Temporal and Spatial Distribution of Juvenile Spring Chinook Salmon off Washington and Oregon

Yearling spring Chinook salmon (Oncorhynchus tshawytscha) along with environmental factors that might control their distribution were sampled along the Washington and Oregon coast every June 1998-2008.  Using the latitude of Columbia River Mouth as a border, the survey area was divided into the north region and south region.  The catch of yearling Chinook salmon varied over space with a high proportion of zeroes.  Chinook salmon showed a high preference for the northern region.  Salmon density and environmental factors exhibited non-linear relationships.  A constrained zero-inflated generalized additive model (COZIGAM) was fitted to the survey data with the result that water temperature, chlorophyll a concentration, zooplankton biomass and spatial factors were significantly correlated with the density of salmon.  To investigate the effects of ocean currents on salmon distributions, the ocean current vectors were decomposed into zonal and meridional components.  When there was more water movement from south as indicated by the cumulative alongshore transport from January-June, yearling Chinook abundance showed a greater difference between the north region and south region.  However, the effect of zonal transport was not significant.  Results suggest that salmon distribution is determined not only by standard habitat parameters (water properties and plankton biomass) but by the strength of coastal currents.  Further study on large scale ocean conditions will be necessary to develop appropriate models for salmon stock assessment and management.