Coho salmon (Oncorhynchus kisutch) are a valuable resource to both commercial and sports fishermen in the north-east Pacific, from Alaska to northern California. They have a three year life cycle (Figure 1). Unlike other salmon species, they spend little time in estuaries, and once in the ocean are thought to remain fairly close to the coast. Survival rates in the ocean vary dramatically from year to year (Figure 2), resulting in large inter-annual fluctuations in the number of adults returning to freshwater.
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Cold water upwelling activity has been found to be strongly associated with the marine survival of coho salmon along the west coast of the United States, from Washington State to northern California. Higher levels of upwelling activity during the spring and summer of the fishes’ first year in the ocean (i.e. during ocean entry) promote survival, and vice versa. Possible mechanisms involve the influence of upwelling activity on food availability and predator distribution. Conditions later during ocean residency may also have some impact; especially during El Niño events, when increased levels of mortality are associated with reduced food availability.
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Sea Surface Temperature (SST) satellite imagery from 1985 onwards has recently been used at PFEL to investigate the effect of coastal ocean conditions on coho survival for both an Early Marine Phase (EMP) and a Late Marine Phase (LMP). SST anomaly charts were derived from a time series of SST images (Figure 3). The relationship between the sum of negative anomalies during the EMP, and the sum of positive anomalies during the LMP are shown in Figure 4. A model which used both the EMP and LMP anomaly sums was able to account for 92% of the variation in coho ocean survival from 1985 to 1996 (Figure 5). The implication from the results in figures 4 and 5 being that cool conditions when the fish first enter the ocean promote survival, and that only very warm conditions later during the marine phase (i.e. El Niño conditions) are detrimental to survival.
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