Shark Depredation and Unwanted Bycatch in Pelagic Longline

More documents

Recommendations

Info

Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries from several of the fisheries reported that when they experience high shark capture, depredation, or gear damage and loss in a haul, if the target species catch rate is not particularly high, they might move to a new location before making another set. For instance, during mahi mahi season, most fishers in the Peru artisanal longline mahi mahi and shark fishery will change position when gear damage and loss from sharks is particularly high (Mangel and Alfaro-Shigueto, this volume). Most of the fishers interviewed in the South Africa longline tuna and swordfish fishery identified moving position when shark catch rates are high (Petersen and Goren, this volume). Fishing position in relation to (i) certain sea surface temperatures, (ii) topographic features such as shelf breaks and sea mounts, and (iii) oceanographic features such as currents, fronts, and gyres, may affect shark interaction rates. Australian fishermen identified setting on the colder side of fronts in order to reduce shark catch levels. Catch rates of blue sharks have been found to decline by 9.7-11.4% in response to only a 0.6 o C increase in sea surface temperature (Watson et al., 2005). Not surprisingly, it has also been shown that blue sharks tend to prefer sub-surface depths that possess cooler temperatures (e.g. Simpfendorfer et al., 2002). However, more comprehensive studies on blue shark distribution according to full water column temperature profiles and thermocline dynamics are necessary before amending fishing practices in accordance with patterns in sea-surface temperatures. 6.2.2. Reduce shark detection of baited hooks Very few interviewed fishers believe that refraining from chumming during the set and not discarding offal and spent bait during the haul will substantially affect shark interactions. Chumming during sets is not a common practice in the pelagic longline fisheries included in this study. Offal and spent bait are typically discarded overboard during hauling operations. Many respondents explained that it would be impractical to retain spent bait and offal to discard at the end of hauling because of the lack of space on the vessels. Some fishers avoid using lightsticks because of the belief that this would increase their shark catch rate (Brothers, this volume). 6.2.3. Reduce shark catch rate through deeper setting, line material or type of bait or hook Some fishers indicated that they avoid the use of certain types of bait to reduce shark interactions, or perceive that avoiding certain bait types will reduce shark capture rates (e.g., Italy and Japan, avoid squid (Clarke, this volume; Piovano, this volume); Australia, avoid oily pilchard and squid (Brothers, this volume)). Few fishers believe that hook shape has a large effect on shark catch rates. Furthermore, some fishers indicated that they set their gear at a certain depth or perceive that setting deeper would contribute to reducing shark interactions (Hawaii, Australia, Italy, Japan). Most fishers believe that the depth of baited hooks and timing of the gear soak influence shark catch rates. The deployment depth of hooks and timing of the soak and haul (day versus night) can have an influence on fish species CPUE, including sharks, perhaps due to different water temperature preferences by each species (Strasburg, 1958; Sciarrotta and Nelson, 1977; Rey and Munoz-Chapuli, 1991; Williams, 1997). For example, Rey and Munoz-Chapuli (1991) found higher mako shark (Isurus oxyrinchus) CPUE on shallower set hooks, and no mako capture on the three deepest hooks in a basket, which were estimated to be set to between 370-460 m deep, in a Spanish tropical eastern Atlantic surface longline swordfish and mako shark fishery. Williams (1997) found that main pelagic shark species, with the main exception being the mako sharks, tend to be taken at a higher rate in more shallow-set gear than vessels setting gear deeper in central and western Pacific pelagic longine tuna fisheries. Blue shark, silky shark, pelagic stingray, and oceanic whitetip CPUE were 2.7, 6.4, 1.1, and 2.8 times higher, respectively, in shallow vs. deep set gear (Williams, 1997). Setting baited hooks below a threshold depth may reduce bycatch and depredation by certain species of sharks in certain areas, but shark interaction rates may also depend on when it is that the hooks are at these depths. One fisher in the Hawaii-based longline tuna fishery tried various types of artificial baits to determine their ability to catch target species and to reduce shark capture. He found that the artificial baits did not catch tuna well and that they were not strong enough as he lost about 90% of the artificial baits after one fishing trip. One fisher in the Peru artisanal mahi mahi and shark longline fishery reported having tried an artificial bait, which did not reduce shark interactions. Artificial baits may hold promise to reduce shark capture and depredation. For instance, the Alaska demersal longline sablefish (Anoplopoma fimbria) and Pacific halibut (Hippoglossus stenolepis) fishery tested an artificial bait and found that the artificial bait caught as many or more target species and reduced bycatch of spiny dogfish shark (Squalus acanthias), skate (Raja spp.), arrowtooth flounder (Atheresthes stomias), and Pacific cod (Gadus macrocephalus) by more than ten times compared to a control of fishing with herring bait, the conventional bait used in this fishery (Erickson et al., 2000). The type of hook and natural bait used affects shark CPUE and may also affect depredation rates (Williams, 1997; Gilman et al., 2006a). Research in the Azores longline swordfish and blue shark fishery found that when non-offset 16/0 circle hooks were used, there was a significantly higher blue shark CPUE than when fishing with a non-offset 9/0 J hook in a 2000 study when blue sharks were not being targeted due to low market demand (Bolten and Bjorndal 2002). In a 2001 study in the Azores fishery, when blue sharks were being targeted, fishing with non-offset 16/0 and non-offset 18/0 circle hooks caught significantly more blue sharks than when fishing with a non-offset 9/0 J hook (Bolten and Bjorndal 2003). Thus, in both Azores studies, fishing with a circle hook results in a significantly higher blue shark catch rate when compared to fishing with a J hook. A study conducted in the U.S. North Atlantic longline swordfish fishery found that use of a non-offset or 10 degree offset 18/0 circle hook with squid bait resulted in a small but significant increase in blue shark CPUE (8% and 9% increases, respectively) compared to fishing with a 9/0 J hook with squid (Watson et al., 2005). Watson et al. (2005) also found that fishing with a 10 degree offset 18/0 circle hook with mackerel bait and fishing with a 9/0 J hook with mackerel bait resulted in a significant and large reduction in blue shark CPUE by 31% and 40%, respectively, compared to fishing 26
Industry Attitudes and Practices Fig. 6.1. Some of the tools used by longline fishermen to immobilize and retrieve terminal tackle from sharks. Hawaii longline fisherman demonstrating how he clips a loop of rope onto a branch line below the weighted swivel at the top of the wire leader to assist with removing ahook from caught sharks (left); ring and electrical cord of a “shocker” device used by a Japanese nearh shore longline fishermen to immobilize sharks (middle); Chilean fisherman’s improvised dehooker (right). (Photos by E. Gilman, J. Mangel, T. Miyamoto) with a 9/0 J hook with squid. Research in an experimental Japanese North Pacific longline fishery found no difference in the capture rate of blue sharks between a circle and Japan tuna hook (Yokota et al., 2006a,b). Thus, results from controlled experiments in the Azores and U.S. North Atlantic longline fisheries indicate that fishing with fish instead of squid for bait causes a significant decrease in shark CPUE, while using a wider circle hook instead of a narrower J hook may cause a significant but small increase in shark CPUE. An assessment of observer data from the Hawaii longline swordfish fishery is consistent with results from the controlled experiments (Gilman et al., 2006a). Shark combined species CPUE was significantly lower by 36% after regulations came into effect, which required the fishery to switch from using a 9/0 J hook with squid bait to using a 10o offset 18/0 circle hook with fish bait (Gilman et al., 2006a). Avoiding certain material for branch lines could also reduce shark depredation and catch rates. For instance, the use of rope/steel (“Yankee”) gangions resulted in lower juvenile sandbar shark catch rates 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’ line, respectively. Stone and Dixon (2001) surmise 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. 6.2.4. Reduce injury to discarded sharks Most sharks caught in pelagic longline fisheries are alive when hauled to the vessel (Chapter 3) (Williams, 1997; Brothers, this volume; Gilman, this volume; Mangel and Alfaro-Shigueto, this volume; Thomson, this volume), suggesting that improved handling and release methods to improve shark post-release survival prospects holds promise to reduce fishing mortality of discarded sharks. When a shark will be discarded, the majority of the time in the fisheries included in this study, fishers will cut branch lines to discard hooked sharks, will cut the hook out of the shark’s mouth or will pull the hook out by force in order to retrieve the terminal tackle before discarding the shark. A minority of interviewed fishermen report occasionally landing and killing sharks in order to recover fishing gear before discarding a shark. Fig. 6.1 shows some of the tools used by longline fishermen to assist with immobilizing and retrieving terminal tackle from sharks. The survival of sharks that are not finned, that are deeply hooked (where the shark has swallowed the hook) and have hooks removed by fishers pulling the hook out likely depends on where they were hooked and how much damage is done by pulling out the hook. In these cases of deeply hooked sharks, as is believed to be the case for sea turtles (Gilman et al., 2006a), prospects for shark post release survival might be improved by having fishers cut the line as close to the shark as safely possible. A large proportion of sharks caught in longline gear that is released after removal of the hook from the mouth are expected to survive. Research conducted by the U.S. National Marine Fisheries Service Pacific Islands Fisheries Science Center using pop-up satellite archival tags found that 97.5% of pelagic sharks released after capture on longline gear survived (1 of 40 captured sharks died), while a study by the U.S. National Marine Fisheries Service Southwest Fisheries Science Center found that 94% of 17 tagged shortfin mako sharks survived beyond two months after release (U.S. National Marine Fisheries Service, 2005). Some vessels in the Australia longline fishery are known to use firearms to safely and efficiently kill caught sharks to retrieve terminal tackle, while some vessels in the Australia and Hawaii fisheries will kill caught sharks in an effort to avoid the inconvenience of their recapture. However in the Hawaii fisheries, observer data show that a very small proportion of caught sharks that are alive when hauled to the vessel are killed before discarding (Gilman, this volume). This is also the case in the Peru longline mahi mahi and shark fishery (Pro Delphinus, unpublished data; Mangel and Alfaro-Shigueto, this volume). Some vessels in the Hawaii longline fleet report that, when they are busy processing commercially valuable species on deck, they will place a tarred rope with a knot at the end off the stern and will clip branch lines containing sharks onto this rope so they can remove the sharks from the gear after they have completed the haul and processed the catch. The majority of 27
Page 3 and 4: Shark Depredation and Unwanted Byca
Page 5: Shark Depredation and Unwanted Byca
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: Industry Attitudes and Practices Lo
Page 43 and 44: Chapter 7 Potential of Deterrents,
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
Page 90 and 91:
Shark Depredation and Unwanted Byca
Page 92 and 93:
Page 94 and 95:
Page 96 and 97:
Page 99 and 100:
Appendix 5 Japan Pelagic Longline F
Page 101 and 102:
Japan Pelagic Longline Fisheries Fi
Page 103 and 104:
Japan Pelagic Longline Fisheries mo
Page 105 and 106:
Japan Pelagic Longline Fisheries Ta
Page 107 and 108:
Japan Pelagic Longline Fisheries is
Page 109 and 110:
Japan Pelagic Longline Fisheries ta
Page 111 and 112:
Japan Pelagic Longline Fisheries In
Page 113 and 114:
Japan Pelagic Longline Fisheries A5
Page 115 and 116:
Appendix 6 Peru Artisanal Mahimahi
Page 117 and 118:
Peru Artisanal Mahimahi And Shark L
Page 119 and 120:
Page 121 and 122:
Page 123 and 124:
Page 125 and 126:
Page 127 and 128:
Appendix 7 South Africa Pelagic Lon
Page 129 and 130:
South Africa Pelagic Longline Tuna
Page 131 and 132:
Page 133 and 134:
Page 135 and 136:
Appendix 8 USA Hawaii-based Pelagic
Page 137 and 138:
USA Hawaii-based Pelagic Longline S
Page 139 and 140:
Page 141 and 142:
Page 143 and 144:
Page 145 and 146:
Page 147 and 148:
Appendix 9 USA Atlantic, Gulf of Me
Page 149 and 150:
USA Atlantic, Gulf of Mexico and Ca
Page 151 and 152:
Page 153 and 154:
Page 155 and 156:
Page 157 and 158:
Page 159 and 160:
Page 161 and 162:
show all

Shark Depredation and Unwanted Bycatch in Pelagic Longline

You also want an ePaper? Increase the reach of your titles

Delete template?

Save as template?