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Changes In Ocean Survival of Coho and Chinook Salmon in the Pacific Northwest by Ray Hilborn and Claribel Coronado, Fisheries Research Institute, University of Washington Idaho Farm Bureau News, May/June 1999

Abundance of salmon has fluctuated greatly. While we tended to concentrate on the general decline of Chinook and Coho salmon from California to British Columbia, we should not lose sight of the fact that even in the 1970s there was a general perception that salmon stocks were declining, and in Canada the Salmonid Enhancement Program was begun with the objective of doubling the number of salmon.

We want to understand what causes the changes in abundance and what the impacts of alternative human actions will be. The traditional explanations for changes in salmon abundance have been the 4-H's: habitat, harvest, hatcheries, and hydropower. We might therefore compare the trends in catch, escapement, total run, etc., to human action in one of these factors, or perhaps several of them.

It has long been recognized that there is variability in ocean survival of salmon, and most analyses of human impacts on salmon will treat ocean survival as a form of uncontrolled noise that confounds the analysis. However, the increasing recognition of large-scale changes in ocean conditions suggests that we might need to do more than simply allow for random ocean survival, but rather try to measure it directly.

Since the early 1970s, there has been an extensive program of tagging Pacific salmon using coded wire tags. At present, approximately 30 million fish are tagged every year, and the salmon management agencies on the Pacific coast conduct an extensive tag recovery program. These data can be used to estimate the survival of tagged fish in the ocean. Coronado (1995) and Coronado and Hilborn (in prep. (a,b)) have used these data to describe the changes in ocean survival.

This analysis suggests that much of the fluctuation in abundance of Chinook and Coho salmon can be explained by changes in ocean survival. This does not mean, however, that we should ignore human impacts and simply hope for better ocean conditions. The major purpose of looking at ocean survival is to eliminate this form of variability from analysis when we consider modification of habitat, harvest, hatcheries, and hydropower.

Thus, while we cannot control the ocean, we must change our management actions as ocean conditions change. We suggest that under present ocean conditions there is little if any sustainable yield for Chinook and Coho salmon in Oregon, Washington, and British Columbia. Harvest rates need to be drastically reduced, and we should expect the escapements to drop. We might choose to maintain escapements at the old levels, but we should recognize that the reason to do this would be to try to retain a large population size until ocean conditions improve.