Shark Depredation and Unwanted Bycatch in Pelagic Longline

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Shark Depredation and Unwanted Bycatch in Pelagic Longline Fisheries Fig. A5.3. Japanese tuna longline effort by ocean, 1952-2004. (Suisan Sougou Kenkyuu Center, 2006). Fig. A5.4. Components of Japanese longline gear and various configurations used historically and currently (see text). 88
Japan Pelagic Longline Fisheries more and more branch lines off the strong but light monofilament. However, at the same time, it appeared that the schools of bigeye began to disaggregate and fishermen returned to a system of placing the hooks at a range of depths by varying the length of the branch line rather than changing the high shortening ratio. In recent years, some Japanese longline fishermen have begun reverting to heavier and stiffer main lines with lower (i.e. deeper) shortening ratios. The reason for this is unclear but may be connected to a desire to target larger and deeper-swimming bigeye, to avoid tangling of the main line, or to vary the fishing strategy from other longline fleets who have learned from the Japanese longline fisheries’ past example. Today’s longline operations consist of a variety of configurations based on different numbers of hpb, shortening ratios, branch and main line lengths and materials. It is thus difficult to characterize the different gear types into generic categories. As an example of materials used, longline leaders used to target tuna or billfish may consist of a buran of rope or cord, followed by a nakatsugi of 3mm nylon monofilament, and an upper tsurimoto of stiff cord with another length of nylon monofilament connecting the cord to the hook (Fig. A5.4). If the fishermen wish to minimize sharks biting off the hook, they may cover the lower length of nylon monofilament with a steel sheath but this may cause the bait to move in the water column in an unnatural manner and thus may result in a lower catch rate for tuna. Fishermen who are even more focused on catching sharks may use a cord buran, followed by a 2mm monofilament nakatsugi, and a tsurimoto of 1mm braided steel wire. Such types of gear are favored by shark fishermen because they attract sharks yet are resilient to shark bite-offs. They have the added advantage of manpower savings in that, unlike monofilament tsurimoto, they do not need to be checked for damage between each deployment. Main lines may be made of rope (e.g. Kesennuma) or nylon monofilament (e.g. Kii-Katsuura). A5.3. Current Fleet Characteristics and Shark Catches As described in the previous section, the Japanese longline fishery has a long history and has undergone a number of major innovations. Despite these general trends, longline operations remain highly diverse and vary considerably by region and fishing master. The following sections attempt to characterize the current state of the Japanese longline fishery in terms of vessel numbers, effort and catch, highlighting important distinctions where relevant. A5.3.1. Number of vessels by size class The number of vessels registered in each size class is shown in Table A5.2. By 2003, the enyo fleet had contracted to 66% of its size in 1993. This reduction is partially attributable to a Japanese government buy-back program implemented in 1999 in response to the FAO International Plan of Action for the Management of Fishing Capacity (IPOA-Capacity) which scrapped 132 tuna longline vessels (FAO 2004). Further reductions in the number of longline vessels are expected in 2006 due to adverse economic conditions resulting from high oil prices, declining catches, competition from farmed tuna and the elimination of government-sponsored financing services (Japan Times 2006). It should be noted that vessel statistics for 2002-2003 show an apparent increase in small-class kinkai vessels but upon closer inspection this increase results merely from a reclassification of largeclass engan vessels and does not halt the overall trend of decline in vessel numbers (Table A5.2, Total column). This trend is also reflected in the finding by Miyake et al. (2004) that Japan’s share of the global tuna catch relative to other countries has declined over time. According to interview information, all of the enyo longliners are equipped with ultra-low temperature (ULT) freezers. Refrigeration in the kinkai fleet is sometimes in the form of freezers; in other vessels ice is used. Some of the engan fleet use a well of chilled water rather than ice. When asked to compare the level of freezer technology in the Japanese enyo fleet to that of other fishing entities, a knowledgeable source stated that rapid development of other fleets is quickly closing any remaining technology gap. A5.3.2. Catch and effort by vessel class and operational behavior Longlining operations represent Japan’s primary fisheries for tuna and billfishes. In addition, in the period between 1993 and 2003, longline gear has consistently been responsible for 70-80% of Japan’s annual reported shark catch (MAFF, 2005). In contrast to shark catches which are generally higher in the latter half of this time period for all vessel classes (Fig. A5.4a), total longline catches of all species show a gradually declining trend (Fig. A5.4b). In terms of sharks, kinkai vessels contribute the largest portion of the catch, followed closely by enyo vessels (Fig. A5.4a). In terms of total catch, however, kinkai catches comprise only 20 to 40% of the catch volume of enyo vessels (Fig. A5.4b). Therefore, kinkai vessels are either hooking a disproportionately large share of sharks due to some aspect of their operational behavior, or merely retaining a greater proportion of hooked sharks. One possibly important factor influencing shark catch rates is hook depth. An early study comparing catch rates between shallow and deep sets observed higher catch rates in the deep sets for bigeye tuna but did not investigate differences in shark catch rates (Suzuki et al. 1977). With the some notable exceptions, e.g. bigeye thresher sharks (Alopias superciliosus, Nakano et al. 2003), the depth preferences of most shark species are not well understood, and thus it is difficult to predict the potential effect of deeper hook setting on shark species. In particular, for species such as blue shark (Prionace glauca), which are believed to be widely distributed in the water column (Nakano and Seki 2003), hook depth may not have a major effect on catch rates. Despite this theory, analysis of shark catch rates between longline sets in the North Pacific characterized as shallow (4-6 hpb) and 2 There are many reasons (e.g. current speed, line setting method, etc.) why the intended fishing depth of the hook is not realized. Please see Shiode et al. (2005) and Miyamoto et al. (2006) for more information 89
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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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Page 115 and 116: Appendix 6 Peru Artisanal Mahimahi
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Page 129 and 130: South Africa Pelagic Longline Tuna
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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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