INTERACTIONS BETWEEN FUR SEAL POPULATIONS AND FISHERIES 



IN THE BERING SEA 



Gordon L. Swartzman and Robert T. Haar 1 



ABSTRACT 



In this paper we consider fur seal-fisheries interaction in the Bering Sea by asking whether the 

 slower than originally predicted recovery of the fur seal stock from female fur seal harvest during 

 1956-68 might be a result of a reduction in carrying capacity because of the large fishery harvest 

 of walleye pollock and Pacific herring— fish which are important fur seal prey. 



The changes we found occurring in the fur seal population did not support the hypothesis that fur 

 seal carrying capacity was reduced by the fisheries. In fact the population parameters changed 

 little, or changed in a direction opposite to that proposed by the hypothesis. 



Study of the fur seal diet data indicated that walleye pollock comprised a larger part of the fur seal 

 diet in the 1970s, after the establishment of the fishery, than earlier, although average pollock size 

 appeared to drop significantly. This trend may have been induced by an increased harvest of older 

 fish. Since walleye pollock are cannibalistic, the removal of the older fish by the fishery could result 

 in lower mortality among the younger pollock stocks, the outcome being an increase in the pollock 

 resource available to both the fishery and the fur seal. 



In this paper we assess and clarify possible rela- 

 tionships between fur seals and fisheries in the 

 Bering Sea. The event most prominent in focus- 

 ing concern on fur seal-fisheries interactions 

 was the failure of the Pribilof Islands' fur seal 

 herd to recover as predicted from large female 

 harvests during 1956-68. While the present herd 

 appears to have stabilized, it has stabilized at a 

 population 30% below the maximum sustained 

 productivity estimates made in 1955 (York and 

 Hartley 1981). A number of possible explana- 

 tions for this have been presented, including re- 

 duced fur seal carrying capacity. 



In this paper we 1) briefly summarize and 

 highlight the available fur seal and fish data, in- 

 cluding studies of cases of other known marine 

 mammal-fish interactions, 2) consider the evi- 

 dence about fur seal population dynamics and 

 seal-fish interactions, and 3) suggest analyses of 

 existing data and further field sampling needed 

 to clarify the effect of the Bering Sea fishery on 

 fur seal populations. 



AVAILABLE DATA 



The relevant data may be divided into fur seal 

 data, Bering Sea fish stock and fishery data, and 

 anecdotal marine mammal-fish interaction data. 



'Center for Quantitative Science, University of Washington, 

 Seattle, WA 98195. 



The fur seal data consist of 1) annual fur seal col- 

 lections at sea during 1958-74 in the eastern 

 North Pacific Ocean and the eastern Bering Sea 

 conducted jointly by the United States and Can- 

 ada under terms of the Fur Seal Interim Conven- 

 tion (Kajimura et al. 1979, 2 1980 3 ); 2) harvests 

 from 1950 to 1978 on the Pribilof Islands of sub- 

 adult males (Lander 1981) and counts of harem 

 and nonharem bulls from 1905 to 1978 on other 

 island rookeries; 3) estimates of pup production 

 on the Pribilof Islands from 1912 to 1924 and 

 from 1951 to 1979 (Johnson 1975; Lander 1981), 

 and counts of dead pups from 1950 to 1979 (Lan- 

 der 1981); and 4) studies of fur seal rookery be- 

 havior (Bartholomew and Hoel 1953; Gentry 4 ), 

 food habits (Spalding 1964; May 1937; Wilke and 



194-1**- 



Manuscript accepted July 1982. 



FISHERY BULLETIN: VOL. 81. NO. 1. 1983. 



2 Kajimura, H.. R. H. Lander, M. A. Perez. A. E. York, and 

 M. A. Bigg. 1979. Preliminary analysis of pelagic fur seal 

 data collected by the United States and Canada during 1958- 

 74. Report submitted to the 22d Annual Meetingof the Stand- 

 ing Scientific Committee, North Pacific Fur Seal Commission, 

 247 p. Northwest and Alaska Fisheries Center National Ma- 

 rine Mammal Laboratory, National Marine Fisheries Service, 

 NOAA, 7600 Sand Point Way NE., Seattle, WA 98115. 



3 Kajimura, H., R H. Lander, M. A. Perez, A. E. York, and 

 M. A. Bigg. 1980. Further analysis of pelagic fur seal data 

 collected by the United States and Canada during 1958-74. 

 Part 1. Submitted to the 23d Annual Meetingof the Standing 

 Scientific Committee, North Pacific Fur Seal Commission, 94 

 p. Northwest and Alaska Fisheries Center National Marine 

 Mammal Laboratory, National Marine Fisheries Service, 

 NOAA, 7600 Sand Point Way NE., Seattle, WA 98115. 



4 R. Gentry, Northwest and Alaska Fisheries Center National 

 Marine Mammal Laboratory, National Marine Fisheries Ser- 

 vice. NOAA, 7600 Sand Point Way NE., Seattle. WA 98115, 

 pers. commun. May 1980. 



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