244 



Dr. Chapman. The results of the La .Toll a workshop as updated 

 have led to the conclusion that the eastern spinner stock is at a level 

 of 5 I percent of its original or preexploitation size. 



Further, it is the consensus of marine mammal biologists that the 

 lower limit of the range of optimum sustainable population for 

 marine mammals is in the range of 50 to 70 percent of the unex- 

 ploited stock- size. Hence, it is concluded either by comparing the 

 present level of 54 percent with the midpoint of this range, GO per- 

 cent, or by more sophisticated analysis using modern operational re- 

 search methods that the Eastern spinner stock is below the optimum 

 level. 



Can a marine mammal stock be taken much below this level and 

 be expected to recover, and/or constitute a viable element of the 

 ecosystem ? 



In regard to cetaceans, the answer is unknown. Only one depleted 

 whale stock has rebuilt — the California gray whale and while there 

 is much uncertainty about its numbers in earlier times, the best evi- 

 dence is that the stock was never reduced below the 50 percent level. 

 Other whole stocks that have been reduced further, for example, 

 blue, humpback, right, and bowhead whales have shown no measur- 

 able recovery. 



In the case of blue and humpback whales, the time for such recov- 

 ery is only 10 to 12 years, but right and bowhead whales have been 

 given substantial protection for 30 or more years and still remain 

 at extremely low levels. 



Furthermore, the Commission reiterates the uncertainty of the 

 estimates that are involved in the calculations. As was pointed out in 

 the La Jolla workshop report and has been reviewed extensively 

 since, the estimates involve basic assumptions that are difficult to 

 quantify in addition to the statistical uncertainties which have been 

 quantified and included by giving various confidence limits. 



A few of the aspects that have not been quantified are : 



1. The validity of the estimates of school size by the tuna purse 

 seiners. The average school size estimate changed radically from 1973 

 to 1974 for an unexplained reason. The larger 1974 estimates were 

 used bv the La Jolla workshop. 



2. The assumption that the net recruitment rate for Eastern spin- 

 ner porpoises is the same as that of the offshore spotted porpoise. 

 The latter is based on a rather shakey comparison with a Western 

 Pacific stock of the same species. On the other hand, the gross repro- 

 ductive rate in the eastern spinner is much less than that of the off- 

 shore spotted porpoise. 



3. The possibility of indirect or delayed mortalities. The observers 

 are able to count only the animals seen to be dead or seriously injured 

 at the time of the set and even the ones counted as seriously injured 

 have not been considered in the La Jolla workshop analysis. If the 

 setting process has caused additional mortalities, the present level of 

 the stock is a lower percentage of the original level than 54. 



4. The assumption that the rate of kill on unobserved purse seiners 

 is the same as on observed purse seiners. 



5. The assumption that the setting process with its possible dis- 

 ruption of the behavior of this social animal has not reduced the re- 

 productive rate. 



