FISHERY BULLETIN: VOL. 81, NO. 1 



land survival of pups. In pinniped populations 

 on other islands— fur seals on the Commander 

 Islands, Robben Island, and St. George Island, 

 and elephant and grey seal populations on other 

 islands— total adult populations are increasing 

 and pup survival is going down. Thus, although 

 pup production is increasing due to increased 

 numbers of adult females, pup survival is re- 

 duced. 



5) There may be reductions in survival and 

 birth rates caused by pollutants and entrapment 

 in fishing gear. 



Among the most serious alternatives (from the 

 standpoint of its implications for man) is that 

 increased fishing intensity in the Bering Sea 

 during the female harvest period has reduced 

 the carrying capacity of the Bering Sea for fur 

 seals. In our discussion of the Bering Sea fishery 

 data we noted that the most probable link, if any, 

 is in depletion by the fishery of pollock and her- 

 ring in the feeding area of nursing females. Dem- 

 onstration or corroboration of this hypothesis 

 directly requires showing that pollock and her- 

 ring stocks have been reduced in rookery fur seal 

 feeding areas and that this has resulted in re- 

 ductions of these foods in female fur seal diets, 

 and reductions in lactating fur seal feeding rates 

 and consequently in pup growth and survival. 

 Present data available on fur seals, while sub- 

 stantial, is not sufficient to attempt so conclusive 

 a test of this hypothesis. For example, fish sur- 

 veys were not made in conjunction with pelagic 

 fur seal surveys, so we do not know how selective 

 fur seals are in their feeding or how dependent 

 their feeding rates are on prey density and rela- 

 tive prey abundance. Also, we do not have direct 

 estimates of the abundance of noncommercial 

 species such as capelin and squid, which com- 

 prise large portions of the seal's diet and may be 

 abundant in the absence of pollock and herring. 



We suggest, and others have suggested before 

 (Fowler 1980; Eberhardt and Siniff 1977), that 

 there is a need to examine the changes in a num- 

 ber of behavioral and physiological indices of fur 

 seal populations which might have presaged or 

 reflected reduced carrying capacity. Measures 

 that we considered are 1) the age at which fe- 

 males attain sexual maturity, 2) the weight at 

 age for harvested preadult males, 3) the number 

 of pup deaths on land compared with total pup 

 births, 4) the average time spent at sea by lactat- 

 ing females (or some composite index of the time 

 at sea plus the time suckling pups), 5) the sur- 



vival rate of pups to age 3 computed from harvest 

 of 3-yr-old males and pup counts 3 yr earlier, and 

 6) changes in diet composition after the develop- 

 ment of the pollock fishery. We also used esti- 

 mates of fur seal abundance, fish stock, and daily 

 food intake to see how great an impact the fur 

 seals actually made on this stock and whether 

 estimated fishery reductions in the stock were 

 sufficient to impact the fur seals. 



Fur Seal Population Indices 



Age at Sexual Maturity 



Kajimura et al. (footnote 3) used a method 

 modified from Lett and Benjaminsen (1977) to 

 compute an average age of maturity for year 

 classes from 1954 to 1964 (from the 1958-74 

 pelagic cruises). These are graphed in Figure 3 

 (from Kajimura et al. footnote 3). The average 

 age at maturity increased sharply for the 1956 

 year class, the first year the females were har- 

 vested. Age at maturity subsequently dropped 

 and remained stable, though at a higher average 

 age than before 1956. The graph in Figure 3, as 

 well as the results of other studies done before 

 1956 on age at maturity, suggests that post-1956 

 age at maturity was greater than pre- 1956 aver- 

 ages. 



There are a number of alternative explana- 

 tions for the apparent increase in age of maturity 

 in addition to the carrying capacity of fur seals 

 being reduced. First, the increase may also have 

 been due to the female harvest on the Pribilofs, 

 selecting a higher fraction of mature females at a 

 given age than actually existed in the population. 

 Since the Pribilofs are a rookery, the presence of 

 mature females in higher proportion than in the 



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 Year class 



Figure 3.— Estimated average age at first reproduction of fe- 

 male northern fur seals based on females pregnant at least once 

 for the 1954-64 year classes. From Kajimura et al. (text foot- 

 note 3). 



126 



