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JPR Advance Access published online on February 2, 2006
Journal of Plankton Research, doi:10.1093/plankt/fbi125
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© The Author 2006. Published by Oxford University Press. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org
Received October 16, 2005
Accepted January 26, 2006

Article

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

Stephanie E. Hampton 1 *, Pia Romare 2, and David E. Seiler 3

1 University of Idaho, Department of Fish and Wildlife, Moscow ID 83843, USA; University of Washington, School of Aquatic and Fishery Sciences, Seattle WA 98105, USA
2 University of Washington, School of Aquatic and Fishery Sciences, Seattle WA 98105, USA; University of Lund, Department of Limnology, Lund, Sweden
3 Washington Department of Fisheries and Wildlife, 600 Capitol Way North, Olympia, WA 98501, USA

* To whom correspondence should be addressed.
Stephanie E. Hampton, E-mail: shampton{at}uidaho.edu


  Abstract

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 a) factors that predict D. pulicaria and D. thorata increase to this threshold "switching" abundance, b) trends in Daphnia dynamics that may affect sockeye foraging, and c) temporal correspondence of Daphnia increase and fry arrival. The winter abundance of Daphnia 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 (vs. diapause hatch) than D. thorata exhibited. In addition, Daphnia pulicariawas 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 climate continues to change.

Communicating Editor: KJ Flynn


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