Steller sea lion research> Other Hypotheses >Over Fishing

Over Fishing and the decline of Steller sea lions

The growth in the commercial fisheries in Alaskan and Aleutian waters is an obvious possible suspect in the mysterious decline of Steller sea lions. A number of researchers have tried to compare the annual amount of fish caught to the numbers of sea lions counted. None have found a convincing relationship between fishing and sea lion declines. A study by Trites, Larkin and Money (UBC) compared the number of fish caught from 1950 to 1990 to the number of sea lions counted in six regions of the Gulf of Alaska. They found some intriguing patterns. In Southeast Alaska where sea lions have been increasing, there is no apparent relationship between sea lion numbers and commercial catches. However, from the Alaska peninsula through the Aleutian Islands, catches of crabs and shrimp are positively correlated with sea lion abundance, while catches of halibut and gadids (pollock and cod) are negatively correlated. This means that sea lions tended to go up and down in parallel with catches of crabs and shrimp, while sea lions declined as catches of halibut and gadids increased. This same pattern is generally reflected in numbers of vessels fishing (which tend to be proportional to the amount of fish caught).

There are two ways the catch statistics might be interpreted. One is that they reflect a depletion of prey available to Steller sea lions. The other is that they are an index of the relative abundance of prey available to Stellers. High catches of pollock, for example, might mean that pollock is extremely abundant or it could be interpreted to mean that Steller sea lions are being out-competed. The key to sorting out these interpretations is to understand what sea lions eat, and what proportion of the various stocks are caught each year.

There is evidence that major changes have occurred over the past two decades in the relative and total abundance of various species of fish in the Gulf of Alaska and the Bering Sea. These changes may reflect long term periodic oceanographic changes and perhaps long term climatic change. Species in relatively high abundance during the 1950s included shrimp, crab and small pelagic fishes. Those in highest abundance in the 1980s included gadids (mostly pollock) and large flatfish (mostly halibut and arrowtooth flounders). Both gadids and flatfish share considerable dietary overlap with marine mammals and may be significant competitors. Furthermore, pollock is the dominant prey being consumed in the areas of sharpest population decline, and are generally poor in energy or nutritional content. They have about half the energy content of herring, and have less usable energy due to various costs of digestion.

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Swimming Related papers:

Rosen D. A. S., and A. W. Trites. 2002. Cost of transport in Steller sea lions, Eumetopias jubatus. Marine Mammal Science 18:513-524.

Stelle L. L., R. W. Blake, and A. W. Trites. 2000. Hydrodynamic drag in Steller sea lions (Eumetopias jubatus). Journal of Experimental Biology 203:1915-1923.