Hinweis zum Urheberrecht
Bericht / Forschungsbericht / Abhandlung zugänglich unter
URL: https://epub.sub.uni-hamburg.de/epub/volltexte/2009/1440/
Field study to assess some mitigation measures to reduce bycatch of marine turtles in surface longline fisheries
MRAG Ltd
| Originalveröffentlichung: |
| (2008) http://ec.europa.eu/fisheries/publications/studies/turtle_bycatch_2008.pdf |
| pdf-Format:
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| SWD-Schlagwörter: |
| Schwertfisch , Langleinenfischerei , Beifang , Meeresschildkröten , Tierschutz |
| BK - Klassifikation: |
| 48.67 |
| Sondersammelgebiete: |
| 21.3 Küsten- und Hochseefischerei |
| DDC-Sachgruppe: |
| Biowissenschaften, Biologie |
| Dokumentart: |
| Bericht / Forschungsbericht / Abhandlung |
| Sprache: |
| Englisch |
| Erstellungsjahr: |
| 2008 |
| Publikationsdatum: |
| 07.05.2009 |
| Kurzfassung auf Deutsch: |
| 1 General Summary
1.1 Background
There are growing concerns about the numbers of turtles being caught in longline
fisheries and the impact this might have on their populations worldwide. All species of marine turtles are protected reptiles and are considered to be endangered or threatened. Depending on geographic region, the two species most commonly caught in longlines are loggerhead turtles (Caretta caretta) and leatherback turtles(Dermochelys coriacea). Turtles become caught on longlines mainly due to their eneral ecology, feeding behaviour and the overlap in their geographical and depth distributions with the target species of major longline fisheries — tuna, swordfish and other billfish.
A number of previous studies have shown that turtle bycatch may be reduced by changing the type of hook from the traditional ‘J hooks’ (a hook shaped like a J) to circle hooks, which may be more difficult for turtles to swallow, or by using different bait, for example mackerel,which may come free of the hook more readily than squid bait, which is swallowed whole together with the hook.
Mackerel bait thawing Squid bait This project worked with fishermen to test hook and bait types in European surface longline fisheries in the Atlantic, eastern and western Mediterranean with the aim of assessing whether they reduce turtle by-catch. Clearly, gear modifications to reduce turtle bycatch may also have an effect on the ability of the gear to catch the target species, swordfish. This is obviously a prime concern of fishermen, so the effect of
gear modifications on target species catch rates were also assessed, allowing
potential measures to be identified which not only reduce turtle bycatch but also minimise any detrimental effect on catches of swordfish.
The project was funded by the European Commission Directorate-General for
Fisheries, and was undertaken by a consortium led by MRAG Ltd (UK), with AZTI
Tecnalia (Spain) and Lamans s.a. (Greece).
1.2 Approach
An initial review of previous studies on reducing turtle bycatch in surface longline fisheries enabled a robust experimental design to be developed for three at-sea experiments using commercial fishing vessels. A survey of fishermen was also J-hook and circle hook 6 FISH/2005/28A Final Report MRAG-Lamans-AZTI undertaken in the three areas to investigate the current gears in use, fishermen’s perception of the problem of turtle bycatch and their attitude to potential changes in hook and bait type.
The fishing experiments tested the effects of different hook and bait types on both turtle and swordfish catch rates. The trials were conducted in collaboration with the fishing industry in the following fisheries:
• Greek longline fishery in the eastern Mediterranean, between 30 May and 8
September 2007;
• Spanish longline fishery in the western Mediterranean, between 11 July
and 17 September 2007;
• Spanish distant water longline fishery in the south-east Atlantic Ocean,
between 28 February and 4 April 2007.
Experimental fishing was carried out by one vessel in each region with observers
collecting data on turtle bycatch and on target species weights and catch rates. Two longlines were set each day, one with squid bait and one with mackerel bait, and each with alternating magazines of J hooks, 0o offset 16/0 circle hooks and 10o offset 18/0 circle hooks. 35 trials (70 lines) were carried out in the Atlantic, and 60 trials in each of the eastern and western Mediterranean fisheries.
1.3 Results
1.3.1 Turtle catches
A total of 124 turtles were caught in the trials — 9 leatherback turtles and 115
loggerhead turtles. More loggerhead turtles were caught in the Atlantic and western Mediterranean (36 and 77 respectively) than in the eastern Mediterranean (2). Most
turtles were released alive, but 8 loggerhead turtles died. 37 of the released turtles had injuries caused by interaction with the fishing gear.
Bycatch rates of turtles were different in each region, and were lowest in the eastern Mediterranean where 2007 was an unusual year with very low catches of both swordfish and turtles in the fishery in general. The results from this region should therefore be treated with caution, as the catches of turtles were too low to undertake meaningful statistical analysis.
Turtle bycatch was significantly affected by bait type. Turtles were consistently caught more frequently on squid bait than on mackerel bait, and 82% of all loggerhead turtles were caught with squid.
There was no significant difference in turtle bycatch rates between circle hooks and J hooks, although there was an indication that 18/0 circle hooks were less likely to be swallowed than J hooks or 16/0 circle hooks and, in the western Mediterranean, that turtle catch rate on circle hooks was slightly lower than on J hooks.
1.3.2 Swordfish catches
Swordfish catch rates were not significantly affected by bait type in any region. However, hook type did have an influence in the western Mediterranean, with significantly higher catch rates of swordfish on J hooks compared to circle hooks.
The size of swordfish caught was not affected by hook type, but bait type did have an effect in the western Mediterranean, where larger swordfish were caught on squid bait compared to mackerel bait.
Effects of hook and bait on other species caught during the trials as secondary target species or bycatch were also monitored. Bluefin tuna catches were significantly lower on mackerel compared to squid bait in the western Mediterranean.
We found that the combination of hook and bait type that resulted in the lowest
bycatch of turtles and the highest catches of swordfish was J hooks with mackerel bait.
1.4 Questionnaire results and fishermen’s opinions
92 questionnaires were carried out with fishermen from the Atlantic, eastern and
western Mediterranean fisheries, representing 7% of the vessels fishing. Their
primary fishing activity was swordfish, with albacore and other tuna of secondary importance.
Attitude to and awareness of the turtle bycatch problem reflected its severity —in the region with highest number of encounters (the western Mediterranean) the majority of respondents (75%) considered that something should be done to reduce turtle bycatch. Although most did not have a clear idea what should be done, some suggestions included changing from squid bait to mackerel in the summer; setting the gear deeper; and stopping fishing in the months when catches of turtles are highest (with some compensation from EU funds), generally July and August.
Fishers were generally resistant to the idea of changing from J hooks to circle hooks,as they regarded circle hooks to be less effective in catching swordfish. Most fishermen use a combination of mackerel and squid bait, and although they were also resistant to changing bait to other species, changing the relative proportions of mackerel and squid is likely to be more acceptable to them.
1.5 Conclusions
Our results suggest that in all regions the greatest reduction in turtle bycatch rates, with the least effect on swordfish catch rates, would be achieved by using only mackerel bait instead of squid bait and continuing to use J.Since most turtles are caught in the summer months (June - September) in the Mediterranean, the greatest benefit to turtles would be from a requirement to use only mackerel bait in these months. Avoiding fishing for swordfish in the peak months of July and August was also suggested by a few fishermen, if compensations were
provided for the negative economic impact of such a measure. Our results suggest
that turtle catches could be reduced by several thousand a year in the Mediterranean simply with a move from mixed bait to mackerel bait on J hooks in the summer.
Our results do not support a move from J hooks to circle hooks as a way of reducing turtle bycatch. Although we did not find any significant differences in catch rates between hook types, other studies have suggested large 18/0 circle hooks are less likely to be swallowed by turtles than J hooks and cause lower mortality to turtles overall. This is one reason they are advocated. There is some evidence from these trials however, that it is more difficult to release turtles from circle hooks than J hooks, which could also affect turtles’ chances of survival and requires further investigation. |
