In 1986, the U.S. tuna fleet reached the allowable mortality 

 level of 2 0,500 in mid-October and was required to cease catching 

 tuna by setting on schools of porpoise. The quota may have been 

 reached because of an increased number of problem sets (i.e. , 

 sets resulting in the death of 15 or more porpoise) and/or a 

 record high catch rate of 25 tons of tuna per set. Large schools 

 of tuna may be associated with large schools of porpoise and, 

 therefore, more porpoise than usual may have been encircled per 

 set. The large number of tuna in the net also may have made it 

 more difficult to release porpoise during the backdown procedure. 



Porpoise mortality in the U.S. tuna fishery was signifi- 

 cantly lower in 1987. The total estimated kill was 13,992 

 porpoise. In 1987, the catch rate of tuna was about 2 tons of 

 tuna per set. This suggests that incidental mortality of 

 porpoise may be correlated with the size of the catch per set. 

 In 1988, porpoise mortality in the U.S. tuna fleet again 

 approached the maximum allowable level. Estimated total mortal- 

 ity was 19,712 porpoise. As in 1986, the fleet experienced a 

 high number of problem sets, those that account for the highest 

 porpoise mortality. The catch rate of tuna in 1988 was approxi- 

 mately 19 tons of tuna per set. 



The 1984 amendments to the Marine Mammal Protection Act 

 established a quota of 2,750 for the eastern spinner stock. In 

 1987, despite a relatively low overall mortality of marine 

 mammals, an estimated 2,688 eastern spinner dolphins were killed 

 compared to 1,608 killed in 1986 when the overall quota of 20,500 

 porpoise was reached. In 1988, the quota for eastern spinner 

 stock was exceeded for the first time with an estimated mortality 

 of 2,832 animals taken by the U.S. fleet. A possible explanation 

 for the high mortality of this stock in 1987 and 1988 is that 

 tuna, and hence fishing effort, were concentrated in the areas 

 where eastern spinner dolphins are more abundant. 



In recent years, there has been a declining trend in the 

 percentage of sets made by U.S. tuna fishermen on porpoise. In 

 1986, 94 percent of the sets made were set on porpoise; in 1987, 

 it was 78 percent of the sets; and in 1988, it was 65 percent of 

 the sets. 



In 1989, the overall take of porpoise, including eastern 

 spinner dolphins, by U.S. tuna fishermen was reduced substan- 

 tially. Observed incidental mortality was 12,643 porpoise, the 

 lowest level since the El Nino year of 1983 and the second lowest 

 ever. The incidental take of eastern spinner dolphins was 1,468 

 animals. The catch of tuna was approximately 21 tons per set and 

 the percentage of problems sets in 1989 was six percent. 

 Although the percentage of problem sets was comparable to the 

 1988 level, the number of sets with mortality greater than 50 may 

 have been reduced substantially. Other factors which may have 

 contributed to the reduced incidental mortality in 1989 were the 



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