Shark Depredation and Unwanted Bycatch in Pelagic Longline

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
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