Thermoregulation by Maturing Adult Chinook Salmon In Marine, Estuary, Lake and Tributary Habitats As Determined by Archival Loggers and Acoustic Telemetry

Very little is known about the movements and thermal experience of Chinook salmon during their homeward migration through nearshore and estuarine waters and in particular for movements through lake systems.  The purpose of this study was to determine how maturing Chinook salmon distributed themselves relative to water temperature in distinct marine, estuarine, and freshwater habitats.  These habitats included the marine waters of Puget Sound, a small estuary above and below a tidal barrier, and distinct lake environments in the Lake Washington watershed.  All fish were tagged with a combination of acoustic transmitters and archival temperature loggers in the lower estuary at the entry to the watershed.  We found that fish selected cold water refuges in three distinct stratified areas, in Puget Sound, the salt wedge above the barrier, and in Lake Washington.  We found that a majority of fish exited out of the small estuary to reside for some period of time (average 7 days) in the cold (12-15 C) nearshore waters of Puget Sound before returning to the estuary.  Upon initial entry into freshwater fish either held in a small marine-influenced cool-water area in the salt wedge or would return below the tidal barrier to hold in tidally influence saltwater. We found that fish remained in the vicinity of the estuary (average 14 days) and apparently conserved energy by remaining below the thermo-halocline in deeper (14-m average depth) coldwater (17-20 C) of the salt wedge (2-15 ppt).  We found that travel time through a 6-km connecting navigation channel to Lake Washington was short, less than 1 day, with fish experiencing the maximum temperatures (20-22.5 C) found during any migration segment.  Low dissolved oxygen levels at depth did not preclude movement through the channel as fish selected shallower depths with higher temperatures.  Maximum temperature experienced was dependent upon tagging date with early migrants experiencing temperatures from 21-22.5 C and later migrants 20-21.5.  In Lake Washington fish exhibited vertical migrations above and below the thermocline and held near river deltas.  Individual fish that utilized different migration paths and areas in the watershed displayed distinct thermal experiences. The ability to exploit multiple cold water refuges provides greater opportunities for thermoregulation during movements through high temperature areas.