We show that cardiac sarco(endo)plasmic reticulum Ca²⁺-ATPase (SERCA) activity differs considerably among sockeye salmon (Oncorhynchus nerka)
populations. Variability in SERCA activity was significantly correlated
with elevation gain and temperature during migration, as well as
maximum cardiac stroke volume. Furthermore, because SERCA activity was
not lowered during the spawning migration, this aspect of the cardiac
contraction machinery is apparently spared during the senescence of
these semelparous salmon, likely because it is essential for these fish
to complete spawning. Only when spawning had been completed was there a
significant reduction in SERCA activity, which was detectable in males
at a 25 °C and in females at a 15 °C assay temperature. Hence, we
propose that migration conditions act as a strong selective force that
has resulted in local adaptation of myocardial SERCA activity among
sockeye salmon populations.