Shark Depredation and Unwanted Bycatch in Pelagic Longline

Japan Pelagic Longline Fisheries
Table A5.3. Effort by vessel class, 1993-2003.
Engan
(days at sea)
Source: MAFF (2005) Source: MAFF (2003) Calculated (see text for assumptions)
Kinkai
(cruises)
Enyo
(cruises)
Kinkai
(sets)
Enyo
(sets)
Engan
(hooks)
Kinkai
(hooks)
Enyo
(hooks)
1993 104,173 1,600 2,041 37,426 154,106 104,173 74,852 308,212
1994 103,538 1,712 2,127 31,785 148,725 103,538 63,570 297,450
1995 101,658 1,435 2,009 28,113 141,744 101,658 56,226 283,488
1996 102,087 1,322 2,026 29,163 146,440 102,087 58,326 292,880
1997 108,097 1,233 1,961 25,396 148,716 108,097 50,792 297,432
1998 105,496 1,173 1,712 22,898 143,208 105,496 45,796 286,416
1999 107,304 1,092 1,423 22,973 127,200 107,304 45,946 254,400
2000 109,088 1,066 1,252 23,228 119,677 109,088 46,456 239,354
2001 110,638 1,036 1,220 22,161 124,614 110,638 44,322 249,228
2002 43,056 4,528 1,148 na na 43,056 na na
2003 46,403 4,443 1,045 na na 46,403 na na
deep (7-20 hpb) showed a striking pattern. In a recent analysis, an
analytical filter was applied to remove data from fishermen who
do not always accurately record shark catches (Nakano and Clarke
2006). The filter works by assuming that at least 80% of all sets in a
single cruise should record at least one shark and excludes data from
cruises which do not meet this criterion. In other words, the number
of sets in a cruise passing the filter can be used as a rough indication
of the frequency of shark catches and when the frequency is too low
the reporting is suspect. When the effect of the filter was examined
separately for shallow and deep sets, the percentage of shallow sets
passing the filter remained consistently above 25% from 1971-1985
and from 1985 until 2002 more than 90% of the shallow sets passed
the filter (Fig. A5.6). In contrast, during the same period (1971-2002)
the proportion of deep sets passing the filter ranged from 7-26%.
These data seem to suggest that the depth of the set may determine
the shark catch rate.
However, when the location of shallow and deep sets was examined
it was revealed that proximity to Japanese ports is confounded with
hooks per basket (a proxy variable for depth). Therefore, it may be
location and not depth which is responsible for the observed trends.
In particular, as the fishery developed (see Section A5.2) the location
of the deep sets moved farther and farther offshore, coincident with
the expansion of the enyo fleet’s targeting of bigeye tuna. Within
the small subset of the deep sets that remained in nearshore waters
(i.e. waters between 15-40ºN and 140-160ºE) a high percentage
(80%) passed the filter (i.e. frequently recorded at least one shark
per set) between 1971-1992 The shallow sets, which had a high rate
of passing the filter were predominantly made in inshore waters
(15-50ºN and east of 180ºE) which are the fishing grounds of the
kinkai fleet. This finding reinforces and partially explains the finding
of disproportionately high shark catches in the kinkai fleet. Given
the available data, we cannot determine whether there is causality
between the higher kinkai shark catch rates and a) the proximity to
a market for sharks in Japanese ports and/or b) the effect of setting
hooks at shallow depths.
Fig. A5.6 indicates that a very high proportion of shallow sets record
catching sharks, whereas a very low proportion of deep sets record
catching sharks (Source: NRIFSF unpublished data).
A5.3.3. Calculation of nominal shark CPUE
Effort for the three longline fleets are currently published (MAFF
2005) in units of cruises for enyo and kinkai fleets, and days at sea for
the engan fleet (Table A5.3, Columns 1-3). In previous years (e.g. MAFF
2003), effort for the enyo and kinkai fleets was published in units of
sets (Table A5.3, Columns 4-5). If one assumes that each engan day at
sea represents one set of on average 1,000 hooks, and each kinkai and
enyo set represents on average 2,000 hooks, an approximate number
of hooks fished by each fleet can be calculated (Table A5.3, Columns
6-8) thereby providing a common unit of effort for all three fleets.
Combining the effort data in Table A5.3 with the annual shark
catch data in Fig. A5.5a (i.e. year-by-year, fleet-specific shark catch
divided by effort) produces nominal catch per unit effort (CPUE)
figures for sharks over the years 1993-2003. The engan and enyo
fleets have a similar nominal shark CPUE of 0.020 (sd=0.0022) for
engan, and 0.021 (sd=0.0090) for enyo. The nominal shark CPUE in
3
For an unknown reason, in 1993 the percentage of deep sets in nearshore waters which pass the filter drops suddenly below 20% and remains at this low level until 2002.
While it may be possible that deep set longline operations changed abruptly in 1993, it is also possible that the introduction of new logbook reporting formats in 1993 may
also be a factor.
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