POLACHECK: YELLOWFIN TUNA CATCH RATES 



evidence that surface yellowfin tuna stocks have 

 declined. However, the time sequence of changes 

 in catches in relation to changes in effort are not 

 those that would be expected if these catch curves 

 were a reflection of the overall population dynamics. 

 Thus, when the temporal sequence of changes in 

 catch and effort are considered by connecting the 

 points in Figure 3, the resulting pattern suggests 

 that during a time interval of one month, a large 

 change in effort results in correspondingly large 

 changes in the catch rates. If these changes in catch 

 rates reflected changes in the overall yellowfin tuna 

 abundance, it would mean that catches of 3,000- 

 5,000 t represented a very significant proportion of 

 the total yellowfin tuna stock and that a very rapid 

 recovery of the yellowfin tuna stocks (i.e., during 

 the course of a month) can occur with reductions in 

 effort. Neither of these conclusions seem reasonable. 

 Also, the fact that there is no evidence that catch 

 rates are lower at the highest effort levels so far 

 experienced when the data are combined into an- 

 nual statistics, further suggests that the catch 

 curves based on monthly and quarterly statistics do 

 not reflect the overall population dynamics. 



The apparent reduction in catch rates at the high- 

 est effort levels based on the monthly or quarterly 

 stratification is an interesting phenomenon warrant- 

 ing further investigation. The reduction in catch 

 rates at these highest effort levels does not appear 

 to be the result of increased handling time at higher 

 effort levels. The number of sets per day has re- 

 mained relatively constant and unrelated to the total 

 number of days fished. Two possible explanations 

 for the decline in monthly catch rates with higher 

 effort are localized depletions and interactions with 

 skipjack tuna catches. In this regard, it is interesting 

 to note that monthly or quarterly catch curves for 

 skipjack tuna, Katsuwonus pelamis, from this same 

 fishery do not show this apparent decline in catch 

 rates at highest effort. The lack of decline in the 

 catch rate for skipjack tuna is another indication 

 that the decline observed for yellowfin tuna is not 

 due to handling time. 



Longline 



Longline catch rates in 1984 and 1985 are substan- 

 tially lower than those in 1979. Whether this de- 

 crease represents a general long-term decline is not 

 possible to determine without a longer time series 

 of data. Interpretation of the temporal trend de- 

 pends partially upon whether the observed rates in 

 1983 are attributable to the large El Nino of 1983 

 or whether they are a measure of the random vari- 



ability in the fishing process. The magnitude of the 

 increase observed in 1983 is much larger than might 

 be expected given the observed variability both be- 

 tween and within months (the latter is indicated by 

 the error bars in Figure 8). While it is tempting and 

 even reasonable to attribute the high rates in 1983 

 as an El Nino effect, the length of the current time 

 series and available information on the effects of El 

 Niiio on yellowfin tuna are insufficient to objectively 

 resolve whether the high 1983 rates are the results 

 of El Nifio. 



Caution in interpreting longline catch rates as 

 directly reflecting changes in population abundances 

 is also warranted. While the operational procedures 

 in tuna longlining would appear not to be very sus- 

 ceptible to inducing a nonlinear relationship between 

 abundance and catch rates (i.e., handling time is not 

 a major factor and the length of a single longline 

 insures that effort can not be highly concentrated 

 in space). However, concerns have been raised about 

 potential hook competition at higher densities 

 (Rothschild 1967; Au 1985). More importantly, long- 

 liners target different depths depending upon local 

 conditions, market factors and the relative abun- 

 dance of different species. In addition, the fact that 

 surface catches in the Atlantic were able to greatly 

 exceed previous catches of large yellowfin tuna 

 by longliners despite the fact that longline catch 

 rates had declined steeply suggests that the rela- 

 tionship between availability to the different gears 

 versus overall abundance is not simple (Fonteneau 

 1981). 



In order to gain a broader temporal perspective 

 to compare the current catch rates, longline hook- 

 ing rates from 1962 to 1980 for the same area con- 

 sidered in this paper are plotted in Figure 14 based 

 on published data by the Fisheries Agency of Japan 

 (1962-80). Longline hooking rates were generally 

 declining through the mid-1970s and then appear 

 to have entered a period of recovery. Because of the 

 commencement of the purse seine fishery in 1980, 

 interpretation of the overall long-term temporal 

 trend is confounded and depends upon whether the 

 apparent increase in the 1970s was a true recovery 

 or a reflection of the variability that can be expected 

 in this fishery. 



Interaction 



The results presented in this paper suggest that 

 the relation between longline and purse seine fish- 

 eries is complex. The above discussion indicates that 

 the current data is insufficient to determine whether 

 a general decline is occurring in longline catch rates. 



139 



