Patterns of co-variability among California Current chinook salmon, coho salmon, Dungeness crab, and physical oceanographic conditions

Abstract

One of the primary motivations for the GLOBEC NEP program was the apparent inverse relationship between the increase in salmon populations in the Gulf of Alaska since the mid-1970s and concurrent declines in salmon populations in the California Current. The increase in abundance of some salmon species in the Gulf of Alaska can be plausibly explained based on mechanisms involving changes in physical structure, biological productivity, and salmon survival. To assess concurrent changes in salmon populations in the California Current and their possible physical and biological bases we examined temporal and spatial patterns of co-variability between biological variables and physical descriptors along the coasts of Washington, Oregon and California, from 1950 to 1990. The biological variables were catch records of coho salmon, chinook salmon and an ecologically related species, Dungeness crab. The physical variables were sea surface temperature, sea surface height (SSH) and the upwelling index (UWI). We found that while California Current coho salmon declined uniformly in the mid-1970s, consistent with the proposed inverse relationship, chinook salmon did not. All three species appear to be driven by the dominant mode of co-variability in the three physical variables, an indicator of warm/cool water conditions, but in different ways. In general, warm conditions have a negative effect on salmon at the age of ocean entry and spawning return, and Dungeness crab during the larval stage, while cool conditions have a positive effect. Differences in spatio-temporal variability between the two salmon species suggest they may respond to ocean conditions differently: coho salmon vary synchronously along the coast on annual time scales, while chinook salmon vary on slightly longer time scales in a specific spatial pattern. Dungeness crab vary on 10-year time scales, synchronously along the coast, except for the most southern areas (central California) where populations collapsed in the late 1950s. The dominant, warm/cool mode of physical co-variability, which drives these populations regionally, is related to basin-scale indices; it appeared to follow these indices in the 1950s and 1975–1990, but differs from them1960–1975, in ways that may be biologically important.