Steller Sea Lion Research: Shooting and Entanglement: Marine Mammal Research Consortium

Steller sea lion research > Other Hypotheses > Shooting and Entanglement

Shooting and Entanglement and the decline of Steller sea lions

Andrew Trites and Peter Larkin(UBC) worked on a mathematical model to evaluate the role that pup harvests, native kills, and incidental and intentional killing by commercial fisheries may have played in the population decline in Alaska. Their model estimate how many sea lions would have been alive since 1956 if no unusual deaths occurred, and how many additional sea lions would have had to die to cause the pattern of population change that was observed in the wild.

Historic harvesting of 45,000 sea lion pups between 1963 and 1972 was substantial

The model indicated that the historic harvesting of 45,000 sea lion pups between 1963 and 1972 was substantial and contributed to stabilizing the numbers of sea lions breeding in Alaska through the 1960s and 1970s. The number of sea lions 'missing' from the model were consistent with estimates of intentional and incidental kills during this time period. However these sources of mortality cannot account for the dramatic population declines through the 1980s. From 1980 to 1990, the model suggests the number of Steller sea lions disappearing each year rose from 5% to 24% of the population. Whether these losses were caused by the removal of prey by fisheries is a circumstantial possibility, but comparisons of catch data and sea lion numbers have not shown a direct linkage.

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Related Publication:

	The status of Steller sea lion populations and the development of fisheries in the Gulf of Alaska and Aleutian Islands.
		Trites, A.W. and P.A. Larkin. 1992.
		In A report of the Pacific States Marine Fisheries Commission pursuant to National Oceanic and Atmospheric award no. NA17FD0177. University of British Columbia, Fisheries Centre, 2204 Main Mall, Vancouver, B.C. V6T 1Z4. pp. 134
		abstract The goal of our study was to assess the status of Steller sea lions in Alaska, review their population biology, and develop a simulation model to explore the role that harvesting and incidental kills by fisheries may have played in the sea lion decline. We also attempted to relate the population declines to the amount of fish caught in the Gulf of Alaska and Aleutian Islands, and to the number of vessels fishing from 1950 to 1990. Using life tables to estimate population size, the numbers of Steller sea lions were estimated for all rookeries for which information was available in each of six areas in the Gulf of Alaska and Aleutian Islands. The total population appears to have risen from 150,000 to 210,000 from the mid 1950s to 1967. The population was then stable for roughly ten years, then increased to 225,000 by 1979. Since then it has decreased to about 85,000. Most of the decline took place in Area 3 (Kodiak region) but there were also significant declines in Areas 4 to 6 (westward of Kodiak). Increases have occurred in the smaller populations of Areas 1 and 2 (southeast Alaska and Prince William Sound). A major growth in domestic fisheries occurred after the declaration of 200 mile zones. The traditional fisheries for salmon, herring and halibut were augmented by major groundfish fisheries. The decline in the numbers of Steller sea lions has been coincidental with the growth in the numbers and size of vessels and the increase in catch. The stabilization in the numbers of Steller sea lions in the Gulf of Alaska from 1956 to 1980 can be attributed to the direct effect of incidental capture in fishing gear, the shooting of sea lions and the harvesting of adults and pups. However, these factors explain but a small portion of the recent population decline, from 1980 to the present. Some sea lions are missing in the arithmetic of population dynamics which cannot be accounted for by movements of animals from one area to another. Whether these losses are caused by the removal of food resources is a circumstantial possibility, but evidence of local abundance of food resources at particular times of the year for particular segments of the population is needed to build a convincing case. Other causes, such as diseases and parasites must also be kept in mind as possible contributing factors. Research on Steller sea lions should focus on the decline in abundance since 1980, changes in body size, the diet at various seasons of the year, bioenergetics and nutritional requirements, and assessment of local abundance of various food items. Long term research on the ecosystem dynamics of the region will be necessary for long term management of all living resources but how best to focus that research is a matter of current scientific debate that will not be resolved quickly.