Ocean habitat use in autumn by Chinook salmon in coastal waters of Oregon and California

Abstract

Describing the ocean habitats used by Chinook salmon Oncorhynchus tshawytscha is an important step towards understanding how environmental conditions influence their population dynamics. We used data from archival tags that recorded time, temperature and pressure (depth) to define the coastal habitats used by Chinook near Oregon and California during the autumns of 2000, 2002 and 2003. We used a clustering algorithm to summarize the data set from each year and identified 4 general habitats that described the set of ocean conditions used by Chinook. The 4 habitats, defined primarily by depth and the time of day that these depths were occupied, were characterized as (1) shallow day, (2) shallow night, (3) deep and (4) deepest. The definitions and use of each habitat were similar across years and the thermal characteristics of all habitats included water temperatures between 9 and 12°C. This temperature range provided the best indicator of Chinook habitat in the coastal ocean. Chinook used 9 to 12°C water at least 52% of the time. Less than 10% of surface waters within the area where Chinook were released and recovered provided these temperatures. Cross sections of subsurface temperatures suggest that between 25 and 37% of the coastal water column was available to Chinook and contained water in the 9 to 12°C range. These results support hypotheses that link salmon-population dynamics to ocean temperatures. Continued monitoring of surface and subsurface thermal habitats may be useful for assessing the extent and quality of conditions most likely to sustain Chinook salmon populations.