sharks of the open ocean || methods to reduce bycatch mortality in longline fisheries

Chapter 36

Methods to Reduce Bycatch Mortality in Longline Fisheries

Daniel L. Erickson and Steven A. Berkeley

Abstract

Potential methods for reducing bycatch mortality in longline fi sheries were examined by two independent studies. Experiments were conducted onboard commercial fi shing boats in the Gulf of Mexico (pelagic longlines; 1994–1997) and the Gulf of Alaska (demersal long lines; 1999). Hook timers, instruments that record the moment when fi sh strike at baited hooks, and motion detectors were used to determine the amount of time that fi sh spent hooked on longlines. For pelagic longlines, which were often soaked longer than 20 hours, hook-timer data revealed that mortality of pelagic fi shes (e.g., swordfi sh, Xiphias gladius) increased with greater time spent on the longline. This mortality varied by species, ranging from 100% within 12 hours for swordfi sh to 30% after 12 hours for sharks. Motion-detector data showed that most demersal fi sh (e.g., Pacifi c halibut, Hippoglossus stenolepis) struck at baited hooks within 3 hours after longlines were set, even though sets were soaked for up to 9 hours. Results of these experiments suggest that soaking longlines no more than some optimal duration (e.g., signifi cantly less than 20 hours for pelagic longlines) may increase the survival of bycatch species while maintaining the catch of target species. Optimal soak-ing duration likely varies by fi shery. The surest method for reducing bycatch mortality in any fi shery, however, is to avoid hooking unwanted bycatch in the fi rst place. One approach is to develop species-selective baits. We provide an example of an artifi cial bait developed for demersal longline fi sheries that caught target species (i.e., Pacifi c halibut and sablefi sh, Anoplopoma fi mbria) as effi ciently as natural bait, while almost eliminating the catch of nontarget species (e.g., squalid sharks and skates).

Key words: artifi cial bait, bycatch mortality, selectivity, bycatch reduction, longline fi sheries, Alaska, Gulf of Mexico, halibut, sablefi sh, tuna, swordfi sh.

Introduction

Shark is the principal “bycatch” species in most pelagic longline fi sheries (Joyce, 1999; Matsunaga and Nakano, 1999; Pawson and Vince, 1999). The high level of shark bycatch,

Sharks of the Open Ocean: Biology, Fisheries and Conservation. Edited by M. D. Camhi, E. K. Pikitch and E. A. Babcock

© 2008 Blackwell Publishing Ltd. ISBN: 978-0632-05995-9

Methods to Reduce Bycatch Mortality 463

coupled with the life-history characteristics of sharks, has resulted in dramatic popula-tion declines for many shark populations throughout the world (Camhi, 1999). Although some researchers (e.g., Matsunaga and Nakano, 1999) have suggested that certain pelagic sharks have shown a constant or increasing catch per unit effort (CPUE) over the past two decades, most report declining catch rates for pelagic sharks in the Atlantic, Pacifi c, Gulf of Mexico, and the Caribbean (e.g., Cailliet et al., 1991; Holts et al., 1998; Cortés, 1999; Cramer, 1999).

Regulatory measures (where they exist) designed to reduce shark bycatch mortality in longline and other fi sheries include time and area closures, catch quotas, and size limits (see Cailliet et al., 1991). Time and area closures are effective at reducing bycatch only when target and nontarget species segregate spatially, which is generally not the case for the pelagic sharks caught in longline fi sheries targeting tunas (Boggs, 1992; Ward et al., 2004). Catch quotas and size limits can be effective for target species, but often result in discarding and subsequent unaccounted mortality of bycatch species in fi sheries with no or limited observer coverage, because discards are usually underreported (Pikitch et al., 1988; Gillis et al., 1995; D. L. Erickson, personal observation). Other management approaches are needed to ensure sustainable shark populations. This chapter presents two alternatives for reducing shark bycatch mortality: shorter soaking periods and species-selective baits.

Methods

These data were collected during two independent fi eld studies: a pelagic longline experi-ment conducted in the Gulf of Mexico during 1994–1997 and a demersal longline study that took place in the Gulf of Alaska in 1999. Instrumentation was used during both studies to describe the effects of soaking duration on catch and bycatch mortality. Species selectivity of a new artifi cial bait was tested in the Alaska trials.

Gulf of Mexico pelagic longline experiment

A pelagic longline experiment was conducted between November 1994 and May 1997 in the Gulf of Mexico onboard commercial fi shing vessels targeting yellowfi n tuna (Thunnus albacares, Scombridae) and occasionally swordfi sh (Xiphias gladius, Xiphiidae). This work was conducted during regular commercial fi shing operations (i.e., vessels were not chartered) onboard four vessels that ranged in length between 20 and 33 m. Up to 1,200 hooks were fi shed in each set on mainlines that were as long as 78 km. Three to twelve hooks were fi shed between fl oats. Buoy drops (length of line between the surface buoy and attachment to mainline) were 18–37 m long; monofi lament gangions (distance from the clip on mainline to hook) were 13–64 m, most often 18–37 m in length. Sets targeting yellowfi n tuna mostly used 15-0 and 16-0 circle hooks, whereas 12-0 J-hooks were used during swordfi sh sets. Lightsticks were used only when targeting swordfi sh. Soak times for each hook were determined as the elapsed time from when the baited hook went overboard to the time it was retrieved.

Hook timers (Boggs, 1992) capable of recording the time a fi sh was caught (hook-up time) were attached to as many as 500 gangions per set. In addition, water temperature

464 Sharks of the Open Ocean

profi les were recorded by expendable bathythermographs (XBTs) deployed at the start and end of each set and haulback. Up to eight time–depth recorders were deployed, spaced along the length of the mainline and attached at the midpoint between buoys, to estimate the depth of each hook during the course of the soak. Instrumentation used during this experiment allowed us to determine hook-up time, hooking duration (hours hooked on the line before landing), temperature and depth of hook-up, and the thermal structure of the water column.

All fi sh were visually assessed as dead or alive immediately after they were brought alongside the vessel. Fish were either landed or released, and their disposition (kept or dis-carded) was recorded. Data recorded for each hook timer included the elapsed time logged by hook timers, time of day when retrieved, and the location on the line that each fi sh was caught. Fish length (measured or estimated), sex (for retained catch), and detailed informa-tion on hook type and gangion length were also recorded. Logistic regression analysis (Cox and Snell, 1989) was used to predict the proportion of fi sh alive (by species or species group). Billfi sh (Istiophoridae) and sharks were analyzed as species groups because there were insuffi cient numbers to allow analysis of individual species. Only one dependent vari-able (hooking duration) was used in this analysis.

Gulf of Alaska demersal longline experiment

Two commercial longline vessels (14.3 and 16.6 m in length) were chartered for this experiment in the Gulf of Alaska in 1999. Both vessels carried individual fi shing quo-tas (IFQs) for retention of Pacifi c halibut (Hippoglossus stenolepis, Pleuronectidae) and sablefi sh (Anoplopoma fi mbria, Anoplopomatidae). Limited bycatch quotas of Pacifi c cod (Gadus macrocephalus, Gadidae) and rockfi sh (Sebastes, Scorpaenidae) were permitted under the IFQs. Spiny dogfi sh shark (Squalus acanthias, Squalidae), various skate species (e.g., longnose skate, Raja rhina, Rajidae), and arrowtooth fl ounder (Atheresthes stomias, Pleuronectidae) constituted most of the bycatch and were discarded.

Sixty-two longline sets were made in the Gulf of Alaska. Two hundred 12-0 circle hooks were deployed per set on 1.7 km of demersal groundline. Soak time in hours was calculated as the time elapsed between the submergence of the last hook while setting gear until the emergence of the fi rst hook during longline retrieval. Gangions consisted of 0.5-m mono-fi lament (136 kg test). Motion detectors (V. Afanasyev, Cambridge, UK), capable of record-ing up to 99 hook motions per minute, were attached to every tenth gangion during certain sets. A space of approximately 10 m on each side of these gangions was free of baited hooks to reduce the chance of recording activity from neighboring hooks. These motion detectors enabled us to determine hook-up time and hooking duration. Hook-up time was defi ned as the moment when more than 50 motions per minute were recorded by the detectors. This threshold level was established after viewing numerous underwater videotapes of Alaskan groundfi sh being captured by longline gear.

Although 62 experimental longline sets were employed to evaluate the catching per-formance of six types (or recipes) of artifi cial baits against the herring bait that is often used by Alaskan longline fi shers, comparisons for only one artifi cial bait type (#6) are presented here. Artifi cial baits were made of natural and biodegradable materials (Alaska Fisheries Development Foundation, 2000), and were developed exclusively for these experiments by Marco Marine (Seattle, WA) and the Center for Applied Regional Studies


(Dr. Susan Goldhor, Cambridge, MA). These baits are not commercially available. Twelve longline sets were conducted for testing artifi cial bait #6; sets comprised 100 hooks baited with herring and 100 hooks baited with artifi cial bait #6. Bait type was alternated on every tenth hook. Groundlines included 20–30 m of free space between bait types to mini-mize potential interactions between different bait types on adjacent hooks.

Results

Gulf of Mexico pelagic longline experiment

Seventy-nine pelagic longline sets were sampled in the Gulf of Mexico between November 1994 and May 1997. Retained catch consisted of yellowfi n tuna (n � 485), swordfi sh (n � 61), dolphin (Coryphaena hippurus, Coryphaenidae; n � 181), wahoo (Acantho cybium solandri, Scombridae; n � 47), escolar (Lepidocybium fl avobrunneum, Gempylidae; n � 153), bigeye tuna (Thunnus obesus, Scombridae; n � 8), and bluefi n tuna (Thunnus thynnus, Scombridae; n � 2). There were 103 sharks recorded in the catch (Table 36.1), nearly all of which were discarded.

The pelagic longlines were often soaked for long periods, in some cases, for more than 24 hours (Fig. 36.1). The proportion of fi sh alive when landed was inversely related to time on the line (Fig. 36.2), hence the mortality of captured fi sh increased as soak time increased. Of the pelagic species shown, sharks were most resistant to hooking mortality; after an initial mortality of approximately 20%, little additional mortality was observed even after 12 hours on the line. The opposite pattern was demonstrated by swordfi sh, most of which died on the line within 6 hours after hook-up. A number of these sword-fi sh were shark-bitten, but it is impossible to know whether they were alive or dead when this occurred. Most swordfi sh in this study were discarded (70%), largely because they were below the minimum size (125 cm lower jaw–fork length). There appeared to be an initial hooking mortality of approximately 40% for yellowfi n tuna on pelagic longlines. This mortality rate increased to approximately 65% after 12 hours on the line. Tuna that died on the line were either discarded or fetched a low price (due to poor quality) relative

Table 36.1 Species composition of sharks caught on pelagic longlines during an experiment conducted in the Gulf of Mexico.

Family Common name Scientifi c name Number

Carcharhinidae Silky Carcharhinus falciformis 53 Oceanic whitetip Carcharhinus longimanus 5 Dusky Carcharhinus obscurus 4 Tiger Galeocerdo cuvier 4 Sandbar Carcharhinus plumbeus 2 Blue Prionace glauca 1Alopiidae Bigeye thresher Alopias superciliosus 6Lamnidae Longfi n mako Isurus paucus 4 Shortfi n mako Isurus oxyrinchus 4Sphyrnidae Scalloped hammerhead Sphyrna lewini 2Unknown Unidentifi ed Unknown 18


Mean soak time (hours)10 15 20 25 30

Perc

ent

freq

uen

cy

0

5

10

15

20

25

Fig. 36.1 Frequency distribution of mean soak times for 79 pelagic longline sets in the Gulf of Mexico. Note that individual hooks may soak much longer than the mean soak time.

Hours after hook-up

Pro

bab

ility

of

bei

ng

aliv

e

0.0

0.2

0.4

0.6

0.8

1.0 Yellowfin

Swordfish

Billfish

Sharks

0 6 8 10 1242

Fig. 36.2 Results of logistic regression analysis showing the proportion of billfi sh (n � 41), swordfi sh (n � 46), yellowfi n tuna (n � 237), and sharks (n � 41) alive versus length of time hooked on a pelagic longline in the Gulf of Mexico.

to individuals that were retrieved while still alive. Of the species analyzed, billfi sh in the family Istiophoridae (which included blue marlin, Makaira nigricans; white marlin, Tetrapturus albidus; sailfi sh, Istiophorous platypterus; and spearfi sh, Tetrapturus angusti-rostris) had the lowest initial hooking mortality (approximately 10%). Mortality for these species increased to 65% after 12 hours on the line.


Gulf of Alaska demersal longline experiment

Two to four demersal longline sets were made on each fi shing day at depths ranging from 71 to 310 m. Soak times were either short (�3 hours) or long (6–9 hours). Equal numbers of short and long soaks were made on each day.

Although we sampled 62 demersal longline sets that were soaked from 2.5 to 8.9 hours, only sets that were soaked in excess of 6 hours were used to examine the distribution of hook-up times for captured fi sh (Fig. 36.3). Furthermore, only gangions containing motion detectors and herring-baited hooks (n � 130) were used to describe this distribution of hook-up times. Fish were landed on 50 of the 130 hooks associated with motion detectors (24 Pacifi c cod, 9 sablefi sh, 7 spiny dogfi sh shark, 4 halibut, 5 arrowtooth fl ounder, and 1 longnose skate); 80 hooks were hauled back with no catch. None of the landed fi sh were hooked beyond 3 hours after bait entered the water, even though sets included in this anal-ysis were soaked from 6.4 to 8.9 hours. As many as 43 of the 80 empty hooks may have caught fi sh temporarily (i.e., for which motion detectors showed �50 motions per minute), but these escaped before longline retrieval. Figure 36.4 shows a fi sh that was hooked about 1 hour after the bait entered the water and that escaped 20 minutes later.

Artifi cial bait #6 nearly eliminated the catch of sharks and skates during demersal long-line fi shing operations in the Gulf of Alaska, whereas the catch of target species (Pacifi c halibut and sablefi sh) using this artifi cial bait was similar to that using herring-baited hooks (Fig. 36.5). The difference in catch between the two baits was signifi cant for spiny dogfi sh shark (paired t-test for 10 sets, p � 0.005), but not signifi cant for the target species (for 12 sets, p � 0.05). Because of a limited sample size, differences were not statistically signifi cant between bait types for catches of longnose skate (for 5 sets, p � 0.05).

0

5

10

15

20

25

0.5 1 1.5 2 2.5 3 4 5 6 7 8 9

Freq

uen

cy

Time elapsed before capture (hours)

Fig. 36.3 Distribution of time elapsed before capture for fi sh caught by demersal longlines baited with herring in the Gulf of Alaska in 1999. Only results for longlines soaked for more than 6 hours are shown (range � 6.5–8.9 hours; n � 130 gangions). Cell ranges are: 0.5 � �0.5 hour; 1 � 0.5 to �1 hour; 1.5 � 1 to �1.5 hours; 2 � 1.5 to �2 hours; and so on.


Discussion

Data presented in this chapter and by Boggs (1992) demonstrate that the longer fi sh are hooked on pelagic longlines, the more likely they will be dead upon longline retrieval. We found that hooking mortality rates were species dependent. For example, mortality of hooked fi sh was slow and constant for sharks but rapid for swordfi sh. Boggs (1992) also

Fig. 36.4 Movements of a single motion detector attached to a gangion on a demersal longline in the Gulf of Alaska in 1999. A fi sh was caught about 1 hour after the set was made and escaped 20 minutes later. This longline was retrieved about 5 hours after the fi sh escaped.

0

25

50

75

100

0:00 1:00 2:00 3:00 4:00 5:00 6:00Time elapsed (hours:minutes)

Mo

tio

ns

per

min

ute

Set

Hooked

Escaped

Haulback

Fig. 36.5 Demersal longline catch (number of fi sh by species) using herring and artifi cial bait #6 in the Gulf of Alaska in 1999. Comparisons were paired. Even though 12 sets were made, only sets that caught the species of interest were included for statistical analyses. Sample sizes were: Pacifi c halibut, n � 12; sablefi sh, n � 12; spiny dogfi sh shark, n � 10 (two sets caught none); and longnose skate, n � 5 (seven sets caught none).

0

50

100

150

200

Pacifichalibut

Sablefish Spinydogfishshark

Longnoseskate

Cat

ch (

nu

mb

ers)

ArtificialHerring


showed that the rate of hooking mortality varied among species, ranging from more than 60% mortality within 3 hours of hooking for spearfi sh to less than 20% mortality after 7–9 hours on the line for bigeye tuna. Overall, hooking mortality is high (greater than 80%) for species such as wahoo (Boggs, 1992), skipjack tuna (Katsuwonus pelamis, Scombridae; Boggs, 1992; Ward et al., 2004), and swordfi sh, whereas it is substantially lower (0–40%) for other species such as pelagic sharks (Boggs, 1992; Ward et al., 2004). Pelagic sharks, however, are sensitive to exploitation (Schindler et al., 2002); hence, even low levels of unaccounted mortality (i.e., discard mortality) could impact certain shark populations.

Pelagic longlines are often soaked for 20 hours or longer during normal commercial fi sh-ing operations (Ward et al., 2004). Our results suggest that a reduction in soak time would increase the survival of discarded bycatch. However, there may be concern from the com-mercial fi shing industry that shortened soaks may also lead to lower catches of target species. Indeed, Ward et al. (2004) showed a tendency for higher catch rates (number of fi sh per 1,000 hooks) as soak times increased to 20 hours. Yet their results were species dependent, and in many cases the soak time either showed no relationship to catch rate or was negatively corre-lated to catch rate. For example, soak time was positively correlated with catch for most shark and billfi sh species, whereas there was no clear relationship between soak time and catch for tuna and various bony fi shes. Ward et al. (2004) showed that there was some amount of loss rate during pelagic longline fi shing operations (e.g., due to escapement and bait loss) and, for many species, an optimal soaking time (less than 20 hours) that would maximize catch.

We have described a study conducted in the Gulf of Alaska that utilized special instru-mentation to estimate optimal soaking times for demersal longlines. This method showed that herring-baited hooks attracted fi sh only during the initial 3 hours of longline sets that were soaked for 6–9 hours. High (1980) and Sigler (2000) also showed a clear inverse relation between catch rate and bait soaking time for demersal longlines targeting Pacifi c halibut and sablefi sh in Alaskan waters. Sigler (2000) suggested that the reduced encoun-ter rate over time was related to the diminishing odor concentration at the edge of the odor plume, whereas High (1980), using direct observations, showed that decreasing hook-up rate over time was caused by bait loss (e.g., bait taken by predators and scavengers such as fi sh, shrimp, and crab) and escapement. This rapid decline in fi shing effi ciency has been documented for other demersal longline fi sheries. Grimes et al. (1982) observed 70% bait loss for hooks soaked for 190 minutes on the ocean bottom in the Mid-Atlantic Bight. They attributed much of this bait loss to predation by starfi sh (Astropectin spp.) and crabs (Cancer spp.).

Our Alaska data reveal that signifi cant numbers of targeted catch escaped, or were eaten by sharks and subsequently unmarketable. Escapement of hooked fi sh represents an additional reason for decreased catch over time on longlines. Data from motion detectors attached to gangions suggested that 43 of 80 empty hooks may have temporarily caught fi sh that managed to escape. As fi sh were landed on 50 hooks, this result suggests a 46% escape rate for fi sh striking hooks on demersal longlines. High (1980) also found signifi cant escapement (25%) for fi sh that were temporarily caught on demersal longlines in Alaska.

Since the incidence of escapement and the number of shark-bitten fi sh likely increase with the time spent hooked on longlines, and because there exists an inverse relationship between soak time and catch rates for many species (demersal and pelagic), it may not be cost-effective to fi sh longline gear for extended soaking periods (e.g., more than 5 hours for demersal longlines in Alaska). In pelagic longline fi sheries, dead tuna are often discarded or


fetch a lower price because of their poor quality. Since catch rates of target species decline as baits are lost, and hooked fi sh die or are attacked by sharks, limiting soak time to some optimal duration may be one simple way to reduce bycatch mortality while minimizing the impact to fi shers’ income. Clearly, the optimal soak time will vary among fi sheries.

The surest method for minimizing bycatch mortality is to eliminate bycatch in the fi rst place. Bait type is one of the most important gear parameters for species selectivity in long-line fi sheries (Løkkeborg and Bjordal, 1992). We showed that an artifi cial bait fi shed as well as (or better than) natural baits for target species, while the same artifi cial bait nearly eliminated the catch of bycatch species (e.g., sharks and rays). Others have shown species-selective responses to bait for various demersal fi sheries (Løkkeborg and Bjordal, 1992). Januma et al. (1999) developed an artifi cial bait for tuna longline fi sheries; however, the hooking rates were lower than that of natural baits, and therefore the bait was not considered successful. Hence, virtually no successful work has been conducted in the area of species-selective artifi cial baits for pelagic longline fi sheries. Much more experimentation is needed to produce species-selective longline baits and longline gear.

We have presented two possible approaches to reducing bycatch mortality in pelagic longline fi sheries: species-selective baits and reduced soak times. However, we believe that the most effective way to reduce bycatch in a fi shery is to provide fi shers with incentives to develop their own solutions and to utilize useful solutions that are already available. The scientifi c community should assist and collaborate with the industry in developing these solutions.

Acknowledgments

The Alaskan fi eld trials were conducted in collaboration with Marco Marine (Seattle, WA), Susan Goldhor of the Center for Applied Regional Studies (Cambridge, MA), the Alaska Fisheries Development Foundation (Anchorage), and the Alaska Sea Life Center (Seward). An underwater camera system was provided by Gary Stauffer and Craig Rose of NOAA Fisheries, Alaska Fishery Science Center (Seattle, WA). We thank the captains and crews of the F/V’s Sebrika and Rocinante. This project was successful because of the work and suggestions of numerous individuals, including Chris Mitchell, Richard Drake, Susan Goldhor, Radu Giurca, Hal Cook, Mimi Fielding, Susan Inglis, Bill Coffer, Chris Moruhn, Chuck Hart, Karl Skrifvars, and Harold Kalve. The Alaska study was funded by the Alaska Science and Technology Foundation. The study conducted in the Gulf of Mexico was funded by NOAA (Salstonstall–Kennedy) Grant No. NA57FD0031 and NOAA (MARFIN) Grant No. NA47FF0019. We thank Randy E. Edwards for his collabo-ration in that research. We also thank the anonymous reviewers for their useful comments.

References

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