Condition dependence in the marine exit timing of sockeye salmon (Oncorhynchus nerka) returning to Copper Creek, Haida Gwaii

Abstract

We examined a small population of sockeye salmon (Oncorhynchus nerka) that enters their natal stream, to hold in their natal lake, months (>130 days) prior to spawning. This effectively decouples the influence of spawn timing requirements and behaviours from river entry (alternately referred to as “marine exit”) timing and is therefore a good model to study the migration strategies specifically associated with marine exit. We found individuals with early marine exit had higher growth rates in the months prior to river entry, had greater lipid density, were more likely male, more likely of the 2.2 versus 1.2 age class, had smaller gonads, and (if female) had more and smaller eggs. Body size at river entry did not vary seasonally. These patterns are explained using a life history model proposing that individual fish exit the sea when the marginal fitness benefits of further growth are outweighed by the marginal fitness cost of further marine residence. This point is reached at different times depending on body size, sex, lipid reserves, and the growth rate.