.44F 0.92 * 0.03

Syngnathus acus – – – – 8 10.54F 2.12 * 0.03

Syngnathus spp. 1 13.8 * 0.03 2 10.35F 2.62 * 0.01

Torpedinidae Torpedo marmorata 1 19.1 * 0.03 – – – –

Torpedo torpedo 75 22.1F 8.9 * 2.60 – – – –

Trachinidae Trachinus araneus 2 14.8F 3.4 * 0.07 – – – –

Trachinus draco 410 17.7F 3.4 * 14.21 2 23F 13.01 * 0.01

Trachinus radiatus 2 13.5F 0.6 * 0.07 – – – –

Trachinus vipera 1195 9.8F 1.4 * 41.41 102 7.43F 1.48 * 0.39

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137132

Family Species Dredge surveys Abundance surveys

N Av. Length

F sd (cm)

%

undersized

%

in catch

N Av. Length

F sd (cm)

%

undersized

Abundance

(njind m� 2)

Triglidae Trigla latrovisa – – – – 1 19.4 * 0.00

Trigla lucerna – – – – 2 13.7F 13.86 * 0.01

Trigla spp. 24 29.8F 6.3 * 0.83 3 13.43F 13.74 * 0.01

Uranoscopidae Uranoscopus scaber 1 15.5 * 0.03 – – – –

*Not regulated by Portuguese law.

Table 1 (continued)

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137 133

sidered for P < 0.05. Significance of p-values was

corrected using the Bonferroni method (Snedecor

and Cochran, 1982).

3. Results

3.1. Survey analysis

A total of 84 surveys were conducted throughout

the sampling period. A total of 37 different species

belonging to 20 fish families were captured during

sampling. Two flatfish families were collected

throughout this period, the Scophthalmidae, Psetta

maxima and Scophthalmus rhombus, and the Soleidae,

Dicologoglossa cuneata, Solea lascaris, Solea sene-

galensis, Solea vulgaris, Synaptura lusitanica, and

Microchirus boscanion (Table 1). The Trachinidae

and the Sparidae were the most abundant non-flatfish

families, and were represented by four and five differ-

ent species each. Trachinus vipera was the most

abundant by-catch fish species, representing 41.4%

of the total fish catch. S. lascaris was the most

abundant flatfish species corresponding to 8.1% of

the total catch (Table 1). Flatfish represented 20.1% of

Fig. 3. Number of specimens collected (both flatfi

the total by-catch, whereas non-flatfish species

accounted for 79.9%. S. vulgaris, S. rhombus and P.

maxima were the flatfish species with the highest

percentages of undersized specimens with 100%,

94.2% and 76.9%, respectively. Two of the non-

flatfish species also had 100% of individuals below

the minimum allowable catch size (Diplodus sargus

and Spondyliosoma cantharus) (Table 1).

By-catch was higher during autumn, decreasing

throughout winter and spring (Fig. 3). Inversely, the

CPUE (number of specimens captured per 100 kg of

captured bivalves) showed higher values during

spring (Fig. 4).

The overall short-term mortality was 63.8%, sim-

ilar to the value observed in the non-flatfish group

(63.0%); however, it was slightly higher in the flatfish

group (67.0%).

3.2. Abundance analysis

A total of 12 surveys were conducted for abun-

dance estimates. Despite the difference between

gears, the area sampled was approximately the same

(F 26000 m2). Descriptive statistics and abundances

of the species collected are presented in Table 1.

sh and non-flatfish species) in each season.

Fig. 4. Number of specimens collected per 100 kg of bivalves (CPUE) (both flatfish and non-flatfish species) in each season.

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137134

The Bothidae, Arnoglossus thori (44.16%) and

Bothus podas (28.28%), were the commonest flat-

fish, followed by S. lascaris (17.7%). Atherina

presbyter (Atherinidae) was the most abundant spe-

cies (Table 1).

The wide majority of the specimens collected were

below the legal minimum catch size. Among the

flatfish group, more than 75% of specimens were

undersized in three (S. rhombus, D. cuneata and S.

lascaris) out of four species (Table 1). The same

occurred with the non-flatfish species, where in seven

of the ten legislated species, all specimens were below

the legal minimum catch size (Table 1).

3.3. Statistical analysis

Significant differences in seasonal size variation

were only found in three of the five analysed flatfish

species (S. lascaris, S. rhombus and S. lusitanica).

Significant differences were only found for the pair-

wise comparisons between the summer samples and

the remaining seasons (S. lascaris, summer/autumn,

P < 0.0004, F-value 12.93, df 1,145, summer/winter,

P < 0.0001, F-value 15.37, df 1,105, spring/summer,

P < 0.0001, F-value 21.35, df 1,54; S. rhombus, sum-

mer/winter, P < 0.008, F-value 7.59, df 1,54; S. lusi-

tanica, summer/winter, P < 0.02, F-value 6.41, df

1,19).

Only S. rhombus and S. lascaris could be com-

pared between the dredge surveys and abundance

surveys. A. thori and B. podas were not captured in

the dredge surveys, and S. lusitanica and S. vulgaris

were not captured in the abundance surveys. The

small number of specimens of P. maxima, D. cuneata,

M. boscanion and S. senegalensis in one of the

samples did not allow statistical comparisons. Signif-

icant differences in size between samples were found

for both species (S. rhombus, P < 0.0001, F-value,

36.18, df 1,126; S. lascaris, P < 0.0001, F-value

528.37, df 1,331).

4. Discussion

By-catch is perhaps the most general environmen-

tally harmful impact of modern fisheries (Dayton et

al., 1995). Recently, several studies have dealt with

the effect of trawling on invertebrate benthic commu-

nities (e.g. Engel and Kvitek, 1998; Collie et al.,

2000; Bergmann and Moore, 2001a,b; Bergmann et

al., 2001), and fish communities (e.g. Van Beek et al.,

1990; Hill and Wassenberg, 1990; McAllister and

Spiller, 1994; Kaiser and Spencer, 1995). Environ-

mental disturbance of the fish communities is in most

cases less significant, since the high mobility of fish

permits them to leave seriously impacted grounds.

Yet, depending on their feeding preferences, fishes

can also be attracted to trawled or dredged areas in

search of benthic invertebrates exposed by these

activities. Both trawling and dredging cause consid-

erable mortality depending on the type of gear (Collie

et al., 2000). However, discards can provide an

important source of food for benthic scavengers

(Bergman and Van Santbrink, 1994). According to

Castro and Sarda (1999), benthic scavengers consume

the entire discard that reaches the bottom. In this

particular case, results indicated that discards were

totally recycled by scavengers including the Norway

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137 135

lobster (Nephrops norvegicus) and shrimp (Parape-

enaeus longirostris) (themselves the target species

that originated the discards), less than 24 hours after

they had been thrown overboard.

This is especially true of trawling activity with a

prolonged tow time performed at greater depths. Under

such conditions, the by-catch arrives dead on board or

dies onboard, not only because of the prolonged tow

time and greater depths, but also due to towing speed,

catch size and composition (Bergman et al., 1998),

enhanced mortality through hypoxia (Spicer et al.,

1990), temperature changes (Zainal et al., 1992), and

physical damage due to handling and compression by

the weight of the catch (Wileman et al., 1999).

In the bivalve dredge fishery on the south coast of

Portugal (Algarve), most of the parameters mentioned

above seem to play a minor role in post-fishing mor-

tality. Hypoxia or temperature changes during on-deck

exposure are reduced, because the stay of the animals

on board does not usually exceed 10 min, and the

fishing activity is only performed in the morning, when

air temperature is relatively low. Depth variation is also

negligible since there is no difference between the

hydrostatic pressure at the dredging depth (approxi-

mately 3–4 m) and at the surface.

The compression of the catch weight also has little

impact on by-catch survival. It was observed that in the

majority of the tows the total weight of the catch did not

exceeded 20 kg, and only rarely was it above 50 kg.

It seems that immediate mortality or lesions in live

specimens were caused by mechanical action of the

gear tooth and/or by abrasion through other compo-

nents of the catch (small rocks and shells) inside the

iron bag. This agrees with the results of Van Beek et

al. (1990) and Kaiser and Spencer (1995), who

indicated mechanical action as the main cause of by-

catch mortality.

Hill and Wassenberg (1990, 2000) found higher

values of immediate fish mortality than those obtained

in this study (63.8%). The overall fish mortality was

91.7% (Palma et al., unpubl. data), which is less than

that presented by other authors (e.g. Hill and Wassen-

berg, 2000) and similar to the results of Van Beek et

al. (1990). Unfortunately, these overall values may be

slightly higher due to the retention on board of larger

specimens by the fishermen, sometimes in perfect

condition with no injuries or loss of scales (J. Palma,

pers. obs.).

High survival rates of fish are usually mentioned

when fish escape from the cod end of the trawl nets

(e.g. Van Beek et al., 1990; Kaiser and Spencer, 1995;

Wileman et al., 1999). In the particular case of the

gear used in the Portuguese bivalve dredging the iron

grids of the receptacle are placed parallel to the

opening of the gear (see Fig. 1), which allows smaller

fishes (especially flatfish) to flow harmlessly out of

the gear, assuring a survival rate of nearly 100%. The

flatfish length is in direct relation to their height, and

consequently species that usually do not attain large

sizes, or small specimens of larger species, can pass

through the bars of the dredge iron bag.

This is confirmed by the results obtained from the

abundance surveys. B. podas and A. thori are the

most abundant flatfish species in the area sampled

and were not captured during the dredge surveys.

The remaining flatfish species captured by both gears

showed highly statistically differences in size

(P < 0.0001), indicating that smaller specimens pass

through the dredge because they were present in the

fishing grounds. Results also indicate that the fishing

grounds are inhabited by the juveniles of most of the

species. The length at first maturity is 35 cm for S.

senegalensis, and 22.5 cm for S. lascaris (Andrade,

1990); 33 cm for S. rhombus and 35 cm for P.

maxima (Bauchot, 1987) and 14 to 16 cm to D.

cuneata (Desoutter, 1990). Specimens with lengths

below the ones mentioned above were found for

each of the species.

Large fishes seem to be less sensitive to sediment

grain size than small fishes because they can be buried

in a wider range of sediments (Gibson and Robb,

1992; Phelan et al., 2001). Nonetheless, the sediment

preferences of small fish change with growth (Stoner

and Ottmar, 2003), leading them to occupy the same

habitats as the adults. This preference might occur in

the study area due to a constant east-western long-

shore drift that promotes sediment transportation

along shore creating homogeneity of the sandy bot-

toms along the oceanic shore of the south coast of

Portugal (Algarve) (Faugeres et al., 1985; Bettencourt

et al., 1989). This environmental constraint leads to a

coexistence of the different life stages in the same

area, which might be targeted by any type of fishing

gear operating in this area.

The analysis of the survey data on a seasonal

basis can induce misleading conclusions due to the

J. Palma et al. / Journal of Sea Research 50 (2003) 129–137136

different number of surveys performed in each sea-

son. However, the results showed that some of the

species have their peak of occurrence in a particular

season (namely during cooler seasons), but not in a

manner that the number of species during autumn

and winter overlap the ones that occur during spring

and summer, when environmental conditions provide

higher growth rates. Such an assumption is corrob-

orated by the fact that when verified, significant

length differences occurred between the specimens

captured during summer and those captured in other

seasons.

From an ecological perspective, the impact of

the bivalve dredge fishery on fish communities is

similar to that of other trawling gears. However, in

this specific case the impact is higher on maturing

and adult specimens and affects a considerably

lower number of specimens than other trawling

gears. Due to the strong hydrodynamics and sedi-

ment type (sandy bottom) of the sampled area, the

environmental impact is also much smaller than for

other trawling activities.

Based on the data stated above, an extrapolation

to the entire fleet is not only necessary but also

inevitable. Although undersized flatfish were caught,

the length composition of the catch was quite dif-

ferent from the faunal composition in the surveyed

areas. This is a clear indication of the good perfor-

mance of the dredge gears. Due to the characteristics

of the gear presently in use, it is quite difficult to

reduce the number of captured flatfishes without

decreasing the bivalve dredge efficiency.

Although there was a moderate impact on flatfish

species, this study shows that the capture of flatfish

and their effective retention on board on an illegal

basis as presently occurs in the bivalve dredge

fishery should be considered in the management

and conservation of the fish stocks of the south

coast of Portugal (Algarve).

Acknowledgements

This work was funded by the EC project DG XIV,

ref. 99/062. The authors are indebted to the crews of

the fishing vessels for their assistance. The manuscript

benefited from the comments of three anonymous

referees.

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