CRITICAL THERMAL MAXIMA OF COHO SALMON (ONCORHYNCHUS-KISUTCH) FRY UNDER FIELD AND LABORATORY ACCLIMATION REGIMES

Juvenile coho salmon (Oncorhynchus kisutch) from three populations in Washington State were captured in the field and tested for critical th ermal maximum (CTM). Tolerances varied among the populations (mean CTM s were 28.21, 29.13, and 29.23 degrees C) and exceeded published data from some laboratory tests. The population from a relatively cool stre am had a lower CTM than the two populations from warmer streams. Howev er, after the salmon had been in the laboratory for 3 months under con stant, common temperature regimes, the CTMs no longer differed, indica ting that the population-specific differences resulted from different acclimation regimes rather than from genetic adaptation.