Archer et al.: Estimates of the incidental kill of Stenella attenuata attenuate calves in the tuna purse-seine fishery 



239 



3.0 



2.5 

 2.0 

 1.5 

 1.0 

 2° 0.5 



CD 

 if) 



I 0.0 



I 3.0 



a 



1 2.5 



c 

 a 



| 2.0 f 

 1.5 

 1.0 

 0.5" 

 0.0" 



-+- 



Northeastern 



• ( .tt 



It 



• * 



Western-southern 



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1970 



1980 1990 



Year 



2000 



Figure 4 



Mean calf deficit per set (D s ) by year. Vertical 

 lines indicate 959? confidence intervals. 



Edwards 3 !, the estimated calf deficit represents an approx- 

 imately 147c underestimate of the reported kill. 



The calf deficit in the present study was estimated from 

 the number of dependent calves and lactating females 

 killed by using age-color frequency tables and data on the 

 stomach contents of weaning calves. Specimens used to 

 derive the age and color table were collected from 1973 to 

 1978 and 1981, and specimens used for the weaning model 

 were collected between 1989 and 1991. If the distributions 

 of these samples were not representative of all years that 

 we examined, then our results may be biased. However, the 

 results of a study to construct the annual age distribution of 

 the kill (Archer and Chivers 4 ) indicated that there is no sig- 

 nificant difference in the age-color frequency table across 

 years. The sample size for the stomach data ( 29 calves) was 

 too small to examine differences between years. 



Our finding of no significant difference between our esti- 

 mates of the number of lactating females and the observed 

 tally of lactating females in sets where the entire kill was 

 sampled validates this portion of our estimation proce- 

 dure. However, because the number of suckling calves 

 present in these 100% -sampled sets was not recorded, we 

 were unable to validate the method used to generate these 

 estimates in a similar manner. 



The results of our paired Ntests indicated that the ob- 

 served number of animals smaller than 122 cm tended to 

 be greater than the number we estimated. This is most 

 likely a result of the difference between how calves were 

 counted in each method. Archer et al. ( 2001 ) considered all 

 animals under a series of cutoff values to be calves that 

 were dependent on suckling for survival. In the present 

 study, the weaning model that we used (Archer and Rob- 



3 Edwards, E. F. 2002. Behavioral contributions to separa- 

 tion and subsequent mortality of dolphin calves chased by tuna 

 purse-seiners in the eastern tropical Pacific Ocean. National 

 Oceanographic and Atmospheric Administration Administra- 

 tive Report LJ-02-28, 34 p. Southwest Fisheries Science 

 Center, 8604 La Jolla Shores Dr., La Jolla, CA 92037. 



4 Archer. F.. and S. J. Chivers. 2002. Age structure of the 

 northeastern spotted dolphin incidental kill by year for 1971 to 

 1990 and 1996 to 2000. National Oceanographic and Atmo- 

 spheric Administration Administrative Report LJ-02-12, 18 p. 

 Southwest Fisheries Science Center, 8604 La Jolla Shores Dr., 

 La Jolla. CA 92037. 



