Shark Depredation and Unwanted Bycatch in Pelagic Longline

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Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries Modified techniques that have been used to land more sharks include fishing in more shallow depths, increasing the numbers of hooks deployed between floats, and generally setting gear closer to the bottom, presumably to land more coastal sharks during opened seasons (Hoey and Moore, 1999). These strategies are likely to yield high shark catch rates under certain conditions, at least in relation to large coastal species. For instance, POP data have revealed that 67% of silky sharks taken in the GOM were caught in depths inferior to 1000 meters, while the majority of dusky sharks (SAB) were captured in less than 500 meters. As a basis of comparison, blue sharks and tuna were caught most frequently beyond the 1000 meter depth contour, while mako and common thresher sharks just around that threshold (Hoey and Moore, 1999). In the GOM, conditions for highest silky shark catches closely resembled those for swordfish, morning gear retrievals from less than 1000 meter depths. The avoidance by tuna-targeting vessels of areas where swordfish catches are high (shallow depths, < 500 meters, in the GOM, SAB, and FEC) should thus simultaneously reduce incidental silky shark catches (Hoey and Moore, 1999). Additional studies have also documented the effects of gear shifts on CPUE. For example, the use of rope/steel (“Yankee”) gangions has yielded a lower CPEU of juvenile sandbar sharks than when using monofilament gangions (Branstetter and Musick, 1993). In another study, percent-capture of blue shark with the use of monofilament gangions (66%) exceeded that when employing multifilament gangions (34%) (Stone and Dixon, 2001). Shortfin mako shark catches adhered to the same pattern (60% and 40% for ‘mono’ and ‘multi’ respectively). Stone and Dixon (2001) surmised that the relative aversion to the multifilament gangion could have been a function of strong visual acuity, a trait shared by pelagic predators that often hunt nocturnally. Investigating the effects of these alternate gears on additional elasmobranch species is certainly warranted for standardizing historical catch rates. A9.3.3. Effects of J-style versus circle hooks As said, shifts in gear regimes can influence the catchability of both targeted and non-targeted species. A plethora of studies have addressed the effects on catch rates, location of hooking (e.g. internal versus external; gut versus mouth hooking), and resulting injuries and mortalities induced by differing hook types in teleosts (e.g. Bacheler and Buchel, 2004; Cooke et al., 2005), pelagic teleosts (e.g. Domeier et al., 2003; Kerstetter and Graves, 2006A; Kerstetter et al., 2006; Kerstetter and Graves, 2006B) and in sea turtles (see Garrison, 2003; Watson et al., 2005). Although sharks have only been directly (Yokota et al., 2006) or secondarily (Watson et al., 2005; Kerstetter and Graves, 2006B) addressed in a few such studies, methodologies in which to do so in future studies are now well established. In general, these studies have found that the use of 18/0 (non-offset) circle hooks compared to 9/0 and 10/0 J-style hooks (in some cases in conjunction with mackerel bait) reduce the bycatch of sea turtles, increase the catch of target species such as yellowfin and bigeye tuna, increase the number of animals hooked in the mouth or jaw as opposed to the esophagus or gut, and mitigate the degree of hooking mortality and (estimated) post-release mortality of bycatch species. In response to these results, NMFS imposed a policy in August 2004 requiring all PLL vessels, time and area-independent (excluding the NED), to use only 16/0 or larger non-offset circle hooks and/or 18/0 or larger circle hooks with an offset not exceeding 10 degrees (NMFS, 2005; NMFS, 2006). Moreover, only whole finfish and squid baits may be utilized in the majority of areas, while only whole mackerel and squid baits may be possessed and/or utilized in the NED (when opened) (NMFS, 2006). The potentially positive impact of circle-hook usage on hooking mortality and ultimate discard survivability is not a trivial issue for sharks. Significant interspecific differences in catch mortality rates have been found in sharks captured by PLL operations (Table A9.8) (Beerkircher et al., 2002). Even if a pelagic species is discarded without overt physical depredatory damage, it can still succumb due to physiological stress (Moyes et al., 2006), and cryptic injuries incurred while on or while being removed from the hook. Excluding tiger sharks and rays addressed in Beerkircher et al. (2002), most species demonstrated moderate to high rates of mortality upon gear retrieval. Even the resilient blue shark has exhibited modest (7-19% as cited earlier) degrees of hooking mortality. Thus, although difficult to distinguish by investigation, there is room for survival enhancement in even the most resilient species. The less injurious potential of circle hooks certainly bodes well for the variety of by-caught sharks if adhering to the same superficial hooking trends as in other species. Among the studies considering shark catchability as a function of hook and/or bait type, the results are inconclusive regarding catch rates and the use of circle hooks. Watson et al. (2002) found circle hooks to yield higher catch rates of blue sharks than with J-style hooks, independent of bait type. This discrepancy, however, was acknowledged as a possible function of increased bite-offs of the monofilament leaders (and thus high unaccounted blue shark catch) due to the swallowing Table A9.8. Catch status of elasmobranchs observed in the pelagic longline fishery off the southeastern U.S., 1992–2000 (Beerkircher et al., 2002) Species Alive Dead Unknown Damaged % Dead Silky 487 949 0 10 66.3 Dusky 348 325 0 6 48.7 Night 110 451 0 11 80.8 Blue 381 49 0 4 12.2 Tiger 255 8 0 0 3.0 Scalloped hammerhead Oceanic whitetip 77 117 1 5 61.0 95 36 0 0 27.5 Rays 113 0 0 0 0.0 Sandbar 82 29 0 1 26.8 Bigeye thresher Shortfin mako 38 43 0 1 53.7 52 28 0 0 35.0 144
USA Atlantic, Gulf of Mexico and Caribbean Pelagic Longline Swordfish and Tuna Fisheries and/or deeper lodging of J-style hooks. Otherwise, studies have uniformly found no effect between the hook types on blue shark catch rates (Yokota et al., 2006; Kerstetter et al., 2006; Kerstetter and Graves, 2006B). Although hooking has been found to be deeper (e.g. esophageal or stomach) in J-style than in circle hooks independent of bait type and offset (in circle hooks), catch mortalities of blue sharks have not differed accordingly (Yokota et al., 2006; Kerstetter and Graves, 2006B; Kerstetter et al., 2006). The lack of difference is likely ascribable to the species already displaying high hooking survivorship, whereby discerning a difference due to hook type is difficult. Anecdotally, the conventional “Japanese (tuna) hooks”, which are currently prohibited from use, may actually reduce shark catch rates more than the circle hooks (John Hoey, personal communication). Interestingly, it has been found that the use of mackerel baits reduced the blue shark catch rates associated with both circle (31%) and J-style (40%) hooks (Watson et al., 2005). Thus, a switch in bait regimes may hold greater influence on mitigating shark bycatch than hook type. In the best interest of management, conservation, and industry, investigating the effects of both hook and bait types in additional species of sharks caught by PLL is essential. The management regime shift to circle hooks in the U.S. PLL fishery in 2004 has likely resulted in a still-unquantified change in shark catch rates. Future analyses of shark CPUE changes via POP/logbook data will require careful comparison to account for the change in catchability between the two hook types. Anecdotally, there are on-going studies assessing the catch rate and hooking injury effects of circle hooks on a wide array of shark species caught in PLL operations. A9.3.4. Ongoing work on hook repellants Historically, there have been numerous measures designed, tested and implemented to repel sharks (see Sisneros and Nelson, 2001). The primary impetus behind the majority of these has been to deter shark attacks on humans. In response to the sometimes negative implications of incidental shark takes in the PLL fishery however, attention related to technologies with potential hook-repelling implications has heightened in recent years. Focusing prominently on semiochemical repellants, SharkDefense, LLC (Oak Ridge, NJ) is actively engaged in field-testing products with applications for the PLL fishery. According to their website1 (http://www.sharkdefense.com/Fisheries/fisheries.html), the company is presently augmenting previously successful demonstrations of chemical aversions in a mix of several (small and large coastal) tropical reef-oriented species by field-testing semiochemicals and other chemical synthetics repellants in pelagic species such as blue sharks. Thus far, results have been promising. 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). However, the methods of application and the deployments of such technologies in real-time remain uncertain. Field investigation into the efficacy of these practices has also proven challenging in that it is supposedly difficult to administer and monitor the repellants in a controlled fashion. Fig. A9.7. Illustration of a magnet prototype that received first prize in the 2006 World Wildlife Fund Smartgear Competition (http://www.worldwildlife.org/ oceans/projects/smartgear.cfm). Most recently, Michael Hermann, a partner of SharkDefense, was awarded the World Wildlife Fund’s (WWF) 2006 International ‘Smart Gear” Competition grand prize for a technology intended to deter sharks from hooks. The invention, a highly powerful but small magnet deployed above the shaft of a hook (Fig. A9.5) is meant to repel sharks through an overstimulation of their acutely sensitive electromagnetic receptors. At present, the magnets have successfully deterred Negaprion brevirostris (the lemon shark) and Ginglymostoma cirratum (nurse shark) with little interference on targeted species. Again however, the efficacy of this deterrent under actual PLL conditions, and on pelagic species which certainly employ very different foraging strategies, has yet to be documented. A9.3.5. Next steps To better evaluate the extent of shark bycatch in the Atlantic and to derive means to reduce incidental interactions with sharks in the PLL fishery, a more thorough analysis of the available data sources (POP and logbook) must be conducted. Of utmost importance is the treatment of seasonality, seawater temperature profiles, depth-offishing, and geographic area as primary variables possibly impacting shark catches (John Hoey, personal communication). The better conditions leading to increased takes of specific shark species are understood, the more feasible it will be to establish mitigation tools and management strategies. The reconciliation of POP and logbook data also represents a major step to better understanding the extent of shark takes and would expound upon advisable future management strategies in relation to sharks. Although there has been some concordance in these data (e.g. Brooks et al., 2005), the fishing industry has anecdotally alluded to an often wide discrepancy between perceptions by fishermen and observers. For example, one anonymous fisherman vicariously described a true case of divergence over how many blue sharks had been caught across several sets on the Grand Banks: “On what I think was the first trip he took an observer on back in the early ‘80s, the observer asked how many blue [sharks] they caught. [The] “unnamed fisherman” guessed about 250. The observer said, 80. The numbers might not be exactly those, but the “unnamed fisherman” was really taken aback that he’d misjudged the number so wildly. I think it just emphasizes how perception seems to alter reality.” This example illustrates that amidst the demand and commotion of fishing operations, the under- and/or over-reporting of non-targeted catch is plausible, in which cases the resulting data can grossly misrepresent what is actually occurring. 145
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Shark Depredation and Unwanted Byca
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Contents Summary and Conclusions 1
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Contents Appendix 3. Fiji Pelagic L
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Contents Appendix 8. USA Hawaii Lon
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Chapter 1 Introduction and Methods
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Introduction and Methods 1.2. Metho
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Fishing Gear and Operational Charac
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Fishing Gear and Operational Charac
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Shark Catch Rates and Disposition T
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Chapter 4 National and Internationa
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National and International Measures
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Economic, Practical, Ecological and
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Economic, Practical, Ecological and
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Chapter 6 Industry Attitudes and Pr
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Industry Attitudes and Practices Lo
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Industry Attitudes and Practices Fi
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Chapter 7 Potential of Deterrents,
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Potential of Deterrents, Hotspot Av
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References Campana, S.E., L. Marks,
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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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Shark Depredation and Unwanted Bycatch in Pelagic Longline

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