SWARTZMAN and HAAR: INTERACTIONS BETWEEN FUR SEALS AND FISHERIES 



Since there were no stomach content data for 

 fur seals near the Pribilofs from 1968 to 1970, the 

 years of the major herring fishery in the eastern 

 Bering Sea, it was not possible to estimate how 

 much interaction there was between fur seals 

 and the herring fishery. Although herring is 

 sometimes a food of fur seals, it may not be com- 

 mon in stomachs of the nursing female fur seals, 

 because in summer the herring are not common 

 in fur seal feeding areas but mostly remain in 

 coastal waters (Wespestad 1978 12 ). 



Studies on Related Systems 



Marine mammals are integrally tied to their 

 environment. They can respond to reduction in 

 competition by increases in abundance, which 

 implies that many marine mammal populations 

 are existing at or near their carrying capacities. 

 Many marine mammals are opportunistic and 

 voracious predators and can strongly affect 

 trophic dynamics of lower trophic levels (Simen- 

 stad et al. 1978). Marine mammals are also fre- 

 quently in food competition with each other. This 

 is demonstrated 1) by the reduction in age of ma- 

 turity of minke whales in the Antarctic Ocean 

 after drastic reduction through harvest of sei 

 and blue whales (Hofman 13 ), 2) by the increase in 

 ringed seal populations after depletion of bow- 

 head whales in the Beaufort Sea (Lowry 14 ), 3) in 

 the fairly heavy predation of sea lions on fur seal 

 pups (3.5 to 5.5% annually on St. George Island 

 according to Gentry footnote 4), and 4) in the 

 feeding overlap on hake by a large number of 

 marine mammals (Fiscus 1979). 



Work by Fowler (1981) showed that /^-selected 

 (low fecundity) animals demonstrate density de- 

 pendence when near their carrying capacities, 

 and from the above arguments it seems probable 

 that most marine mammals exhibit density de- 

 pendence in at least some of their population or 

 growth parameters. Also, temporary reductions 

 of a marine mammal population might provide 

 an opportunity for a food competitor to reduce 

 the carrying capacity of that marine mammal 

 population. An important question in this case 



12 Wespestad, V. G. 1978. Exploitation, distribution and 

 life history features of Pacific herring in the Bering Sea. Proc. 

 Rep.. 26 p. Northwest and Alaska Fisheries Center, National 

 Marine Fisheries Service, NOAA. 2725 Montlake Blvd. E 

 Seattle. WA 98112. 



13 R. Hofman, Marine Mammal Commission, 1625 Eye Street 

 NW„ Wash., DC 20006, pers. commun. May 1980. 



l4 L. Lowry. Alaska Department of Fish and Game. Fair- 

 banks, AK 99701, pers. commun. May 1981. 



is whether the density-dependent effects experi- 

 enced by a population at or near its carrying 

 capacity are primarily a behavioral or a physio- 

 logical phenomenon. The term "density-depen- 

 dent" generally means that a population variable 

 varies nonlinearly with changing population den- 

 sity. This does not, in itself, imply a direct cause 

 or mechanism for this response. However, it may 

 occur through increased mortality, reduced fe- 

 cundity, reduced weight gains, or changes in ani- 

 mal condition. Each of these population parame- 

 ters may be affected by a variety of density 

 related factors. 



In the case of the fur seal it has been hypothe- 

 sized that reduction in fur seal populations due to 

 female harvest gave the competing fishery an 

 opportunity to increase harvest rates and there- 

 by reduced the fur seal's carrying capacity. If 

 this hypothesis is true, we should see a change in 

 one or several of the population parameters dis- 

 cussed earlier. 



EVIDENCE OF CHANGES IN 

 FUR SEAL CARRYING CAPACITY 



Fur Seal Population Trend 



The fur seal population appears at present to 

 be dropping. After the female harvests from 

 1956 to 1968, an increase in pregnancy rate and 

 survival was expected. This expected response of 

 the population did not materialize, and popula- 

 tion numbers are reduced over model population 

 projections. 



Hypotheses to explain the reduction since 1956 

 (Fowler footnote 10) are: 



1) The discrepancy is mainly due to overesti- 

 mates of pup abundance during the tagging 

 studies (1951-61) and underestimates in the sub- 

 sequent pup shearing studies. 



2) There has been a reduction in carrying ca- 

 pacity because of reduced available food in the 

 Bering Sea, resulting from overfishing of major 

 food sources for the fur seal— pollock and her- 

 ring—in the feeding areas of the rookery seals. 



3) The reduced pup abundance may be a tran- 

 sient effect of the female harvest. This is in spite 

 of the observation that the direct effect of the ani- 

 mals removed has by now largely passed through 

 the population (Lander 1981). 



4) Increased abundance of nonharem adult 

 males and an increase in ratio of these males to 

 harem bulls on St. Paul Island may have reduced 



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