Hematocrit Is Associated with Thermal Tolerance and Modulated by Developmental Temperature in Juvenile Chinook Salmon.

To evaluate whether oxygen-carrying capacity influences thermal tolerance in fishes, we reared four Chinook salmon families in present-day (+0°C) and possible future (+4°C) temperatures and assessed the response of hematocrit (Hct) to acute temperature stress. In the +4°C treatment, Hct increased above control levels when juvenile fish were exposed to their critical thermal maximum (CT). Conversely, no effect of temperature stress on Hct was found in the +0°C treatment. Hct was positively associated with CT ([Formula: see text]; [Formula: see text]), contributing to the CT of the +4°C treatment being significantly higher than that of the +0°C treatment (mean ± SD, [Formula: see text] and [Formula: see text], respectively). The association between CT and Hct found here supports the hypothesis that thermal tolerance is affected by oxygen supply to tissue. Moreover, the developmental plasticity of CT and Hct could represent an adaptive mechanism for salmon faced with climate change.