Shark Depredation and Unwanted Bycatch in Pelagic Longline
Shark Depredation and Unwanted Bycatch in Pelagic Longline
Shark Depredation and Unwanted Bycatch in Pelagic Longline
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Japan <strong>Pelagic</strong> Longl<strong>in</strong>e Fisheries<br />
Fig. A5.2. <strong>Shark</strong> catch (<strong>in</strong> mt) by Japan, 1894-2004. Data from 1894-1949 (t), when Japan’s fish<strong>in</strong>g activities were limited to the Pacific, were sourced from<br />
Okamoto (2004). Data from 1950-2004 (t) are Japan’s catches for all oceans <strong>and</strong> subset, to the maximum extent practical, to exclude non-shark elasmobranches<br />
(FAO 2006). Japan’s reported catches for the Pacific (only) are denoted with o (FAO 2006). The beg<strong>in</strong>n<strong>in</strong>g (1941) <strong>and</strong> end<strong>in</strong>g (1945) of WWII are shown with<br />
open circles (o).<br />
outer limit of the fish<strong>in</strong>g grounds accessible to Japan, known as the<br />
“MacArthur L<strong>in</strong>e”, occurred over the follow<strong>in</strong>g few years (1946-<br />
1949) <strong>and</strong> paralleled rapid <strong>in</strong>creases <strong>in</strong> shark catches. Dur<strong>in</strong>g this<br />
period, despite the lack of refrigeration facilities onboard vessels,<br />
shark meat was commonly used for food <strong>and</strong> the dem<strong>and</strong> for shark<br />
f<strong>in</strong>s <strong>in</strong> Ch<strong>in</strong>a supported a valuable export trade. The value of shark<br />
products, even <strong>in</strong> the early days of the fishery, is responsible for the<br />
existence of statistics on shark catches over such a long time series<br />
(Okamoto 2004).<br />
In April 1952 the MacArthur L<strong>in</strong>e restrictions were completely lifted,<br />
allow<strong>in</strong>g for the full-scale launch of Japan’s far seas tuna longl<strong>in</strong>e<br />
fishery. The sharp decl<strong>in</strong>e <strong>in</strong> shark catches from a peak of 120,000<br />
mt <strong>in</strong> 1949 to below 80,000 mt <strong>in</strong> 1952 is l<strong>in</strong>ked to this expansion<br />
of offshore operations for two reasons. First, many of the longl<strong>in</strong>e<br />
vessels which had previously been target<strong>in</strong>g sharks off northern Japan<br />
converted to more valuable distant water tuna fish<strong>in</strong>g operations,<br />
thus the shark-directed fishery effort was reduced. Second, sharks<br />
caught by distant water operations target<strong>in</strong>g tuna were not reta<strong>in</strong>ed<br />
because of a desire to maximize hold space availability for tuna<br />
(Okamoto 2004), thus the shark h<strong>and</strong>l<strong>in</strong>g practices changed.<br />
Dur<strong>in</strong>g the Japanese longl<strong>in</strong>e fish<strong>in</strong>g ground expansions of 1960s<br />
through 1980s (see Myers <strong>and</strong> Worm (2003) for maps), reported shark<br />
catches cont<strong>in</strong>ued to fall (Fig. A5.2). Nevertheless, the proportion of<br />
reported shark catch deriv<strong>in</strong>g from the Pacific was ma<strong>in</strong>ta<strong>in</strong>ed at a<br />
disproportionately high level. While longl<strong>in</strong>e effort <strong>in</strong> each ocean<br />
s<strong>in</strong>ce 1960 shows that Pacific effort was no more 77%, <strong>and</strong> sometimes<br />
as little as 47% of total effort <strong>in</strong> each year (Fig. A5.3), the ratio of<br />
Pacific to non-Pacific shark catches averaged 0.94 from 1950-2004<br />
<strong>and</strong> never dropped below 0.86 <strong>in</strong> any year (Fig. A5.2).<br />
At the end of the 1960s the development of flash freez<strong>in</strong>g <strong>and</strong> supercold<br />
(m<strong>in</strong>imum -40°C) storage facilities led to a shift <strong>in</strong> tuna species<br />
targeted by the longl<strong>in</strong>e fishery away from albacore <strong>and</strong> yellowf<strong>in</strong> for<br />
cann<strong>in</strong>g, toward bluef<strong>in</strong> <strong>and</strong> bigeye for sashimi (Miyake et al. 2004).<br />
As a result, <strong>in</strong> the early 1970s, small- <strong>and</strong> medium-sized Japanese<br />
vessels <strong>in</strong> the western <strong>and</strong> central Pacific began to favor deep sett<strong>in</strong>g<br />
of hooks to better target bigeye tuna (Thunnus obesus). In contrast<br />
to the st<strong>and</strong>ard (or shallow) sets <strong>in</strong> which hooks would extend to a<br />
maximum depth of 120m, the new form of deep sett<strong>in</strong>g placed hooks<br />
<strong>in</strong>itially as deep as 250m <strong>and</strong> as time went by even deeper depths<br />
were fished. By the mid 1980s over 80% of all sets by the Japan fleet<br />
were deep sets (Suzuki et al. 1977).<br />
Fig. A5.4 illustrates the components of longl<strong>in</strong>e gear <strong>and</strong> several<br />
generic configurations that have been used over the years. Although<br />
hook depth is very important <strong>in</strong> characteriz<strong>in</strong>g types of longl<strong>in</strong>e<br />
fish<strong>in</strong>g operations, hook depth is not recorded <strong>in</strong> logbooks <strong>and</strong> for<br />
various reasons may not always be well understood 2 . Instead the<br />
number of branch l<strong>in</strong>es between floats (also called hooks per basket<br />
or hpb) is often used as a proxy. However, as shown <strong>in</strong> Fig. A5.4, the<br />
depth of the hook is not necessarily related to the hpb, i.e. a shallow<br />
set may have few or many hpb. Instead, the shorten<strong>in</strong>g ratio, or<br />
the ratio of sea surface distance between floats to the length of the<br />
ma<strong>in</strong> l<strong>in</strong>e between floats may be a more accurate, though usually<br />
unavailable, descriptor of longl<strong>in</strong>e configuration.<br />
In the 1960s the shorten<strong>in</strong>g ratio was 0.65-0.7 <strong>in</strong>dicat<strong>in</strong>g that the<br />
catenary curve between floats had a steep slope <strong>and</strong> the range of hook<br />
depths was quite broad. This may have been because fishermen’s<br />
knowledge of target species’ depth was not very precise <strong>and</strong> sett<strong>in</strong>g<br />
hooks over a broad depth range <strong>in</strong>creased the chance of “hitt<strong>in</strong>g” the<br />
appropriate depth. When target<strong>in</strong>g of bigeye tuna began <strong>in</strong> the 1970s,<br />
fishermen observed that 2 or 3 bigeye could be caught on consecutive<br />
branch l<strong>in</strong>es, often twice with<strong>in</strong> a s<strong>in</strong>gle basket at the same depth,<br />
i.e. along the descent <strong>and</strong> ascent of the ma<strong>in</strong> l<strong>in</strong>e’s catenary curve.<br />
Once tuna’s depth preferences were better understood, the objective<br />
was thus to place all of the hooks at the same depth us<strong>in</strong>g a<br />
higher shorten<strong>in</strong>g ratio (e.g. 0.8). In the early 1990s technological<br />
advances suddenly allowed the use of monofilament as a material <strong>in</strong><br />
longl<strong>in</strong>es. Because monofilament was lighter than other materials,<br />
the shorten<strong>in</strong>g ratio became even higher (0.9) as the length of the<br />
ma<strong>in</strong>l<strong>in</strong>e approached the sea surface <strong>and</strong> fishermen were able to hang<br />
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