Body size is the primary regulator affecting commencement of smolting in amago salmon Oncorhynchus masou ishikawae

The present study retrospectively examined relationships between growth trajectory and smolting in order to identify the key regulators in commencement of smolting using groups of all-female homozygous clonal amago salmon. We found that (1) regardless of specific growth rate, the majority of fish above the threshold size (12–13 g) by the end of the decision window (early fall) became smolt in the future; (2) even if fish smaller than the threshold size during the decision window exceeded that size afterwards, they were not recruited to smolt; (3) smolting of fish that attained sufficient size during the decision window could not be suppressed by subsequent restriction of growth rate; (4) smaller fish that had not attained the threshold size during the decision window could not smoltify, even though they showed faster growth rates than that of future smolt during/after the decision window. Taken together, these findings indicated that fish mass by the end of the decision window was responsible for the determination of whether individual amago salmon could smoltify or not and that the faster growth rates observed in future smolt after the decision window occurred as a result of commitment of smoltification.