342 



Fishery Bulletin 103(2) 



surveys. Seasonal differences may exist, but data in 

 Feldkamp et al., (1989, 1991) and Melin (2002) indicate 

 that these differences are negligible. Feldkamp et al. 

 (1991) showed differences in diving behavior during El 

 Nino and non-El Nino, but Melin (2002) did not find as 

 much difference in diving behavior during El Nino and 

 non-El Nino (with the exception of longer transit time 

 to foraging grounds during El Nino). 



Error in age- and sex-class abundance estimates at 

 haulouts is also affected by subjectivity and inter-ob- 

 server differences in age and sex classification of sea 

 lions. Therefore, age- and sex-class counts provided 

 in these surveys, although conducted by a single ex- 

 perienced observer (M. Lowry), serve as approximate 

 indices of sea lion age- and sex-class distributions in 

 central and northern California. These indices will be 

 useful for future attempts to estimate consumption of 

 prey by sea lions along central and northern California, 

 given that nutritional requirements differ among age 

 and sex classes. 



By estimating abundance of sea lions on land as 

 well as at-sea, we were able to derive a multiplier for 

 estimating total abundance from counts of animals 

 hauled out on land. This multiplier can be applied to 

 future land counts of California sea lions in central 

 and northern California to estimate total abundance, 

 as has been done for harbor seals in California, Or- 

 egon, and Washington (Huber et al., 2001; Barlow 

 et al. 6 ; Forney et al. 2 ). It may also be useful for es- 

 timating total abundance from counts of sea lions 

 hauled out in Oregon, Washington, and British Co- 

 lumbia because the age- and sex-class structure of sea 

 lions is similar to that found in central and northern 

 California. However, the multiplier should not be used 

 for smaller areas (such as the zones in the inshore 

 stratum) or for other species, because regional and 

 interspecies differences may exist. In particular, it 

 would not be appropriate for regions where sea lions 

 reproduce, such as in the Southern California Bight 

 (SCB) and in Mexico, because adult females that are 

 rearing pups may spend a different proportion of their 

 time at sea. For that reason, it would be judicious to 

 conduct concurrent offshore and haulout surveys in the 

 SCB and Mexico to derive a correction factor for each 

 geographical region of the sea lion's range. Multipliers 

 could also be derived for smaller areas (such as our 

 zones) by conducting suitably designed smaller-scale 

 at-sea surveys in conjunction with counts of animals 

 hauled out, or by using satellite or radio telemetry 

 tags to directly measure the relative times at sea and 

 on land. 



The multiplier for deriving total abundance from 

 haulout counts provides researchers and resource man- 

 agers with an alternative method for estimating total 

 population abundance or abundance of a stock. Abun- 

 dance estimates derived with this new approach can 

 be compared to abundance estimates obtained with 

 more conventional methods (such as the life history 

 model), and may provide a means for estimating to- 

 tal abundance when life history data are unavailable. 



The approach used in the present study may be par- 

 ticularly useful for estimating abundance at times and 

 places unrelated to breeding activities, or for periods 

 when breeding is disrupted, as with El Nino conditions. 

 Abundance estimates and distributional data provided 

 by these methods can be used to determine where and 

 when the greatest effects on salmon and other prey spe- 

 cies may occur. Diet studies at major hauling areas in 

 conjunction with abundance surveys to derive consump- 

 tion estimates are required to determine the effect of 

 California sea lions on salmon and other sea lion prey 

 species of the region. 



Acknowledgments 



This research was supported financially by the Office 

 of Protected Resources, National Marine Fisheries Ser- 

 vice. We greatly appreciate the assistance given by Jim 

 Gilpatrick, Charlie Stinchcomb, and, especially, Scott 

 Benson of Moss Landing Marine Laboratories during the 

 surveys. Jay Barlow provided guidance. Special thanks 

 to Morgan Lynn of the Southwest Fisheries Science 

 Center who kept the photographic equipment functioning 

 properly. Henry Orr of the Southwest Fisheries Science 

 Center helped with illustrations. Research within Gulf of 

 the Farallones National Marine Sanctuary and Monterey 

 Bay National Sanctuary was conducted under National 

 Marine Sanctuary Permit GFNMS/MBNMS-20-98. This 

 research was conducted under MMPA Research Permit 

 No. 774-1437. We greatly appreciated the reviews and 

 comments by Jay Barlow, Jeff Laake, Jim Harvey, and 

 three anonymous reviewers. 



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