Abstract. — Dolphins (Delphin- 

 idae) have been killed incidentally 

 by the purse-seine fishery for yel- 

 lowfin tuna, Thunnus albacares, in 

 the eastern tropical Pacific since at 

 least 1959. Annual estimates of the 

 number of dolphins killed from 

 each stock are used by the National 

 Marine Fisheries Service in mak- 

 ing management decisions about 

 the population status of affected 

 stocks. Mortality estimates from 

 the period with the greatest kill of 

 dolphins, 1959-72, are important 

 for estimates of the level of deple- 

 tion of these stocks from their 

 unexploited population sizes. A re- 

 definition of the geographical 

 boundaries of offshore stocks of 

 pantropical spotted dolphins, Sten- 

 ella attenuata, makes it necessary 

 to estimate annual kill for these 

 newly defined stocks for 1959-72. 

 I estimated the number of dolphins 

 killed annually from 1959 to 1972 

 for the northeastern and western/ 

 southern stocks of spotted dol- 

 phins, using the methods of Lo and 

 Smith ( 1986 ). I also revised the es- 

 timates of annual kill for the east- 

 ern and whitebelly stocks of spin- 

 ner dolphins, S. longirostris, by cor- 

 recting minor problems in previous 

 data and analyses. Additionally, I 

 estimated a coefficient of variation 

 (CV) for each stock-specific esti- 

 mate of incidental kill, which had 

 not previously been done. Esti- 

 mates of total kill were similar to 

 previous estimates: 4.9 million dol- 

 phins are estimated to have been 

 killed by the purse-seine fishery 

 over the fourteen year period con- 

 sidered here, an average of 347,082 

 per year. Nearly all of the fisheries 

 kill of pantropical spotted dolphins 

 was of the northeastern stock, to- 

 taling 3.0 million (211,612 per 

 year). Estimates of kill for the east- 

 ern stock of spinner dolphins were 

 similar to previous estimates, to- 

 taling 1.3 million (91,739 per year). 

 As expected, CVs of the kill for each 

 stock were higher than those previ- 

 ously reported for the total kill. 



Revised estimates of incidental kill of 

 dolphins (Delphinidae) by the 

 purse-seine tuna fishery in the 

 eastern tropical Pacific, 1959-1972 



Paul R. Wade 



Scnpps Institution of Oceanography 

 University of California, San Diego 

 LaJolla. CA 92093 



Southwest Fisheries Science Center* 

 National Marine Fisheries Service. NOAA 

 RO. Box 27 1 , La Jolla. CA 92038 



Manuscript accepted 22 September 1994. 

 Fishery Bulletin 93:345-354 (1995). 



Dolphins (Delphinidae) have been 

 killed incidentally by the purse- 

 seine fishery for yellowfin tuna, 

 Thunnus albacares, in the eastern 

 tropical Pacific (McNeely, 1961) 

 since at least 1959 (Perrin, 1969). 

 Purse seiners catch tuna by locat- 

 ing and capturing dolphin schools, 

 taking advantage of an association 

 between these species (Au, 1991). In 

 spite of attempts to release dolphins 

 alive using a procedure called the 

 backdown (Barham et al., 1977), 

 dolphins are killed when they be- 

 come entangled in the net. Dolphins 

 from several species are killed; the 

 majority represent either pan- 

 tropical spotted dolphins, Stenella 

 attenuata, or spinner dolphins, 

 Stenella longirostris. Several stocks 

 of each species are impacted. 



Annual estimates of the number 

 of dolphins killed from each stock 

 are used by the National Marine 

 Fisheries Service (NMFS) in mak- 

 ing management decisions about 

 the population status of affected 

 stocks. For example. Wade (1993) 

 used annual estimates of mortality 

 and variance in mortality to con- 

 clude that eastern spinner dolphins, 

 Stenella longirostris orientalis, were 

 likely below 60% of their unex- 

 ploited population size in 1959. This 

 led to the listing of eastern spinner 



dolphins as a depleted species un- 

 der the U.S. Marine Mammal Pro- 

 tection Act. 1 During the period of 

 greatest dolphin mortality, 1959- 

 72, the kill of spotted dolphins was 

 estimated to be twice that of spin- 

 ner dolphins (Smith, 1983). There- 

 fore, it is also important to investi- 

 gate the management status of 

 stocks of spotted dolphins. Wade 

 (1993) showed that the estimated 

 decline of the eastern spinner dol- 

 phin was mostly due to the early 

 period of high mortality. Thus, esti- 

 mates of incidental kill from 1959 

 to 1972, along with a measure of 

 their uncertainty, are crucial for 

 assessing whether spotted dolphin 

 stocks are also depleted. 



Recently, Dizon et al. (1994) es- 

 tablished new geographical bound- 

 aries for the offshore stocks of 

 pantropical spotted dolphins (Fig. 1) 

 on the basis of a reexamination of 

 cranial morphology (Perrin et al., 

 1994). Estimates of the number of 

 spotted dolphins killed from each 

 stock must be revised to reflect this 

 stock structure. Therefore, my first 

 objective was to estimate annual 

 kill of the northeastern and west- 



: Address for correspondence. 

 Federal Register Vol. 58, No. 164, August 

 26. 1993 (58 FR 45066). 



345 



