Shark Depredation and Unwanted Bycatch in Pelagic Longline

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Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries sharks placed on this line will have fallen off before the end of the haul, while most sharks remaining on the line when it is retrieved are dead. In the Italy Mediterranean industrial longline swordfish fishery, some fishermen reported that, in the past when shark capture rates were higher, they would routinely kill all caught sharks in order to reduce local shark abundance, which they believed would result in an increase in their swordfish catch by reducing shark predation on the swordfish (Piovano, this volume). Reducing such practices that result in the mortality of discarded sharks would contribute to preventing the unsustainable exploitation of sharks. A few of the interviewed fishers report killing all caught sharks in order to prevent the inconvenience of recapturing them, reduce shark depredation, and in one case, to reduce shark populations to reduce the likelihood of being killed by a shark if he ever has to abandon ship (Brothers, this volume; Gilman, this volume; Thomson, this volume). While this attitude was not held by the majority of the fishers interviewed, it is useful to identify as a potential obstacle to introducing new techniques to improve the post-release survival prospects of caught sharks that are discarded. While circle hooks may result in a higher shark CPUE than J and Japan tuna style hooks (Section 6.2.3), because fish caught on circle hooks are more likely to be hooked in the mouth versus swallowing the hook, removal of circle hooked sharks is easier and may result in reduced injury to the shark than sharks caught on other hook types. None of the interviewed pelagic longline fishermen use dehookers to discard live sharks. Only two fishers, one from the Chile artisanal mahi mahi and shark longline fishery and one from the Peru artisanal mahi mahi and shark longline fishery, report that they use a dehooker to recover hooks from sharks when these are onboard and already dead (Alfaro-Shigueto et al., this volume; Mangel and Alfaro- Shigueto, this volume). The Peruvian fisher possessed the dehooker initially for the purpose of releasing sea turtles. A benefit of using a dehooker might include reduced time to retrieve terminal tackle. The U.S.-based company Aquatic Release Conservation produces a 0.8 m long “Big Game” and a 0.4 m long “Bite Block” dehooker, which are designed to remove hooks from sharks brought onboard. For sharks that are too large to bring onboard, this company also produces a 2.4 m long and 3.7 m long dehooker on a pole. The effect of dehookers on the post-release survival prospects of sharks compared to crew cutting a hook from the shark or cutting the branch line and leaving a hook and trailing line attached to the shark has not been assessed. Almost all of interviewed fishers from all fisheries included in the study, except South Africa where all the fishers thought use of a dehooker would reduce shark injury, believe that it would be inefficient to use dehookers to remove sharks from gear, either because this would increase the safety risk, or because it is simply not feasible to use this device to remove sharks. Some caught sharks will twist and spin when hauled to the vessel, which could cause the dehooker to be lost overboard and be a hazard for crew to handle before being lost overboard. Because sharks may be on the sea surface when being hauled, some fishers were concerned that the incidence of having branch lines break if a shark pulls the line would increase with use of a dehooker because to use a dehooker requires bringing the shark close to the vessel. Gear soak time also likely affects the proportion of caught sharks that are alive when hauled to the vessel. Changes to fishing gear and methods that reduce soak time could contribute to reducing fishing mortality of discarded sharks. 6.2.5. Reduce retention of sharks on branch lines Retention of sharks on branch lines with wire leaders or other durable material is substantially higher than in gear with no wire leader where nylon monofilament is connected directly to the hook (Chapter 4) (e.g., Williams, 1997). For instance, an owner-operator of a vessel in the Japan nearshore pelagic longline fishery who seasonally will target sharks stated that he does not use a wire leader (nylon monofilament is used through to the hook) when he is targeting tunas and billfish because he believes that this gear design maximizes the catch of these target species. He uses a 1 m wire trace at the hook during seasons when he targets sharks (Clarke, this volume). Also, fishers in the Chile and Peru artisanal longline mahi mahi and shark fisheries will add a wire leader to increase shark catch rates (Alfaro-Shigueto et al., this volume; Mangel and Alfaro-Shigueto, this volume). 28
Chapter 7 Potential of Deterrents, Hotspot Avoidance and Incentive Instruments to Reduce Shark Interactions in Pelagic Longline Fisheries 7.1. Shark Deterrents For fisheries with an incentive to reduce shark interactions, chemical, magnetic, electropositive rare earth metals and electrical repellents are promising shark deterrents. Some of these strategies are concepts requiring substantial investment to develop the technology for application in longline gear. Research and commercial demonstrations are needed to assess their efficacy at repelling sharks and effects on target and incidental species. Research and development is also needed to reduce the per-unit cost of these repellents to make them economically viable for use in pelagic longline fisheries. Chemical deterrents, including a protein extract called ‘pardaxin’ from an excretion from the moses sole (Pardachirus marmoratus), sodium and lithium lauryl sulfate (components of common soap and shampoo) and sodium dodecyl sulfate, a related compound, have been found to repel some species of sharks under certain conditions. The moses sole extract was found to be heat sensitive and not stable when stored at room temperature; the only form in which pardaxin proved stable is freeze-dried, but in that form is only 30% as effective as fresh. The soap compounds attack fatty molecules of cell membranes in a shark’s gill filaments, causing an influx of sodium ions from surrounding seawater. Smith (1991) found that shark reactions to sodium lauryl sulfate vary with concentration of soap used and from species to species. Some fishers from the Peru artisanal mahi mahi and shark fishery, where sharks are an incidental or target species, report retaining shark offal (viscera and heads) until the end of a haul because they believe that discharging offal during the haul would deter sharks and reduce their shark catch rate (Mangel and Alfaro-Shigueto, this volume). Shark Defense LLC (Oak Ridge, New Jersey, USA-based company) has identified semiochemical-based repellents that have been shown to cause six species of sharks to leave an area after the chemical was disbursed without repelling teleost fish such as pilot fish and remora accompanying the sharks (Eric Stround, personal communication, Shark Defense LLC, 3 April 2006). Presumably, this can be ascribed to an apparent aversion in sharks to certain chemicals, including ammonium acetate (a major component in decaying shark flesh) and other semiochemicals emitted from predators (Sisneros and Nelson, 2001). When mixtures of semiochemicals were introduced into feeding aggregations of sharks and teleost fish in reef habitat, sharks quickly left the feeding area while bony fish stayed in the area and continued feeding (U.S. National Marine Fisheries Service, 2005). The semiochemicals, originally derived from decayed shark carcasses, but now produced synthetically, may elicit a flee response in elasmobranchs but not in bony fish, but research results are not yet available in a peer-reviewed publication. A preliminary study was conducted by the U.S. National Marine Fisheries Service Pacific Islands Fisheries Science Center in Hawaii in early 2005 comparing catch of target species and sharks in sets using bait injected with synthetic shark semiochemicals produced by Shark Defense LLC to sets using untreated bait. Results were inconclusive, in part, because the research design prevents conclusions to be drawn on the single factor effect of the presence or absence of semiochemicals in the bait on shark and target species CPUE, and because it was not possible to confirm that bait injected with semiochemicals retained the chemical throughout the gear soak (U.S. National Marine Fisheries Service, 2005). Since conducting this 2005 trial in Hawaii, the chemical is now available in a hydroxypropyl cellulose and glycol ether ester gel matrix, where the viscosity of the gel prevents it from being poured (Eric Stround, personal communication, Shark Defense LLC, 3 April 2006). In current ongoing trials the gel is placed in biodegradable, porous muslin bags filled with 100 ml of the gel at 30.5 cm above a bait. The gel has been observed to dissolve evenly over an 8-hour period while the fishing gear soaks. The gel could also be syringed directly into a bait or a bag of the gel could be stuffed into a bait (Eric Stround, personal communication, Shark Defense LLC, 3 April 2006). One bag filled with 100 ml of the gel matrix would cost about USD 1.05 (Eric Stround, personal communication, Shark Defense LLC, 3 April 2006). 29
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Page 9 and 10: Contents Summary and Conclusions 1
Page 11 and 12: Contents Appendix 3. Fiji Pelagic L
Page 13: Contents Appendix 8. USA Hawaii Lon
Page 19 and 20: Chapter 1 Introduction and Methods
Page 21 and 22: Introduction and Methods 1.2. Metho
Page 23 and 24: Fishing Gear and Operational Charac
Page 25 and 26: Fishing Gear and Operational Charac
Page 27 and 28: Shark Catch Rates and Disposition T
Page 29 and 30: Chapter 4 National and Internationa
Page 31 and 32: National and International Measures
Page 33 and 34: Economic, Practical, Ecological and
Page 35 and 36: Economic, Practical, Ecological and
Page 37 and 38: Chapter 6 Industry Attitudes and Pr
Page 39 and 40: Industry Attitudes and Practices Lo
Page 41: Industry Attitudes and Practices Fi
Page 45 and 46: Potential of Deterrents, Hotspot Av
Page 47 and 48: References Campana, S.E., L. Marks,
Page 49: References Neves dos Santos, M., Ga
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Appendix 5 Japan Pelagic Longline F
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Japan Pelagic Longline Fisheries Fi
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Japan Pelagic Longline Fisheries mo
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Japan Pelagic Longline Fisheries Ta
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Appendix 6 Peru Artisanal Mahimahi
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Peru Artisanal Mahimahi And Shark L
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Appendix 7 South Africa Pelagic Lon
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South Africa Pelagic Longline Tuna
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Appendix 8 USA Hawaii-based Pelagic
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USA Hawaii-based Pelagic Longline S
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Appendix 9 USA Atlantic, Gulf of Me
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USA Atlantic, Gulf of Mexico and Ca
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Shark Depredation and Unwanted Bycatch in Pelagic Longline

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