SWARTZMAN and HAAR: INTERACTIONS BETWEEN FUR SEALS AND FISHERIES 



entire population would leave the nonrookery 

 population with a higher proportion of imma- 

 tures which would then affect the samples taken 

 at sea. Another difficulty with these data is that 

 only 2 yr of pre- 1956 age class data were avail- 

 able from the pelagic cruises, and the other pre- 

 1956 data reported by Kajimura et al. (footnote 3) 

 may not have used the same index of maturity 

 as Kajimura et al. (footnote 3). Other possible 

 sources of bias in the age at maturity estimate 

 were the tendency of the pelagic fur seal samples 

 to contain a higher number of older individuals 

 than expected, and the underlying assumption 

 that survival rates of pregnant and nonpregnant 

 females are the same (Kajimura et al. footnote 

 3). 



Growth With Age 



Preliminary analysis by the National Marine 

 Mammal Laboratory (NMML) (Fowler footnote 

 10) of the data from 3-yr-old males harvested on 

 the Pribilof Islands showed a statistically signifi- 

 cant increase in weight over time from 1964 to 

 1970 in contrast to growth rate reductions to be 

 expected under a reduced fur seal carrying ca- 

 pacity. 



Kajimura et al. (footnote 2) plotted the average 

 length of pregnant females against age for the 

 time periods 1958-62, 1963-68, and 1969-74. Their 

 results (Fig. 4) indicate that growth rates were 

 greater from 1963 to 1974 than from 1958 to 



130 



125 



120 



1963-68 \ ^ ,-<X s \ / 



12 



16-26 



A9e (years) 



1962. These results raise the possibility that the 

 fur seal might actually have experienced an in- 

 crease in carrying capacity since 1963. However, 

 Berdine 15 noted that if fur seal population den- 

 sity and carrying capacity both decline, growth 

 rate could still show an increase. As mentioned 

 earlier, changes in carrying capacity can result 

 from a variety of causes, and until stronger links 

 are established between fur seal populations 

 and their controlling processes, arguments that 

 carrying capacity changes are reflected by cer- 

 tain changes in population parameters will be 

 incomplete. 



Pup Deaths on the Rookery 



Counts of dead pups on the rookeries of the 

 Pribilof Islands are an indication of the survival 

 rate of pups, when these are used in conjunction 

 with total pup birth estimates. Gentry (footnote 

 4) estimated that dead pup counts include around 

 95% of the actual dead pups on the islands. From 

 1970 to 1979 pup death estimates on the Pribilof 

 Islands varied between 4,500 and 54,000, aver- 

 aging about 25,000— about 7% of the average pup 

 population (Lander 1981). Earlier pup count 

 data indicated extremely high pup mortality in 

 1954, 1956, 1960, and 1961; the last three years 

 were also the years when mature females were 

 harvested— this may account for the high pup 

 mortalities. 



Several facts about the dead pup counts are: 1) 

 Large pup losses appeared more frequently be- 

 fore 1956 than after, although this bears further 

 corroboration; 2) the year-to-year variability in 

 pup mortality was large; and 3) pup mortality on 

 St. Paul Island did not appear to be correlated 

 with that on St. George Island, while temporal 

 patterns of pup mortality from one rookery to the 

 next on either island were more closely corre- 

 lated with each other. The last fact seems to 

 argue against food limitation as the controlling 

 factor for pup survival through the rookery peri- 

 od and suggests, instead, some more local effects 

 on the populations. 



Average Time at Sea for Mother Seals 



Bartholomew and Hoel (1953) recorded time at 

 sea and nursing for 12 nursing fur seals in 1952 



Figure 4. — Comparison of average lengths of pregnant fur 

 seal females age 5 to 16-26 for combined months of January to 

 April 1958-62, 1963-68, and 1969-74. Sample size >10 seals. 

 From Kajimura et al. (text footnote 2) 



15 J. Berdine, Judson Hall, Room 621, 53 Washington Square 

 South, New York, NY 10012, pers. commun. August 1980. 



127 



