Kathryn E. Kostow, Technical Analyst, Fisheries/Stock Assessments, Oregon Department of Fish and Wildlife
Broad-scale decreases in the size and age of Pacific salmon (Oncorhynchus spp.) over time are well documented. Several theories attempt to explain this dynamic, including exploitation by fisheries that selectively remove larger, older-maturing individuals; changes or variation in the ocean environment that affect ocean productivity which in turn may affect salmon growth and maturation rates; variable fish abundance in the North Pacific that influence density-dependence effects on salmon growth and maturation; hatchery programs that promote smaller, younger-maturing fish; and loss of historic production areas that produced larger, older fish. Changes in the size and age of Chinook salmon (O. tshawytscha) caught in lower Columbia River terminal fisheries have not previously been reviewed to determine whether these populations follow the general regional trends observed elsewhere. Although industrial European fisheries have occurred on the Columbia River since the mid 1800s, systematic data on length, weight and age of Chinook salmon are only available since the 1960s. By the 1960s, many events that might influence fish size and age had already occurred. Our analysis indicates that the size and age of Columbia River Chinook salmon caught in river fisheries significantly increased from the 1960s to the 1990s, contrary to trends reported elsewhere, but then declined from the 1990s to the 2000s. By the 2000s, fish that were age 4 and older were smaller at age than ever seen previously, even though older age classes were still more frequent than they were in the 1960s. The declining trend from the 1990s to the 2000s was particularly steep for spring-run Chinook salmon which are currently the smallest and youngest ever recorded. Some of the variation over time is likely due to changes in the structure of the fisheries itself that affect the way it samples the populations. However, biological changes are also indicated that lend some support to theories about the effects on size and age of fishing pressure, hatchery influence, variation in the physical environment and density-dependent growth rates. The dynamics of size and age variation in Columbia River Chinook salmon are complex most likely because multiple influences are having confounding effects.
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