FISHERY BULLETIN: VOL, 87, NO. 1 



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Figure 14.— Estimates of annual catch rates for Japanese longliners (number of yellowfin 

 tuna per 1.000 hooks) in the western Pacific. The solid line represents stratified estimates 

 based on five degree square areas from published data by the Fisheries Agency of Japan 

 (1962-80). The dotted line represents the estimates stratified by area from Table 5 based 

 on data held by the Tuna and Billfish Assessment Programme of the South Pacific Commission. 



Even if a general decline is occurring, it would not 

 be possible to evaluate whether the purse seine catch 

 is a likely cause of the decline without either more 

 detailed information on the age structure of the 

 catches or a much longer time series of data. 



Based on the comparison of catch rates within the 

 same area and time period, yellowfin tuna do not 

 appear to be a homogeneous stock with respect to 

 purse seining and longlining. The lack of any rela- 

 tionship at a fine spatial and temporal scale could 

 be due to 



1 . factors affecting vulnerability to surface gear are 

 unrelated to factors affecting vulnerability to 

 longline gear, or 



2. those portions of the yellowfin tuna population 

 being exploited by the purse seine fishery (i.e., 

 primarily 2-3 year old fish) have a spatial-tem- 

 poral distribution which does not coincide with 

 that for the older and larger yellowfin tuna be- 

 ing harvested by longliners. 



In reality, probability both of these factors, plus ran- 

 dom elements in the fishing process are contributing 

 to the apparent lack of any relationship. 



The fact that the observed changes in longline 

 catch rates within areas appear not to be related to 

 the purse seine catches taken from that area may 



be due to any number of factors. Some possible 

 hypotheses include 



1. The level of exploitation by purse seiners within 

 any of the areas considered has been insufficient 

 to affect a significant decline in longline catch 

 rates. 



2. Given the difference in the size and age of the 

 fish exploited by the two fisheries, a time lag 

 would be expected before any effect could be ob- 

 served and the presently available time series 

 may be too short to detect the effects. 



3. There is a large amount of movement of yellow- 

 fin tuna so that the yellowfin being harvested by 

 longliners are not merely the escapement from 

 the purse seine fishery within that area. 



4. There are two independent stocks or substocks 

 of yellowfin tuna— a deep and a surface one- 

 each of which is primarily vulnerable to only one 

 gear type. 



5. The available purse seine catch statistics are in- 

 complete and areas in which the greatest decline 

 in longline catch rates have occurred may in fact 

 be areas where large, unreported purse seine 

 catches have occurred. 



6. The main areas in which the largest decline in 

 longline catch rates have occurred border the 

 EEZ's of the Philippines and Indonesia. The 



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