Winter ichthyoplankton biomass as a predictor of early summer prey fields and survival of juvenile salmon in the northern California Current

Abstract

Diets of juvenile coho Oncorhynchus kisutch and Chinook O. tshawytscha salmon are made up primarily of winter-spawning fish taxa in the late-larval and early juvenile stages that are undersampled in plankton and larger trawl nets. Although we have no direct measure of the availability of fish prey important to juvenile salmon during early marine residence, we do have data on the larval stage of their prey that may be used as a surrogate for the later stages. Data on these prey items were obtained from ichthyoplankton samples collected along the Newport Ore- gon Hydrographic line (44.65° N) during January-March in 1998-2010. We explored winter bio- mass of prey fish larvae as a potential indicator of marine feeding conditions for young salmon the following spring. The proportion of total winter ichthyoplankton biomass considered to be com- mon salmon fish prey fluctuated from 13.9% in 2006 to 95.0% in 2000. The relationship between biomass of fish larvae in winter and subsequent coho salmon survival was highly significant (r 2 = 50.0, p = 0.004). When the 2 outlier years of 1998 (El Nino) and 1999 (La Nina) were removed, this relationship was also highly significant for spring Chinook (r 2 = 70.7, p = 0.0002) and significant for fall Chinook salmon (r 2 = 34.0, p = 0.03) returns. Winter larval fish composition showed a high degree of overlap with juvenile salmon diets during May, but less overlap in June. Larval fishes appeared to be an early and cost-effective indicator of ocean ecosystem conditions and future juvenile salmon survival.