Shark Depredation and Unwanted Bycatch in Pelagic Longline

Japan Pelagic Longline Fisheries
Fig. A5.2. Shark catch (in mt) by Japan, 1894-2004. Data from 1894-1949 (t), when Japan’s fishing activities were limited to the Pacific, were sourced from
Okamoto (2004). Data from 1950-2004 (t) are Japan’s catches for all oceans and subset, to the maximum extent practical, to exclude non-shark elasmobranches
(FAO 2006). Japan’s reported catches for the Pacific (only) are denoted with o (FAO 2006). The beginning (1941) and ending (1945) of WWII are shown with
open circles (o).
outer limit of the fishing grounds accessible to Japan, known as the
“MacArthur Line”, occurred over the following few years (1946-
1949) and paralleled rapid increases in shark catches. During this
period, despite the lack of refrigeration facilities onboard vessels,
shark meat was commonly used for food and the demand for shark
fins in China supported a valuable export trade. The value of shark
products, even in the early days of the fishery, is responsible for the
existence of statistics on shark catches over such a long time series
(Okamoto 2004).
In April 1952 the MacArthur Line restrictions were completely lifted,
allowing for the full-scale launch of Japan’s far seas tuna longline
fishery. The sharp decline in shark catches from a peak of 120,000
mt in 1949 to below 80,000 mt in 1952 is linked to this expansion
of offshore operations for two reasons. First, many of the longline
vessels which had previously been targeting sharks off northern Japan
converted to more valuable distant water tuna fishing operations,
thus the shark-directed fishery effort was reduced. Second, sharks
caught by distant water operations targeting tuna were not retained
because of a desire to maximize hold space availability for tuna
(Okamoto 2004), thus the shark handling practices changed.
During the Japanese longline fishing ground expansions of 1960s
through 1980s (see Myers and Worm (2003) for maps), reported shark
catches continued to fall (Fig. A5.2). Nevertheless, the proportion of
reported shark catch deriving from the Pacific was maintained at a
disproportionately high level. While longline effort in each ocean
since 1960 shows that Pacific effort was no more 77%, and sometimes
as little as 47% of total effort in each year (Fig. A5.3), the ratio of
Pacific to non-Pacific shark catches averaged 0.94 from 1950-2004
and never dropped below 0.86 in any year (Fig. A5.2).
At the end of the 1960s the development of flash freezing and supercold
(minimum -40°C) storage facilities led to a shift in tuna species
targeted by the longline fishery away from albacore and yellowfin for
canning, toward bluefin and bigeye for sashimi (Miyake et al. 2004).
As a result, in the early 1970s, small- and medium-sized Japanese
vessels in the western and central Pacific began to favor deep setting
of hooks to better target bigeye tuna (Thunnus obesus). In contrast
to the standard (or shallow) sets in which hooks would extend to a
maximum depth of 120m, the new form of deep setting placed hooks
initially as deep as 250m and as time went by even deeper depths
were fished. By the mid 1980s over 80% of all sets by the Japan fleet
were deep sets (Suzuki et al. 1977).
Fig. A5.4 illustrates the components of longline gear and several
generic configurations that have been used over the years. Although
hook depth is very important in characterizing types of longline
fishing operations, hook depth is not recorded in logbooks and for
various reasons may not always be well understood 2 . Instead the
number of branch lines between floats (also called hooks per basket
or hpb) is often used as a proxy. However, as shown in Fig. A5.4, the
depth of the hook is not necessarily related to the hpb, i.e. a shallow
set may have few or many hpb. Instead, the shortening ratio, or
the ratio of sea surface distance between floats to the length of the
main line between floats may be a more accurate, though usually
unavailable, descriptor of longline configuration.
In the 1960s the shortening ratio was 0.65-0.7 indicating that the
catenary curve between floats had a steep slope and the range of hook
depths was quite broad. This may have been because fishermen’s
knowledge of target species’ depth was not very precise and setting
hooks over a broad depth range increased the chance of “hitting” the
appropriate depth. When targeting of bigeye tuna began in the 1970s,
fishermen observed that 2 or 3 bigeye could be caught on consecutive
branch lines, often twice within a single basket at the same depth,
i.e. along the descent and ascent of the main line’s catenary curve.
Once tuna’s depth preferences were better understood, the objective
was thus to place all of the hooks at the same depth using a
higher shortening ratio (e.g. 0.8). In the early 1990s technological
advances suddenly allowed the use of monofilament as a material in
longlines. Because monofilament was lighter than other materials,
the shortening ratio became even higher (0.9) as the length of the
mainline approached the sea surface and fishermen were able to hang
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