233 



Abstract— We estimated the total 

 number of pantropical spotted dolphin 

 (Stenella attenuata) mothers killed 

 without their calves ("calf deficit") in 

 all tuna purse-seine sets from 1973-90 

 and 1996-2000 in the eastern tropical 

 Pacific. Estimates were based on a 

 tally of the mothers killed as reported 

 by color pattern and gender, several 

 color-pattern-based frequency tables, 

 and a weaning model. Over the time 

 series, there was a decrease in the calf 

 deficit from approximately 2800 for 

 the western-southern stock and 5000 

 in the northeastern stock to about 60 

 missing calves per year. The mean 

 deficit per set decreased from approxi- 

 mately 1.5 missing calves per set in 

 the mid-1970s to 0.01 per set in the 

 late-1990s. Over the time series exam- 

 ined, from 75% to 95% of the lactating 

 females killed were killed without a 

 calf. Under the assumption that these 

 orphaned calves did not survive with- 

 out their mothers, this calf deficit rep- 

 resents an approximately 14% increase 

 in the reported kill of calves, which is 

 relatively constant across the years 

 examined. Because the calf deficit as 

 we have defined it is based on the kill 

 of mothers, the total number of mis- 

 sing calves that we estimate is poten- 

 tially an underestimate of the actual 

 number killed. Further research on 

 the mechanism by which separation 

 of mother and calf occurs is required 

 to obtain better estimates of the unob- 

 served kill of dolphin calves in this 

 fishery. 



Annual estimates of the unobserved 

 incidental kill of pantropical spotted dolphin 

 {Stenella attenuata attenuato) calves 

 in the tuna purse-seine fishery 

 of the eastern tropical Pacific 



Frederick Archer 



Tim Gerrodette 



Susan Chivers 



Alan Jackson 



Southwest Fisheries Science Center 



National Marine Fisheries Service 



8604 La Jolla Shores Dr. 



La Jolla, California 92037 



E-mail address (for F Archer): enc.archeriainoaa.gov 



Manuscript approved for publication 

 7 January 2004 by Scientific Editor. 



Manuscript received 20 January 2004 

 at NMFS Sceintific Publications Office. 



Fish. Bull. 102:233-244 (2004). 



In the eastern tropical Pacific (ETP), 

 yellowfin tuna (Thunnus albacares) 

 are frequently found swimming under 

 schools of pantropical spotted ( Stenella 

 attenuata) and spinner (S. longirostris) 

 dolphins. For the past four decades, 

 the ETP yellowfin tuna fishery has 

 made use of this association by chasing 

 the more visible dolphins at the sur- 

 face and using purse-seines to encircle 

 the schools "carrying" the tuna (NRC, 

 1992). The large bycatch of dolphins in 

 this fishery has become widely known 

 as the "tuna-dolphin issue" (Gerro- 

 dette, 2002). During the 1960s, the 

 number of dolphins killed by the fishery 

 was estimated to be 200,000-500,000 

 per year (Wade, 1995), and two stocks 

 of spotted and spinner dolphins were 

 reduced to fractions of their previous 

 sizes (Smith, 1983; Wade et al. 1 ). Along 

 history of technological innovations by 

 fishermen, laws and fishing regula- 

 tions, dolphin quotas, eco-labeling of 

 "dolphin-safe" tuna, and a comprehen- 

 sive international observer program 

 (Gosliner, 1999; Hall et al, 2000; Ger- 

 rodette, 2002) has reduced the dolphin 

 bycatch to less than 1% of its former 

 level. The reported bycatch in recent 

 years is less than 2000 dolphins per 

 year for all species combined (IATTC, 

 2002). 



Although the reported kill has dra- 

 matically decreased, recent studies 



suggest that there is little evidence 

 that the stocks are growing close to 

 expected rates (Wade et al. 1 ). One hy- 

 pothesis for this lack of recovery has 

 been that there are unobserved kills of 

 dolphins during tuna purse-seine sets. 

 Archer et al. (2001) presented evidence 

 of an under-representation of suckling 

 spotted and spinner dolphin calves in 

 a sample of tuna purse-seine sets in 

 the eastern tropical Pacific. Given that 

 some of these missing calves are still 

 dependent on their mothers for nutri- 

 tion, it is likely that once separated 

 they would die and this under-repre- 

 sentation represents some degree of 

 unobserved kill. 



In Archer et al. (2001), the sample 

 of sets examined was limited to those 

 sets in which all of the animals killed 

 had biological data collected by techni- 

 cians aboard the tuna vessel. Calves 

 still dependent on their mothers in the 

 kill were identified by five intervals of 

 body length, chosen to cover a range of 



1 Wade, P. R.. S. B. Reilly. and T. Gerro- 

 dette. 2002. Assessment of the popula- 

 tion dynamics of the northeastern offshore 

 spotted and the eastern spinner dolphin 

 populations through 2002. National 

 Oceanographic and Atmospheric Admin- 

 istration Administrative Report LJ-02- 

 13. 58 p. Southwest Fisheries Science 

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

 CA 92037. 



