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