INCIDENTAL MORTALITY OF 

 DOLPHINS IN THE EASTERN TROPICAL PACIFIC, 1959-72 



N. C. H. Lo 1 and T. D. Smith 2 



ABSTRACT 



The estimates of the number of dolphins killed annually from the beginning of the U.S. tuna purse seine 

 fishery in the eastern tropical Pacific are used by the National Marine Fisheries Service in developing 

 management advice for the U.S. purse seine fleet. We estimated the annual number of dolphins killed 

 incidentally in the tuna purse seine fishery for 1959-72. Kill data were available for only a few years prior 

 to 1970. Because no obvious trend was shown with the existing data, kill rates were averaged over those 

 years and stratified by various categories: large and small vessels, sets with large catch of tuna and small 

 catch of tuna, sets which used backdown (a dolphin-releasing procedure), and sets which did not use 

 backdown. These kill rates, combined with estimated number of sets, produced the estimated annual kills. 

 Because data were available only for some of the years, they had to be pooled to obtain annual estimates. 

 As a result, the annual estimates were highly correlated. Because the total as well as the annual estimates 

 are of interest, it is necessary to compute the variance-covariance of the estimated annual kills. The an- 

 nual kill from 1959 to 1972 varied from 55,000 in 1959 to 534,000 in 1961. There were three distinct 

 maxima of 534,000, 460,000, and 467,000, corresponding to peaks in number of sets made on dolphins 

 in 1961, 1965, and 1970. The total kill from 1959 to 1972 was estimated to be about 4.8 million, with 

 a coefficient of variation of 17%. 



The eastern tropical Pacific tuna purse seine fleet 

 began to develop rapidly in the late 1950's and has 

 grown to over 100 U.S.-registered vessels and a 

 substantial number of non-U.S.-registered vessels in 

 recent years. This fleet fishes primarily for yellow- 

 fin tuna, Thunnus albacares, and skipjack tuna, Kat- 

 suwonus pelamis. Majority of the yellowfin tuna are 

 taken while the tunas are schooling with dolphins 

 primarily of the species Stenella attenuata and S. 

 longirostris. Birds and dolphins are frequently used 

 as cues in finding the tuna. During the capture of 

 the tuna, some of the dolphins are killed or drown- 

 ed by becoming tangled in the net webbing (Perrin 

 1969). The number of dolphins killed has been 

 estimated to have been greater than one-half million 

 in some of the years in the 1960's (Smith 1983). Cur- 

 rently, fewer animals are killed each year due to im- 

 provements in the fishing gear and in procedures to 

 release dolphins. 



Estimates of the total number of dolphins killed 

 each year in this fishery are used as a basis for 

 management advice by the National Marine 

 Fisheries Service (NMFS). In this paper we describe 

 in detail the method used in Smith (1983), including 



estimation of the variances and covariances of the 

 annual kill estimates so that the variance of the total 

 kill for the period can be estimated. Additionally, we 

 reexamine the data used in previous estimates (Per- 

 rin 1970; Perrin and Zweifel 1971 3 ; Perrin et al. 1982; 

 Smith 1983; Smith and Lo 1983), and we present 

 revised estimates of the total numbers of dolphins 

 killed. 



MATERIALS AND METHODS 



The model used to estimate the total annual in- 

 cidental kill of dolphins (T t ) in the eastern tropical 

 Pacific tuna purse seine fishery is 



T t = R t X t 



(1) 



where t denotes the year (1959 to 1972), R denotes 

 the number of dolphins killed per set, and X denotes 

 the number of sets made involving dolphins. The rate 

 of kill (R) varies between larger and smaller vessels, 

 and in dolphin sets where fewer and greater amounts 

 of yellowfin tuna are caught (Lo et al. 1982). In addi- 

 tion, the rate of dolphin kills is generally less if 



Southwest Fisheries Center La Jolla Laboratory, National 

 Marine Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La 

 Jolla, CA 92038. 



2 Northeast Fisheries Center Woods Hole Laboratory, National 

 Marine Fisheries Service, NOAA, Woods Hole, MA 02543. 



Manuscript accepted February 1985. 

 FISHERY BULLETIN: VOL. 84, NO. 1, 1986. 



3 Perrin, W. F, and J. R. Zweifel. 1971. Porpoise mortality in 

 the eastern tropical tuna fishery in 1971. Unpubl. manuscr., 22 p. 

 Southwest Fisheries Center La Jolla Laboratory, National Marine 

 Fisheries Service, NOAA, 8604 La Jolla Shores Drive, La Jolla, CA 

 92038. 



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