Heat exchange rates between adult sockeye salmon (Oncorhynchus nerka) and the environment

Body temperatures of salmon do not always match the temperatures of their immediate environment. We investigated the relationship between varying ambient water temperatures and body temperatures of adult sockeye salmon Oncorhynchus nerka using Newton's law of cooling and non-linear mixed models. The median estimated coefficient of heat exchange $k$ for O. nerka in this study was 0.09 min^-1 with corresponding temperature change halftime and thermal time constant values of 7.7 and 11.1 min, respectively. There was little support for salmon condition factor and mass impacting $k$ , or differences in $k$ when the body was cooling or warming. Metabolically produced heat had a very small effect on body temperature. The bodies of the salmon did not instantaneously equilibrate with ambient temperatures. Model-averaged predictions showed that it would take approximately 60 min for the core body temperature of an adult O. nerka to equilibrate with ambient temperature (i.e., rate of body temperature change <0.01°C min^-1) after moving into water temperatures 10°C cooler or warmer than their initial body temperature. Greater differences in body and ambient temperatures were predicted when the salmon experienced rapid and larger changes in ambient temperature. The differences in body and ambient temperatures experienced by Babine Lake O. nerka in this study highlight a need to exhibit caution when equating water temperature to the body temperature of fish in thermally heterogeneous environments across short timescales. The estimated values of $k$ and $T_m$ from this study could be applied to other research requiring body temperature predictions for free-ranging adult O. nerka or possibly other similarly sized and shaped salmonids without using internal temperature loggers.