INCIDENTAL DOLPHIN MORTALITY IN THE EASTERN TROPICAL 

 PACIFIC TUNA FISHERY, 1973 THROUGH 1978 



Bruce E. Wahlen 1 



ABSTRACT 



Since the late 1950's, large numbers of dolphins have been killed incidentally in the yellowfin tuna purse 

 seine fishery in the eastern tropical Pacific. Estimates of numbers of dolphins killed incidentally in this 

 fishery from 1973 through 1978 were made previously using a stratified ratio estimator. Previous 

 estimates were revised by reducing the number of strata and incorporating revisions in the data. Revised 

 estimates of total mortality, which are consistently more precise than previous estimates, declined from 

 about 100,000 dolphins per year from 1973 through 1976 to about 25,000 and 15,000 during 1977 and 

 1978. The decline in estimated mortality between 1976 and 1977 was primarily the result of a decline 

 in the kill rate which coincided with a significant management action in late 1976. Other examples dur- 

 ing the 1964 through 1982 period of such a temporal correspondence between a change in the number 

 or distribution of dolphins killed and legal or management actions are discussed. 



Since the late 1950's, tuna purse seine fishermen 

 operating in the eastern tropical Pacific Ocean 

 (ETP) have exploited several dolphin species— pri- 

 marily spotted dolphins, Stenella attenuata, and 

 spinner dolphins, S. longirostris, and also striped 

 dolphins, 5. coeruleoalba, and common dolphins, 

 Delphinus delphis— to locate and catch yellowfin 

 tuna, Thunnus albacares. Perrin (1969) described 

 the process of deploying, or setting, the net around 

 the tuna and dolphins, and then releasing the 

 dolphins while retaining the tuna. During this pro- 

 cess, however, large numbers of dolphins have been 

 killed incidentally by becoming entangled in the 

 purse seines (Smith 1983). 



The U.S. Marine Mammal Protection Act of 1972 

 mandated the Secretary of Commerce to make 

 periodic assessments of the condition of dolphin 

 populations involved in this ETP fishery. As a result 

 of a 1976 ruling by a U.S. District Court regarding 

 regulations promulgated under the Act, the Federal 

 Government established annual dolphin mortality 

 limits for the U.S. registered fleet (Fox 1978). Esti- 

 mates of annual dolphin mortality have been an in- 

 tegral component of periodic assessments (Smith 

 1983). 



Estimates of cumulative dolphin mortality made 

 throughout the year are used to monitor mortalities 

 relative to the annual limits (Lo et al. 1982). When 

 a particular limit is reached, regulations prohibit 

 U.S. registered vessels from fishing on the affected 

 populations for the remainder of the year. In Octo- 



ber 1976, the National Marine Fisheries Service 

 (NMFS) issued a prohibition notice for the first time 

 (Federal Register 1976), but because of litigation the 

 notice did not become effective until November 

 1976. 



In recent years, researchers have published sev- 

 eral estimates and revisions of estimates of dolphin 

 mortality incidental to this fishery. For the period 

 1959-78, estimates have been made by Smith 

 (1979 2 ), Lo et al. (1982), Smith (1983), and Lo and 

 Smith (1986); for the years 1979-83, see Allen and 

 Goldsmith (1981, 1982), Lo et al. (1982), Hammond 

 and Tsai (1983), Hammond (1984), and Hammond 

 and Hall (1985). 



Lo et al. (1982) suggested that previous estimates 

 of dolphin mortality incidental to this fishery were 

 based on a stratification scheme with an unneces- 

 sarily large number of strata. In this paper, I revise 

 the 1973-78 estimates for U.S. registered purse 

 seiners by reducing the number of strata and by in- 

 corporating revisions in the data. 



DATA 



Sample data were obtained from recorded obser- 

 vations of scientific observers who had been placed 

 by the NMFS aboard selected U.S. registered tuna 

 purse seine vessels fishing in the ETP. Data re- 

 corded by these observers included the type, date, 



'Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, P.O. Box 271, La Jolla, CA 92038. 



Manuscript accepted September 1985. 

 FISHERY BULLETIN: VOL. 84, NO. 3, 1986. 



2 Smith, T. D. (editor). 1979. Report of the status of porpoise 

 stocks workshop, La Jolla, Calif., 27-31 August 1979. Southwest 

 Fish. Cent. La Jolla Lab., Natl. Mar. Fish. Serv., NOAA, Admin. 

 Rep. LJ-79-41, 120 p. 



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