A wide window of migration phenology captures inter-annual variability of favourable conditions: Results of a whole-lake experiment with juvenile Pacific salmon

Abstract

Survival of post-larval and juvenile organisms can depend on multiple seasonally-fluctuating features, including thermal conditions and food availability. Alterations in the phenology of either consumers or their prey might decouple trophic relationships and reduce consumer growth and survival, and this potential for mismatches is especially pronounced in migratory organisms that change habitats. Juvenile sockeye salmon (Oncorhynchus nerka) migrate from natal streams to lake habitats, and we hypothesised that their in-lake growth and survival depends on whether timing of migration matches suitable lake conditions. We used mixed effects models to test the hypotheses that: (a) the most successful entry times correspond to favourable prey densities and lake thermal conditions; and (b) a mismatch in fish entry time and seasonal lake dynamics can reduce fish growth and survival. These models were applied to an 11-year time series of migration timing, capture rate and size at smolting for juvenile sockeye salmon in Lake Washington, WA, USA. Juvenile sockeye salmon entering Lake Washington later in the season encountered higher zooplankton abundance and warmer water, and experienced higher average rates of growth and survival, but the optimal date for lake entry ranged across years by up to a month. Zooplankton abundance and bloom timing affected fish growth and relative survival more than lake temperature, and survival was most influenced by zooplankton abundance in the spring when fish entered the lake. In contrast, fish size at seaward migration from the lake depended primarily on zooplankton abundance in the spring as they left the lake rather than the previous spring when they entered. These results suggest that maintaining a wide window of lake entry dates allows this population to capture the annually-varying period of favourable conditions, minimising mismatches between juvenile fish and their resources and thereby enabling successful recruitment in an unpredictable environment.