244 



Fishery Bulletin 102(2) 



growth rates and lower estimates of the current size of 

 the population in relation to carrying capacity. Wade et 

 al. 1 did not model the calf deficit estimated in our present 

 study, but the effect of 14/r additional mortality would 

 probably be less than the 50 f > additional mortality that 

 was modeled. The 50^ mortality was spread over all age 

 classes, and additional mortality due to missing calves 

 should be assigned to the first two year classes only. The 

 important question is whether the calf deficit in the kill 

 represents the main effect of mother-calf separation by 

 the fishing process. As outlined in Archer et al. (2001t, 

 the mechanism by which suckling calves are separated 

 from their mothers is unknown. If separation is simply a 

 function of the number of lactating females killed, then the 

 deficit presented here is an accurate representation of the 

 number of "missing" calves. 



However, there is some evidence that separation can 

 occur without the mother being killed. In the early days 

 of the backdown procedure, purse-seine skippers reported 

 that "Babies swim around the outside of the net pushing to 

 get back in probably because their mothers are still inside" 

 i Gehres p (. It is unclear whether these calves were sepa- 

 rated prior to encirclement or were released early during 

 backdown, prior to their mothers. Regardless, given that 

 dolphins exhibit some of their fastest swimming during 

 a set immediately upon release from the net tChivers and 

 Scott' ), separated calves waiting immediately outside the 

 net may risk separation if their mothers join the rest of the 

 school rapidly swimming away from the net. If this, or any of 

 the other scenarios regarding the manner in which perma- 

 nent separation can occur without the mother being killed 

 i Archer et al.. 2001 1. then the calf deficit underestimates the 

 actual number of orphaned calves. Future research should 

 focus on the mechanism of calf separation because a better 

 understanding of this process is the only way we will be able 

 to estimate the magnitude of the unobserved calf mortality 

 and its subsequent effects on the population. 



Acknowledgments 



The authors wish to thank Michael Scott and Xick Vogel 

 of the IATTC for providing data as well as Jay Bar- 



B Gehres. L. E. 1971. Letter of July 2 to Alan R. Longhurst. 

 2 p. Southwest Fisheries Science Center, 8604 La Jolla Shores 

 Dr.. La Jolla. CA 92037. 



7 Olivers. S. J., and M. D. Scott. 2002. Tagging and tracking 

 of Stenella spp. during the 2001 Chase Encirclement S 

 Studies cruise. National Oceanographic and Atmospheric 

 Administration Administrative Report LJ-02-33. 21 p. South- 

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

 Jolla. CA 92037. 



low and Bill Perrin for helpful reviews and analytical 

 suggestions. 



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