Spatial variations in the distribution of yearling spring Chinook salmon off Washington and Oregon using COZIGAM analysis

Yearling Chinook salmon Oncorhynchus tshawytscha were sampled off Washington and Oregon, USA, along with environmental factors, every June from 1998 to 2010. The abundance of yearling Chinook salmon varied over space with a high proportion of zero catches. Positive catches were more numerous north of the Columbia River, likely because most yearling Chinook salmon turn north after leaving the Columbia River. Using the latitude of the Columbia River mouth as a geographical border, the survey area was divided into 2 regions: north and south of the Columbia River. We hypothesized that (1) the spatial distribution pattern within each region was related to local environmental factors and (2) the difference between north and south was related to large-scale ocean processes. A constrained zero-inflated generalized additive model (COZIGAM) was applied to examine the non-linear relationships between juvenile salmon abundance and environmental factors. Results from the COZIGAM suggested that water temperature, chlorophyll a (chl a) concentration, copepod biomass and spatial factors were significantly correlated with the density of salmon in the northern region, and only chl a concentration was correlated significantly with yearling Chinook density in the southern region. The difference between the abundances north and south of the Columbia River was significantly correlated with alongshore ocean currents, with weaker alongshore currents leading to greater difference between north and south. Results suggest that salmon distribution is determined not only by standard habitat parameters (local biotic and abiotic factors) but by ocean conditions such as the strength of alongshore coastal currents.