Hastings and Sydeman Population status of Etimetopias /ubahis at the South Farallon Islands, California 



55 



f2 30 60 W 120 150 180 210 240 270 300 330 360 



30 60 90 120 150 ISO 210 240 270 300 330 360 

 Julian J.ile 



S 100 -- 



B 



30 60 90 120 150 180 210 240 270 300 330 360 



1 50 



100 -- 



50 -- 



D 



30 60 90 120 150 180 210 240 270 300 330 360 

 Julian date 



Figure 2 



Seasonal variation in counts of Steller sea lions at the South Farallon Islands for (A) both sexes and all age 

 classes; (B) adult females; (C) subadult and adult males; and (Dl immature mdividuals and yearlings. Data from 

 1974 or 1977 to 1996 were pooled. Black dots indicate counts: black lines indicate predicted values from the 

 regression model. Divisions on the .v-axis approximate months. The best regression model for each group included 

 the variables date and date'^ through (A) date^ for total counts (adjusted r-=0.28); (B) date^ for adult females 

 (adjusted r-'=0.22); (C) date^'~ for males (adjusted r'^=0.77); and (D) date*^ for immature individuals (adjusted 

 r2=0.13l. 



breeding season averaged approximately 100 animals, 

 ranging from 50 to 200 animals; whereas numbers from 

 the late fall through early winter peak were more vari- 

 able, averaging slightly less than 100 and ranging from 

 less than 10 to 300 animals (Fig. 2A). 



Seasonal patterns varied among sexes and age classes 

 (Fig. 2, B-D). Counts of adult and subadult males peaked 

 only during the breeding season (Fig. 2C), whereas counts 

 of adult females and immature sea lions were bimodal 

 (Fig. 2, B and D). When models including the variables 

 date through date'^ were fitted to data for adult females 

 and immature individuals separately, the seasonal pat- 

 tern differed significantly between the two groups (age 

 class, year, and all interaction terms; all P<0.001). Counts 

 of immature sea lions were less peaked during the breed- 

 ing season than those of adult females and, in contrast to 

 the average adult female pattern, numbers during winter 

 peaked on average slightly higher than during the breed- 

 ing season (Fig. 2, B and D). 



The seasonal pattern varied significantly among years 

 for all age classes (year and yearxdate interactions for 



adult females and immature individuals; P<0.001, and for 

 subadult and adult males; P<0.05). Variation in seasonal 

 pattern among years was complex but several general pat- 

 terns could be noted. A gradual shift in the peak breeding 

 season count from the beginning of May in 1974 to the be- 

 ginning to middle of June in 1979 was evident (Hastings 

 and Sydeman"). The late fall-winter peak was very pro- 

 nounced from 1984 to 1986, with maximum counts of 200 

 to 300 animals (Hastings and Sydeman'), most of which 

 were immature individuals. From 1992 to 1996, the sea- 

 sonal abundance pattern was muted with equal or higher 

 numbers in the winter than in the breeding season (Hast- 

 ings and Sydeman'). 



Hastings, K. K., and W. J. Sydeman. 1998. Status, seasonal 

 variation and long-term trends in numbers of Steller sea lions, 

 Eumetopias jubatus. at the South Farrallon Islands, California: 

 1927-1996. Final report to the National Marine Fisheries Ser- 

 vice, Southwest Fisheries Science Center, La Jolla, CA, 30 p. 

 (Available from Point Reyes Bird Observatory, 4990 Shoreline 

 Hwy., Stinson Beach, CA 94970.) 



