Persistent habitat use by Chinook salmon Oncorhynchus tshawytscha in the coastal ocean

We used temperature and depth data from 25 archival tags carried by Chinook salmon Oncorhynchus tshawytscha at sea to explore whether and how these fish alter their patterns of habitat use in response to variable oceanographic conditions off the coasts of Oregon and California. The Chinook salmon persistently used a narrow range of thermal habitats (8 to 12 degrees C) during all months of the year, irrespective of location, time or year of release. In general, individuals appeared to adjust their vertical position in the water column to maintain this persistent thermal experience. There was noticeable individual and seasonal variation in the depths used, with the deepest habitats being used during winter. The patterns of depth use were related to the annual cycles of surface temperatures and surface productivity. Chinook salmon synchronously responded to anonymously warm surface temperatures in August 2003 by using relatively deeper habitats. Declines in surface productivity during autumn were accompanied by an apparent switch from relatively shallow habitats, to deeper, presumably benthic, habitats. The persistent use of a narrow range of temperatures suggests that variation in oceanographic conditions do not necessarily correspond to variation in the temperatures that Chinook salmon use. The effects of environmental variability on their growth and maturation in the California Current may, therefore, be relatively independent of temperature-mediated physiological responses. Rather, it seems relatively more important to understand how variable ocean conditions affect the food-web topology in the thermal habitats that Chinook salmon use.