FISHERY BULLETIN: VOL. 81. NO. 1 



DISCUSSION AND CONCLUSIONS 



The three populations of dolphins involved 

 with the yellowfin tuna purse seine fleet in the 

 eastern tropical Pacific have declined since 1959 

 and the decline was not arrested until recently 

 (Figs. 1, 2). Assuming the historical kill level, 

 and the central values for R m and MNPL, the 

 whitebelly spinner dolphin population has de- 

 clined to between 58 and 72% of its pre-exploita- 

 tion levels; the offshore spotted dolphin popula- 

 tion has declined to between 35 and 50% of its 

 pre-exploitation size; and the eastern spinner 

 dolphin population has declined to around 20% of 

 its pre-exploitation size. 



Examination of Figures 3 and 4 shows that the 

 numerical values of the estimates of relative 

 abundance in 1979 for offshore spotted dolphin 

 and whitebelly spinner dolphin are relatively 

 more sensitive to changes in the maximum net 

 recruitment rate and the maximum net produc- 

 tivity level parameters than are the estimates for 

 the eastern spinner dolphin. Also, the sensitivity 

 of these calculations to the maximum net pro- 

 ductivity level increases markedly as the value of 

 the maximum net recruitment level increases. 

 The sensitivity (in percent change) in the ratio of 

 present to pre-exploitation abundance, however, 

 is largest for the offshore spotted dolphin and 

 least for the whitebelly spinner dolphin. This is 

 due in part to the shorter time span over which 

 the whitebelly spinner dolphin has been exploit- 



N t 

 ed, and in part to the lower — — ratio for the east- 



ern spinner dolphin ratio, which makes smaller 

 differences result in a larger percentage. 



Although there are a number of uncertainties 

 about specific parameter estimates used in these 

 calculations, the general declines in abundance 

 change relatively little over the ranges of param- 

 eter estimates explored. For example, rather 

 rapid declines in the 1960's, followed by decreas- 

 ing rates of decline in the 1970's, are evident for 

 all parameter values considered. Specific aspects 

 of these declines in abundance, however, depend 

 to a greater degree on the actual parameter val- 

 ues. For example, the estimated changes in popu- 

 lation sizes from 1975 to 1978 vary with the spe- 

 cific values of maximum net recruitment rate, 

 while the estimated changes in population sizes 

 in the 1960's are relatively insensitive to this 

 parameter. 



In order to improve our estimates of reproduc- 

 tive and mortality rates, a complete review of 



vita! rates for these dolphin populations and for 

 cetaceans in general should be carried out. Sev- 

 eral approaches to this problem have been identi- 

 fied, including a detailed review of the eastern 

 tropical Pacific dolphin data and of the existing 

 data for other cetacean populations. Given the 

 gaps in our knowledge of cetacean reproductive 

 processes, analyses of alternate mathematical 

 models of such processes will be fruitful. 



Although improvements in estimates of abun- 

 dance and kill levels are needed, these areas are 

 generally much better understood than the re- 

 cruitment process. Population-size estimation 

 techniques are still being improved upon; cur- 

 rent emphasis is on testing the assumptions 

 needed in applying line transect theory to aerial 

 sighting survey data and in estimating dolphin 

 school size. Future work will emphasize im- 

 proved shipboard sighting methodology for pos- 

 sible application of line transect theory. 



Marked improvements in the estimates of num- 

 bers of dolphins killed are not anticipated; key 

 areas needing additional information are the kill 

 rates both in the non-U. S. fleet and on unobserved 

 fishing trips. Neither of these areas is readily 

 amenable to study, although further analysis of 

 the kill rates on unobserved trips may provide 

 some basis for exploring this uncertainty. The 

 possible levels of indirect mortality or debility 

 due to the stress of chase and capture are also of 

 concern. Because of the large numbers of dol- 

 phins captured and released each year, even very 

 low rates of indirect mortality could have a sig- 

 nificant effect on the population. 



ACKNOWLEDGMENTS 



This assessment of the status of the dolphin 

 populations is built on data collected by many in- 

 dividuals. The collection of these data has been 

 made possible in large measure by the coopera- 

 tion of the U.S. tuna fishing fleet. In addition, 

 many individuals have contributed to the analy- 

 sis of the data, including National Marine Fish- 

 eries Service staff and numerous scientists from 

 various organizations. It is not possible to ac- 

 knowledge the contributions of specific individ- 

 uals to information presented here because of the 

 large numbers of people who have been involved, 

 but without their efforts the present analysis 

 would not be possible. I also wish to acknowledge 

 the very helpful reviews of an earlier draft of 

 this paper by Douglas Chapman, John Gulland, 

 Linda Jones, and Jeff Breiwick. 



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