Environmentally controlled Daphnia spring increase with implications for sockeye salmon fry in Lake Washington, USA

In Lake Washington, juvenile sockeye salmon (Oncorhynchus nerka) strongly prefer Daphnia over other prey, switching uniformly to Daphnia when the threshold abundance of 0.4 Daphnia L-1 is achieved. Using long-term Lake Washington data (1978-2001) and fry trap data (1992-2001) from a major tributary, we examined the following: (i) factors that predict Daphnia pulicaria and Daphnia thorata increase to this threshold "switching" abundance, (ii) trends in Daphnia dynamics that may affect sockeye foraging and (iii) temporal correspondence of Daphnia increase and fry arrival. The winter abundance of D. pulicaria, in combination with basic parameters of spring conditions, was an important predictor of the date of D. pulicaria spring increase, indicating greater reliance on pelagic population dynamics (versus diapause hatch) than D. thorata exhibited. In addition, D. pulicaria was a more consistent prey than D. thorata, the latter exhibiting larger population fluctuations. Thus, recently increasing D. thorata prominence could decrease diet consistency for sockeye fry. Additionally, the timing of sockeye arrival to Lake Washington and Daphnia's increase to the switching threshold has become less concordant, so that fry in recent years have had to rely upon less profitable prey for longer periods. Long-term trends and species-specific differences in Daphnia phenology may affect fry through altering diet composition, with additional implications for other zooplankton withstanding greater predation pressure in Daphnia's absence. Recent decades of warming in Lake Washington are consistent with the warming of lakes worldwide, and complex phenological responses such as those reported here may be common as the climate continues to change.