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A review of existing and potential longline gear modifications to reduce sea turtle mortality (Svein Løkkeborg)

Svein Løkkeborg
Institute of Marine Research
P.O.Box 1870, 5817 Bergen, Norway

Abstract

Fisheries using pelagic longlines incidentally catch sea turtles and they may represent a serious threat to depleted sea turtle populations. The work to develop gear modifications to substantially reduce sea turtle bycatches in longlines has begun and this report presents a brief overview of the results obtained so far, with special emphasis on the efficiency of the measures tested. Furthermore, areas where further research is needed are identified.

INTRODUCTION

Sea turtles are a global resource and several anthropogenic activities have been identified to cause declines in sea turtle populations. One of these causes is harvest, either direct through the harvest of eggs and nesting females or indirect caused by incidental bycatches in fishing operations. Among fisheries that incidentally capture sea turtles, certain types of trawl, gillnet and longline fisheries generally pose the greatest threat. Several reports have documented incidental catches of sea turtles by these gear types, but unfortunately few studies have been conducted to develop and evaluate mitigation measures to reduce such bycatches.

The work to develop solutions to reduce the sea turtle bycatch in longlines and trawls has begun, however, and some promising mitigation measures have been identified, tested and in some cases implemented. This report describes the technological details of mitigation measures tested in longline fisheries. The efficiencies of these measures are reviewed and evaluated, and areas where further research is needed are identified.

Development of mitigation measures

Fishing gear and modifications

The most comprehensive work on development of mitigation measures to reduce sea turtle bycatch in longlining is a 3-year study conducted in the U.S. Atlantic pelagic longline fisheries (Watson et al., 2004). This project has been carried out by NOAA Southeast Fisheries Science Center in cooperation with the U.S. pelagic longline fishing industry. The fishing experiments were conducted in 2001-2003 at Grand Banks, on board commercial longliners targeting swordfish.

In order to reduce incidental catch of sea turtles, circle hooks and mackerel bait were compared with the traditional J-hook and squid bait. The pooled results for the two last years of the project are given in Table 1.

Table 1. Percentage decrease in bycatches of loggerhead and leatherback turtles in the Grand Banks mitigation experiment (pooled data for 2002 and 2003). The treatments were compared with traditional J-hook baited with squid. (After Watson et al., 2004).

Treatment

Loggerhead

Leatherback

18/0 Circle hook with squid

74%

75%

18/0 Circle hook with mackerel

91%

67%

Circle hook with mackerel bait gave a 90% reduction in the bycatch of loggerhead turtle and the bycatch of leatherback turtle was reduced by 75% using circle hook with squid bait compared to the traditional J-hook baited with squid. When baited with mackerel, the circle hook gave catch increases (8-12%) for the targeted swordfish, but a decrease in target catch rate (29%) when baited with squid. The effectiveness of circle hooks has also been compared with that of the typical J-hooks in the swordfish longline fishery in the Pacific (Boggs in press.). Circle hooks were only 40% as effective as J-hooks at catching swordfish, but compared nearly equal (94%) with J-hooks at catching tuna.

Alternative types of mitigation measures were also tested in this project based on results from other studies. Blue-dyed squid bait, which is likely to be less visible, has been shown to reduce seabird bycatches (Boggs, 2001), and captive sea turtles ignored blue-dyed squid bait when presented with a choice between blue and normal baits (Swimmer and Brill, 2001; Swimmer et al., 2002 cited in Boggs, in press.). Results of the fishing experiments conducted in the NOAA project, showed no effect of blue-dyed squid bait on incidental take of turtles. Furthermore, although historical data have indicated higher turtle bycatch on hooks nearest floats, moving the branch line 20 fathoms away from the buoy did not reduced the incidental catch of loggerhead or leatherback turtles.

Fishing practice modifications

Changing the way the fishing gear is operated is an alternative method to reduce incidental bycatch. There is a big difference in the operation of longline gear for swordfish and tunas. Swordfish is found in much shallower waters than tunas, and consequently the gear is set closer to the surface, whereas tuna longlines is set much deeper (down to 300-400 m when targeting bigeye tuna). Analysis of observer data collected by the Secretariat of the Pacific Community (SPC) indicates that shallow-set longline gear takes 10 times more sea turtles than deep-set gear (SPREP, 2001; Simonds, 2003). Turtles caught on deep-set gear were almost always taken on the shallowest hooks. The Japanese longline vessels, which is the dominant longline fleet in the Pacific, have gradually switched from shallow-set gear targeting yellowfin tuna and albacore to deep-set gear to catch bigeye tuna (Bjordal and Løkkeborg, 1996). This trend, which started in the 1970s, has probably lead to a sharp reduction in sea turtle bycatches in the Japanese longline fishery.

Sea turtle distribution has been shown to be dependent on surface water temperature, and the NOAA fishing experiments on the Grand Banks showed that fishing in colder water could reduce turtle interactions. Bycatches increased with temperature, especially for loggerheads, and this effect of temperature was reversed for swordfish catch rates. Thus fishing in temperatures below 20°C can reduce turtle interactions and increase target catch rates.

The results from the first year of the NOAA project also indicated that bycatches of loggerhead increased with increasing daylight soak time. The experiments conducted during the second year, however, did not give reduced catches of loggerhead by reducing the daylight soak time.

The Honolulu Laboratory of NMFS has conducted experiments in the Hawaii longline fisheries designed to complement similar research being conducted in the Atlantic Ocean (Boggs, in press.). However, permit has not been given to carry out the proposed fishing experiments due to the incidental sea turtle mortality such experiments will cause. This project has therefore been designed to test how modified fishing gear and fishing operations will affect target catches, i.e. to test for viability in maintaining target species catch rates. Two modifications were tested: (1) Stealth (camouflaged) gear that was designed to reduce the visibility by using narrow-frequency, yellow light-emitting diode-based, down-welling (shaded on the upper half) light sticks, dull blue painted lines and blue-dyed bait; and (2) Deep-set daytime gear using the same depth configuration as that for tuna gear, i.e. longer main line and more hooks between floats.

The stealth gear caught significantly fewer swordfish than the control gear reducing the revenue by 30%. The deep-set daytime gear gave a much more pronounced catch reduction than the stealth gear. The catch of swordfish was 85% less for this gear configuration compared with the control gear. The depth of the deep daytime sets averaged 244 m, whereas control and stealth sets averaged 19 m. These results may be improved by testing another colour of light stick and setting at greater depths, respectively.

De-hooking and release of captured turtles

In the NOAA sea turtle project “Careful Release Protocols” were developed with guidelines for releasing hooked and entangled sea turtles with minimal injury. New de-hooking and release techniques were developed to increase survival rates among the turtles that were incidentally caught. A dipnet used as a turtle elevator was used to bring larger turtles on board for de-hooking using different types of dehookers specially designed for removing hooks depending on location in the mouth cavity. Also line cutters were designed to assist in removing line from entangled sea turtles.

Records of hooking positions (hooked in the mouth versus ingestion) showed that a lower proportion of loggerhead turtles caught on circle hooks had ingested the baited hook compared to those caught on J-hooks. Consequently a higher proportion of turtles could be removed for circle hook (87%) compared with J-hook (36%). This is likely to reduce the post-hooking mortality associated with the interactions. Leatherbacks were most often foul hooked (i.e. external).

Also as a part of the NOAA project, behaviour observations were conducted in the laboratory to study the effects of hook size on ingestion of hooks by loggerhead turtles. Squid baited hooks of different sizes were presented to captive reared turtles, and the proportions of individuals attempting to swallow the hooks were recorded. These observations showed the loggerhead turtles had a much lower tendency to ingest hooks larger than 51 mm in width compared to hooks smaller than this size. Hooks smaller than 51 mm in width are predominantly used in the pelagic longline fisheries. Thus using larger hooks has the potential to reduce mortality of loggerhead turtles.

Future research

The Second International Fishers Forum (Honolulu, Hawaii, 19-22 November 2002) concluded that the major challenges standing in the way of finding a means to reduce sea turtle bycatches in longline operations include effective gear modifications and fishing tactics, research facilitation and dissemination, public awareness and industry incentives for action (Anon., 2002). Mitigation research should focus on bait types and size, hook design and size, and float design to make the gear more “stealthy”. Also the use of sensors to determine when turtles and target species encounter the baited gear was suggested.

The International Technical Expert Workshop on Marine Turtle Bycatch in Longline Fisheries (Seattle, Washington, 11-13 February 2003) identified six overarching strategies as key elements to mitigate sea turtle bycatch in longline fisheries of which one was modifications of gear and fishery practices (Anon., 2003). Under this strategy highest priority was given to:

1. Deploy and implement items that have shown to be effective in reducing bycatch in shallow swordfish fisheries (reduced daylight soak time, leaders longer than float line, leaded swivels, circle hooks, mackerel baits, avoidance of areas of high turtle densities).

2. Direct additional and immediate research in the major ocean basins to fine tune recent findings (effects of circle hooks on target species, bait type, weighted leaders, repellents, branch line materials, attractiveness of gear, deep sets).

3. Establish an international fund for longline bycatch mitigation experiments.

4. Increase research on post-hooking mortality.

Most importantly, however, the encouraging mitigation measures tested in the comprehensive study conducted at the Grand Banks may prove to be a viable solution for longline operations in other regions. However, it is important to take into account that separate experiments are required because there are differences between the major ocean basins among species, in pelagic longline strategies and tactics, and in oceanic structure and ecology. Thus, bycatch-reduction techniques that have shown promise should be tested and evaluated by nations having longline fisheries where sea turtles are incidentally caught.

REFERENCES

Anon. 2002. Executive Summary - Second International Fishers Forum, Honolulu, Hawaii, USA, 19-22 November 2002. http://www.wpcouncil.org/documents/Final.pdf

Anon. 2003. Executive Summary - International Technical Expert Workshop on Marine Turtle Bycatch in Longline Fisheries, Seattle, Washington, USA, 11-13 February 2003. http://www.nmfs.noaa.gov/prot_res/readingrm/Turtles/bycatch_report_feb_2003.pdf

Bjordal, Å. & Løkkeborg, S. 1996. Longlining. Fishing News Books, Oxford, 156 pp.

Boggs, C.H. 2001. Deterring albatrosses from contacting baits during swordfish longline sets. In Seabird Bycatch: Trends, Roadblocks, and Solutions, E.F. Melvin & J.K. Parrish (eds.), pp. 79-94. University of Alaska Sea Grant, AK-SG-01-01, Fairbanks.

Boggs, C.H. Hawaii fishing experiments to reduce pelagic longline bycatch of sea turtles. In Proceedings of The International Technical Expert Workshop on Marine Turtle Bycatch in Longline Fisheries, Seattle, 11-13 February 2003. NOAA Tech Memo: (In press).

Simonds, K.M. 2003. Managing turtles and pelagic fisheries on the high seas. Presented at Conservation and Sustainable Management of Sea Turtles in the Pacific, Bellagio, Italy, 17-22 November 2003.

SPREP. 2001. A review of turtle bycatch in the western and central Pacific Ocean tuna fisheries. A report prepared for the South Pacific Regional Environment Programme (SPREP) by the Oceanic Fisheries Programme, Secretariat of the Pacific Community (SPC). 26pp.

Swimmer, J.Y. & Brill, R. 2001. Methods aimed to reduce marine turtle interactions with longline fishing gear (Abstract). In: Proceedings of the 21st Annual Workshop on Sea Turtle Conservation and Biology. NOAA-Tech Memo-NMFS-SEFSC.

Swimmer, J.Y., Brill, R. & Laurs, M. 2002. Behavior and physiology experiments aimed at reducing pelagic longline interactions with marine turtles. Presented at the meeting of the American Society of Limnology and Oceanography, Victoria, British Colombia, Canada, June 10-14, 2002.

Watson, J.W., Foster, D.G., Epperly, S. & Shah, A. 2004. Experiments in the western Atlantic northeast distant waters to evaluate sea turtle mitigation measures in the pelagic longline fishery. Report on experiments conducted in 2001-2003. http://www.mslabs.noaa.gov/mslabs/docs/watson4.pdf


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