AINLEY ET AL.: POPULATION FLUCTUATIONS OF SEA LIONS 



winter. Average monthly population size in- 

 creased slightly from one year to another (Fig. 3). 

 The proportion of adults present each month 

 ranged between 73 and 95%. 



Since 1977, population fluctuations of the Cali- 

 fornia sea lions have been markedly different in 

 several ways. First, except for April and May, 

 average monthly population size began to in- 

 crease rapidly from one year to the next (Fig. 3). 

 This was especially evident for the summer and 

 fall and thus, secondly, by 1978 the timing of the 

 annual maximum population had shifted and 

 fall counts were exceeding those of the spring 

 peak (Fig. 2). In fact, for each month except 

 April and May, average monthly numbers in- 

 creased geometrically from 1971-73 to 1980 

 (least squares; r ranged 0.7745 to 0.9537, P<0.01). 

 Finally, the percentage of adults during 1977-80 

 was reduced to a range between 15 and 35%. 

 These differed significantly from percentages of 

 adults in the period 1971/73-76 (P<0.01; per- 

 centage test, Sokal and Rohlf 1969:608). Young 

 animals were thus migrating north rather than 

 remaining in southern California and Baja Cali- 

 fornia waters as Bartholomew (1967) had noted 

 in earlier years. 



Seasonal population fluctuations and age 

 ratios at the Farallones from 1971 to 1976 were 

 largely similar to those at coastal sites, as mea- 

 sured at Ano Nuevo Island (80 km away, Orr and 

 Poulter 1965; Lance and Peterson 1968), and 



1971/73 74 



Figure 3. — The average number of California sea lions hauled 

 out annually at Southeast Farallon Island. Dots above each 

 year are monthly averages; the curve is described by the geo- 

 metric equation: y = a\ x , where a = 9.5 X 10" 8 and A = e 02979 ; 

 r = 0.6557, P<0.01. 



sites farther north (Mate 1975). Exceptional at 

 the Farallones was the fact that there was almost 

 no fall peak, whereas at coastal sites it greatly 

 exceeded the peak in spring. When the fall peak 

 increased in 1977 the Farallon pattern became 

 similar to coastal sites. However, it is possible 

 that the age composition, for which few compar- 

 ative data are available, and the size of the spring 

 peak were changing then at the Farallons. At 

 coastal sites there is a small spring peak and a 

 large fall peak, but at the Farallones the two 

 peaks became equal in magnitude. 



The diet of California sea lions at the Faral- 

 lones, as revealed by regurgitated items, has 

 been comprised of at least 20 species of prey 

 (Table 1) (some otoliths could actually have come 

 from the stomachs of sea lion prey). Outstanding 

 were the predominance of Pacific whiting, par- 

 ticularly from April to August, and the diversifi- 

 cation in diet from September to March. The 

 whiting eaten averaged 25 to 36 cm in length and 

 were 2 to 3 yr of age (Bailey and Ainley in press). 

 Except for the short period during summer 

 when they were away at breeding sites, Califor- 

 nia sea lions were most abundant when whiting 

 predominated in their diet. At coastal sites of 

 central California, the market squid, Loligo 

 opalescens, along with whiting and northern 

 anchovies, Engraulis mordax, are dominant 

 prey of this pinniped (Morejohn et al. 1978). 



California Sea Lions and 

 the Pacific Whiting Fishery 



From 1967 to 1972 most Pacific whiting were 

 caught off the coasts of British Columbia, Wash- 

 ington, and Oregon (Fig. 4). After 1972, catches 

 increased off the California coast, and especially 

 high catches of around 100,000 t occurred from 

 1974 to 1976. This southward shift of fishing is 

 believed to be due to a depletion of large adults in 

 the Pacific Northwest. Fishing off central Cali- 

 fornia targeted juvenile whiting. 5 After the 

 FCMA restriction on fishing south of lat. 39°N, 

 the total whiting catch dropped significantly 

 (Fig. 4). 



Whiting prevalence in the diet of Farallon sea 

 lions was directly correlated to the average 

 monthly number of trawlers fishing for whiting 

 in the Farallon area (Table 2; r = 0.747, t = 3.55, 



5 Anonymous. 1976. Summary of National Marine Fish- 

 eries Service views on the status of the Pacific hake resource. 

 Unpubl. rep., 4 p. Northwest and Alaska Fisheries Center. 

 NMFS. NOAA, 2725 Montlake Blvd. E., Seattle. WA 98115. 



255 



