Table 5.--Correlation coefficients between northern sea lion numbers and 0-5 year lags of pollock catch. Numbers in ! represent sea lioncounts.

Lag year Island 2

Marmot ! -. 121 . 044 . 490 .894 -.804 . 876 Ugamak! . 394 . 817 .788 .827 .062 .886 Seguam! -.937 -. 825 .666 .583 -.601 Bogoslof! .436 .345 .326 -.504 .159 .948 Walrus! . 585 .239 -.162 .232 . 948 .138 Chirikof! 1. 000 .328 .461 .944 -.264 .855 Chowiet! .887 .401 .536 .952 .210 .706

' Significant at p<0.05.

Z -0.716; for D+1 r = -0.150, and for the direct comparison g = 0.244 * = significant at p<0. 05! .

The catch of pollock caught within the Seguam study area increased by an order of magnitude from 1976-1979 to 1986 Table 6!. The level of take within the Walrus Island and Ugamak Island study areas remained somewhat constant, but at high levels, and the take in the Bogoslof area was consistently low but with a marked increase in 1986. Finally, catches within the INPFC 2 area which includes the Chirikof area and Shelikof area! increased five fold from the late 1970's to 1985, but are now zero. Our analysis of MT/hour showed no apparent relationship to the amount of fish caught over time to sea lion abundance by area. The only exception was at Shelikof Strait where MT/hour and sea lion numbers both declined. The average fish weight of individual pollock caught in the JV fishery from 1980-1986 increased in each study area Table 7!. For the Chirikof and Shelikof study areas average fish weight has increased steadily since 1980 and at Seguam since 1982. For the study areas average fish. weight was more variable but still indicated an increasing trend. There were significant trends at four of the six study areas for increasing average fish weight. Nonsignificant r values occurred at Ugamak F, = 0.655!, Bogoslof g = 0.831, and Walrus 0.729 Table 7!. Sample sizes were insufficient to compare pollock size changes with sea lion numbers.

690 Table 6.--Catch of walleye pollock metric tons! by year and study area for independent and JV fish- eries in the eastern Bering Sea, 1976-86. For Forrester Island the annual catch of Pacific Ocean perch is also shown !.

Bering Bogoslof Walrus Ugamak Forrester Year Sea I Island Island Island Island

1976 241 28,507 191, 618 1977 133 19,654 107,159 1978 365,715 159 22,224 95,891 990 160! 1979 367,636 250 15,904 97,927 555 217! 1980 437,253 262 30,549 114,181 434 912! 1981 714,585 499 20,660 149,717 1001 297! 1982 713,912 710 14,440 142,980 ! 1983 687,504 223 36,549 150,119 0 ! 1984 442,733 834 24,360 133,358 0 ! 1985 604,465 288 23,835 166,241 0 ! 1986 594,997 19, 863 124,114 71,297 0 !

Table 6 cont.! .--Catch of walleye pollock metric tons! by year and study area for independent and JV fish- eries in other areas, 1976-86.

Bering Seguam INPFC Chirikof Shelikof Year Sea IV Island !62 Area Area

197 6 82 6, 587 5, 895 0 1977 1, 718 27,743 8,238 0 1978 6/ 283 1, 257 43,801 12,769 108 1979 9,447 1, 634 29,184 21,050 0 19SO 58,127 15,853 35,853 20,S09 504 1981 55,516 11,183 81,930 14,922 15,158 1982 57,754 18,613 114,918 2,886 69,864 1983 59/022 5,067 157,912 27,553 98,984 1984 77,595 25,322 214,315 37,641 145,3&3 1985 58,147 12,962 211,381 5,752 204,621 1986 45,439 17,639 42,104 0 42,104

691 Table 7.--Meanweight gm! of individual pollock caught in joint-venture fisheries 1980-1986and year by study area, and correlation g! of changesin mass with time.

SeguamBogoslof Walrus Ugamak Chirikof Shelikof Year Island Island Island Island area area

540 328 499 1980 437 426 1981 473 395 503 456 451 1982 643 511 765 581 566 641 498 1983 576 594 1984 797 658 509 697 489 512 703 701 1985 817 617 1986 863 670 656 764 0.938* 0.831 0.729 0.655 0.970* 0.827+

* = significant at p<0.05 ' Abnormal CPUE

Our analyses showedhigh positive and high negative correlations betweensea lion numbersand the commercial harvest of walleye pollock. In three instances where a low direct correlation occurred Marmot,Walrus, and UgamakIslands!, a high lagged-correlationexisted. The only sites with consistently low correlations and Forrester Island! were the only sites with little commercialexploitation during the study period. The results of this study, however,were inconclusive and neither totally supportednor rejected the hypothesis that commercial catch of walleye pollock influences sea lion abundance. The calculated r values listed in Table 5 were not statistically significant when grouped togetherand could be attained by randomvariability. Nonetheless,many of the values whentested independently werehigh enoughfor statistical significance suggesting that a cause and effect relationship betweencommercial fish catch and sea lion abundance may exist. In some cases the calculated g value changes from a negative value with the direct comparisonto a positive value with the time lag e.g., Ugamakand !, two explanations are possible. The first explanationis that it is an artifact of the waywe have assignedfishery catch data to each year of sea lion count. If we had, for example, addedthe catch statis- tics for a fishery occurring before the sea lion counts e.g., Augustto December1979! to the fishery data for

692 the year of the sea lion count e.g., January to July 1980!, then the value for Z reverses. The second explanation is related to the time frame of our analysis. In the eastern , the largest decline in sea lion abundance occurred prior to 1976 the beginning of our analysis period! t pollock catch was also declining in the mid 1970's from the earlier peaks. Both sea lion abundance and pollock catch were declining at the same time - and when we consider the pre-1976 period in the lag analysis, we find a positive correlation. when we consider 1976 to the present, there is an initial decline in northern sea lion numbers followed by level numbers. Pollock catch, however, is slowly increasing through most of the period resulting in a negative correlation. Part of the difficulty in analyzing fishery data is the geographical differences in the utilization or recruit- ment of pollock spawning "groups" in the study area, of which four have been postulated. The four groups include the eastern Bering Sea group and Aleutian Basin group in the Bering Sea and the Shelikof Strait group and Shumagin group in the Gulf of Dawson, 1989; Mulligan et al., 1989!. Each of our study areas can be grouped into these pollock spawning groups: Walrus Island with the eastern Bering Sea groups Seguamand Bogoslof islands with the Aleutian Basin group; Chirikof and Chowiet islands with the Shumagin group; and Marmot Island with the Shelikof group. The Forrester Island rookery is considered separately. The exception is UgamakIsland which behaves statistically as if part of the eastern Bering Sea group but, because of its location, could be included in either the Aleutian Basin group or eastern Bering Sea group. Each of the pollock "groups" is complex and the relationships between recruitment, seasonal movements, effects of oceanographic events, and the amount and effect of exploitation are difficult to examine and may mask the simple type of relationships examined in our study. Irrespective of the possible relationship between pollock catch and sea lion abundance, there are indications that changes in the size and abundance of pollock stocks caused by variable recruitment may have influenced observed trends in sea lion numbers. The observed increase in the average weight of walleye pollock caught in JV fisheries during the 1980's reflects similar trends in the weight of pollock caught in assessment surveys in the Bering Sea Bakkala, 1989!. One explanation for this trend is that during the 1970's, when commercial fishing pressure was at its peak, most of the large fish were removed by the fishery which resulted in 2-4 year old fish dominating the fishery through the 1980's. However, the 1978 year class showed very high recruitment and, coupled with lower fishing pressure in the 1980's, dominated the fishery over the next seven years. This resulted in a steady increase in overall size of fish

693 caught as muchas three times larger than fish eaten by sea lions! in the fishery and assessment surveys in the mid-1980's Bakkala, 1989; Table 7!. Lowry et al. 989! demonstrate that northern sea lions generally preY on large pollock and they argue that the catch of large pollock during the 1970's in the Bering Sea may have reduced the availability of preferred sea lion prey resulting in reducedsea lion abundanceby causingnutri- tional stress. Presumably, as the 1978 year class began to dominate the population and more large fish were available, the level of nutritional stress for sea lions should have ameliorated, providing the fish were season- ally present in sufficient numbers to be eaten. calkins and Goodwin988! found that the size of pollack eaten by sea lions near Kodiak Island our Shelikof study area! had shorter fork lengths during the mid-1980's x 25.4 cm! than during the mid-1970's x = 29.8 cm!- Also, consumptionof pollock by sea lions in their study area increased in percent frequency of occurrence up 20%!as did the estimated numberconsumed up 378!. They argued that the increased take of smaller pollock resulted in increased energy costs for the pursuit ancX captureof moreand smaller prey. Theapparent disparity between the increasing size of pollock caught in the fishery, as discussed above, and decreasing size eaten by sea lions maybe explained by differential selection Commercialpollock fishermen in the Shelikof area catch larger, older fish than are eaten by sea lions Table 7!- Northern sea lions, however, select from the pollock population over a broader area, which presently is' dominated by smaller fish than was the case during the mid-1970's. Interestingly, during the later years 985- 86!, more large pollock were found on the Pacific Ocean side of Kodiak Island and smaller fish in Shelikof Strai< G. Stauffer, NOAA,NMFS, RACE Division, pere. commun-! Adding to the notion that more small pollock dominate the prey resource available to sea lions in the Kodiak Islan«X area is the general declining trend in the status of pollock in Shelikof Strait. Annual biomass of pollock@ in Shelikof Strait has declined from a 20-year high of about 3.77 million tons in 1981 to 0.33 million tons in 1988 Nunnallee and Williamson, 1988!. The average length of 3-year-old pollock in 1983 was about 35 cm; in 19BS they were less than 30 cm. Likewise, average weight of 3-year-old Pollock in 1983 was about 300 gm while in 198S it was less than 200 gm Nunnallee and Williamson, 1988! . One explan-ation for the reduced mass was that during Me 1983 survey, nearly 758 of the 3-year-old and 80% of tlute 4-year-old female pollock were mature, while 0% and 21% of the 3 and 4-year-old, respectively, female pollock were mature in the 1988 survey Nunnallee and Williamson, 1988!. Thus, pollock available to sea lions in t4e Kodiak area are less numerous, smaller in length, weigh less, and when consumed during the pollock spawning

694 period, have less energy content fewer fish with high energy eggs!. Concomitant to the decline of the pollock stock in Shelikof Strait is the observation that sea lions in the area are currently smaller in mass and length, particularly animals 1-5 years of age, then they were in the mid-1970's Calkins and Goodwin, 1988!. As discussed by Calkins and Goodwin 988!, the observed reduction in mass and growth may be a result of reduced nutritional condition at an early age. Similar results have been reported in numerous large mammals, both aquatic and terrestrial Fowler, 1987!. Coincidentally, the decline in abundance in northern sea lions at Marmot Island and nearby areas began in the early to mid-1980's Merrick et al., 1987!, soon after the declining trends in pollock in Shelikof Strait. Marmot Island's sea lion decline may typify the mechan- isms by which the commercial fishery and the declines in sea lion numbers are linked and is depicted in a flow diagram Fig. 3!. Many adult female both pregnant and postpartum! and yearling sea lions stay relatively close to their natal rookery site throughout the year resulting in a need for an abundant food supply near the rookery. At Marmot Island, the concentrations of pollock in Shelikof Strait during January through April are impor- tant because they occur at a time whenpregnant sea lions have increased nutritional demands from their growing fetus coupled with the contin-uing energetic demandof nursing their pup from the previous year. In addition, manyyoung sea lions first begin to forage for themselves in April, May, and June and the reduced availability or quality of prey maycompromise survival. Reductionsin prey availability or quality, though probably not a direct source of mortality for adult females, could compromise the health of females resulting in some abortions as is presumed in the El Nino example! or extend the duration of a female's foraging trips result- ing in fewer nursing bouts which could retard the pup's growth. The later case could explain the observed decrease in young sea lion growth rates in the Kodiak area Calkins and Goodwin, 1988!. Perhaps the most serious potential effect on the popula- tion is on yearlings when they begin to forage. Reduced availability of pollock either through declining numbers in general or of a specific prey size! exacerbated by competition with adult sea lions for the sameprey would make foraging difficult for inexperienced sea lions. If yearlings switched to other fish e.g., rockfish!, any increase in the energetic costs of foraging could effect their health resulting in nutritional stress. Many could be compromisedphysiologically, becoming more vulnerable to other factors e.g., disease and storms! which under normal circumstances does not occur Loughlin, 1987!. Some may have their growth retarded, while others may die. This could explain the decreased growth rates, and

695 Figure 3.--Flow diagram depicting the relationship between a declining prey base and its possible effect on northern sea lions in Alaska.

696 the lack of yearlings observed on sea lion rookeries in recent years Merrick et al., 1988! NMMLfiles!. Such an effect on yearlings would become most obvious in surveys three to four years after the factor first effected them. In our study, comparisons of sea lion numbers with pollock catches lagged by three to four years had the strongest correl-ations at Marmot Island. This supports the notion that yearlings were affected by changes in prey stocks. That other sites show high correlations with a three to four year lag indicates similar mechanisms may also be operating there. The overall effect to sea lions of reductions in pollock available as prey, a result of poor recruitment or through removal by fisheries, probably retards growth, lowers reproductive potential, or reduces survival.

Staff of the Northwest and Alaska Fisheries Center, Resource Ecology and Fisheries ManagementDivision J. Burger, S. Murai, and H. Weikart! provided most of the fishery data. The manuscript was improved by comments from G. Antonelis, W. Aron, J. Balsinger, H. Braham, D. Rice, G. Stauffer, G. Thompson, and A. York.

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699 Pitcher, K. W. 1981. Prey of the Steller sea lion, XQR~t ~ IB~, in the Gulf of Alaska. Fish. Bull., U.S. 79:467-472. Pitcher, K. W., and D. G. Calkins. 1981. Reproductive biology of Steller sea lions in the Gulf of Alaska. J. Mamm., 62:599-605. Rae, B. B. 1960. Seals and Scottish fisheries. Marine Res. Dep. of Agriculture and Fisheries Scotland! . 2:1-39. Shaughnessy, P. D., and I. L. Payne. 1979. Incidental mortality of Cape fur seals during trawl fishing activities in South African waters. Fish Bull., South Africa!. 12:20-25. Stirling, I., M. Kingsley, and W. Calvert. 1982. The distribution and abundance of seals in the eastern Beaufort Sea, 1974-79. Canadian Wildlife Service, Occassional Paper No. 47. 25 pp. Strombom, D. B. 1981. Marine mammal-fishery inter- actions in the northeast Pacific. M.S. thesis, Univ. Washington, Seattle, WA. 189 p. Swartsman, G. L., and R. T. Haar. 1985. Interactions between fur seal populations and fisheries in the Bering Sea. ~ J. R. Beddington, R. J. H. Beverton, and D. M. Lavigne editors!, Marine Mammals and Fisheries, p. 62-93. George Allen 6 Unwin, London.

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