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FISHERY BULLETIN: VOL. 87, NO. 1 

 1984-85 CATCH RATE RELATIVE TO 1979-81 





5000 10000 15000 



Purse seine yellowfin catch 1979—1983 (t) 



Figure 10.— The relationship between yellowfin tuna catches by Japanese purse seiners and 

 the percentage change in yellowfin longline catch rates for rectangular areas of 2.5° of lati- 

 tude by 10° of longitude in the western Pacific. 



longline fishery. Thus, concentration indices for the 

 combined catch of the major tuna species are gen- 

 erally greater than the value for any individual 

 species (unpubl. results). 



Purse Seine 



The results of this paper suggest that there is no 

 evidence that the western Pacific yellowfin tuna sur- 

 face stocks vulnerable to purse seining have de- 

 clined. Catch rates have remained relatively con- 

 stant despite a 10-fold increase in catches since 1978. 

 However, some cautions are warranted in interpret- 

 ing the catch rates from this fishery in terms of 

 indices of abundances. There are a number of fac- 

 tors specific to this fishery which are likely to result 

 in nonlinear relation or lack of relation between 

 changes in catch rates and changes in the size of the 

 population. These couid result in catch rates remain- 

 ing high despite significant changes in abundances. 

 Many of these have been discussed previously in con- 

 nection with catch rates for schooling populations 

 and for purse seine gear (e.g., Neyman 1949; Palo- 

 heimo and Dickie 1964; Quinn 1980; Mangel 1982; 

 Gulland 1983). Probably the most important factor 

 for the Japanese purse seine fishery is that a high 

 proportion of the catch comes from early morning 

 sets on naturally occurring flotsam (called logs by 

 the fishermen) or manmade, free-floating fish aggre- 



gating devices (referred to as payao's in recognition 

 of their Philippine origin). Generally, Japanese purse 

 seiners tend to make a single early morning set on 

 a log or payao located the previous day. Often 

 vessels will return to the same log or payao over a 

 period of several weeks (Gillett 1986; Farman 1987). 

 Thus, purse seine catch rates will be a fimction both 

 of the density and detection rate for logs and more 

 importantly the renewal rate of fishable tuna schools 

 under a log. Little is known about any of these pro- 

 cesses, but they are not likely to be a simple linear 

 function of yellowfin tuna densities. 



Other factors which also might cause a nonlinear 

 relation between catch rates and population density 

 are the nonrandom distribution of searching effort 

 and the sharing of fishing information among 

 vessels. The fact that the concentration indices 

 of Gulland discussed above are generally low does 

 not indicate that the nonrandom distribution of 

 searching effort (e.g.. Figure 12) is not a major con- 

 cern. Purse seine effort during any given month 

 occurs only in a small portion of the range for 

 surface yellowfin tuna. Thus, even when stratified 

 by area, it is not possible to determine whether 

 the catch rates are representative of overall abun- 

 dance. 



The catch curves based on monthly and quarter- 

 ly statistics (Figs. 3, 5) might be interpreted as 

 contradicting the above conclusions that there is no 



136 



