Our l.iviiix Ri'siiiiiTfs — Mammals 



III 



24 



34 44 



54 64 

 Year 



94 



Fig. 1. Trends in abundance of the California sea otter 

 population. 1914-93. 



direction of range expansion was predoininale- 

 ly southward before 1981. but northward there- 

 after. Comparison between spring surveys con- 

 ducted in 1983 and 1993 (Fig. 2) is sufficient to 

 draw several conclusions. First, the population's 

 range limits changed little during this 10-year 

 period. e\en though large numbers of individu- 

 als accumulated near the range peripheries. 

 Second, population density increased through- 

 out this time, although rates of increa.se were 

 lowest near the center of the range. Finally, the 

 relative abundance of individuals has remained 

 largely unchanged (compare Fig. 2a [1983] 

 with Fig. 2b |1993], noting the similarity in 

 forms of distributions for kilometer segments 

 10-21). 



Although the number of dependent pups 

 counted in spring surveys almost doubled 

 between 1983 and 1993, the geographic range 

 within which these pups were bom has changed 

 very little (Fig. 2). Rate of annual pup produc- 

 tion ranged from 0.14 to 0.28, but in most years 

 it varied between 0.18 and 0.21. There are no 

 obvious trends in rate of annual pup production 

 between 1983 and 1993. Although the incre- 

 mental change in the population from one year 

 to the next appeared positively related to the 

 annual number of births, this relationship can- 

 not be shown to be statistically significant. 



Implications 



From the mid-1970"s to the early 1980's. the 

 California sea otter population ceased growing 

 and probably declined. Entanglement mortality 

 in a coastal set-net fishery was the likely cause 

 of this decline (Wendell et al. 1985). 

 Restrictions were imposed on the fishery in 

 1982. and the population apparently responded 

 by resuming its prior rate of increase. 



The maximum rate of increase for sea otter 

 populations is about 20% per year. Except for 

 the California otters, all increasing populations 

 for which data are available have grown at about 



this rate (Estes 1990). These patterns, coupled 

 with the absence of any size- or density-related 

 reduction in growth rates, make the relatively 

 slow rate of increase in the California popula- 

 tion perplexing. 



Although the ultimate reason for disparate 

 growth rates among sea otter populations is 

 unknown, we believe that causes relate more to 

 increased mortality than diminished reproduc- 

 tion. While it is difficult to compare popula- 

 tion-level reproductive rates between sea otters 

 in Alaska and California, longitudinal studies of 



Spring 1983 census 



200 



175 



125 ■ 



100 - 



75 ■ 



50 



25 



3 5 7 9 11 13 15 17 19 21 23 25 27 29 



-- Independents 

 Spring 1 993 census ■ Pups 



II 



,1 



III 



II 



I.. 



5 7 9 11 13 15 17 19 21 23 25 27 

 20-km segment, north (1) to south (29) 



29 



Fig. 2. Distribution and abun- 

 dance of California sea otters in 

 1983 (a) and 1993 (b). Data are 

 from the spring surveys. 



