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The Relationship of Snake River Stream-type Chinook Survival Rates to In-river, Ocean and Climate Conditions Howard Schaller and Charlie Petrosky Science Policy Exchange, September 12, 2007

Snake River salmon and steelhead have substantially declined since the completion of the Columbia River hydrosystem. Most survival rate declines were in the smolt-to-adult life stage rather than in the spawner-to-smolt life stage.

A key remaining uncertainty for evaluating recovery options for upper basin salmon populations relates to the source of mortality that fish experience while in the estuary and early ocean. Sources of estuary and early ocean mortality include not only elements of the natural ocean environment, but also effects of earlier life-stage experiences.

Multiple analytical approaches are presented addressing this mortality for Snake River spring/summer Chinook and steelhead. We found that Snake River stream-type Chinook salmon populations continued to exhibit survival patterns similar to those of their downriver counterparts but survived only one-fourth to one-third as well.

The water velocity conditions in the river (water travel time) and ocean/climatic conditions are considered in describing the variation in survival rates. In all results water travel time proved to be a significant factor in explaining the variation in survival. The FCRPS has increased water travel time and delayed migration of in-river fish; with later arriving components of the population exhibiting lower survival rates.

The results of these multiple analyses provide compelling evidence that passage through the FCRPS, along with ocean/climatic conditions, strongly influences levels of mortality of in-river migrants for Snake River populations.

Related Sites:
Index of NW Council's Salmon Policy Exchange Presentations (Habitat, Mainstem, Estuary, Ocean Conditions)