550 



Fishery Bulletin 97(3), 1999 



Combining H (s) with the combined-fleet cap- 

 ture frequency estimates (Fig. 5, upper cui-ve), 

 schools of 1000 animals or greater were esti- 

 mated to be set on at least once every ten days 

 and contained an estimated 9% of dolphins (Fig. 

 6). Schools set on most often by tuna purse-sein- 

 ers, containing from about 250 to 500 dolphins, 

 were estimated to be set on between 2 and 8 times 

 each per year on average; these schools repre- 

 sented just under an estimated one third of the 

 stock. An estimated one half of NE offshore spot- 

 ted dolphins occurred in schools smaller than 250 

 animals; schools of this size were estimated to be 

 set on less than twice per year each. 



We note that//(s) should not be used to quan- 

 tify the school size preferences of individual 

 dolphins. For example, although we estimated 

 that schools larger than 1000 animals contained 

 an estimated 9% of dolphins at any given time, 

 this does not imply that the same 9% of dol- 

 phins always made up such schools. 



Capture frequency for very large schools 



Although no kernel estimate of nis) was pos- 

 sible for s greater than 1000 animals, the esti- 

 mated detection probability for those schools 

 was essentially one out to the truncation dis- 

 tance w, making a rough calculation for capture 

 frequency possible. Because of the rounding 

 tendency of tuna vessel observers, we made an 

 estimate for schools greater than or equal to 

 1000. With the assumption that the effective 

 strip halfwidth is equal to the truncation dis- 

 tance, an estimate of the average capture fre- 

 quency due to the entire fleet is 



J^(large) 

 N , {S> 1000) = ^Z^ 



N. 



lUS. large) 



schtmh 



ft US I 

 y I trips 



I Inns 



bill 1 

 ■ips 



2Li 



^"■schools 



The estimated average capture frequency for these 

 very large schools was 51.3 sets per year, or just un- 

 der once a week. 



Discussion 



Capture rates for individual dolphins 



To interpret these capture frequency results in terms 

 of individual dolphins, we must consider the size 

 range of the schools with which a given individual 



tends to associate. If one assumes that dolphins have 

 a strong fidelity for a characteristic school size, then 

 the above results indicate that a fixed but relatively 

 small percentage of the dolphin population was con- 

 sistently subjected to a high rate of capture in purse- 

 seine nets, whereas the majority of dolphins were 

 subject to relatively little disturbance from the fish- 

 ery However, little is known about the spatial and 

 temporal dynamics of dolphin schools and their sizes, 

 and a range of other assumptions are possible. 



If school membership is completely fluid and dol- 

 phins mix perfectly among schools, then over the long 

 term, all dolphins would experience the same cap- 

 ture rate. We made a rough estimate of this rate by 

 estimating the total annual number of dolphins set 

 on and the total number of dolphins. Using data from 

 Table 2, we made a rough estimate of 7610 for the 



