Density-dependent mortality is mediated by foraging activity for prey fish in whole-lake experiments

Authors

  • Peter A. Biro,

    Corresponding author
    1. Division of Ecology, Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1 N4; and
      Peter A. Biro (tel. +1 403 220 5263, fax +1 403 289 9311, e-mail pbiro@ucalgary.ca).
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  • John R. Post,

    1. Division of Ecology, Department of Biological Sciences, University of Calgary, 2500 University Drive NW, Calgary, Alberta, Canada, T2N 1 N4; and
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  • Eric A. Parkinson

    1. British Columbia Ministry of Water, Air and Land Protection, University of British Columbia, Vancouver, British Columbia, Canada, V6T 1Z4
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Peter A. Biro (tel. +1 403 220 5263, fax +1 403 289 9311, e-mail pbiro@ucalgary.ca).

Summary

  • 1Whereas the effects of density-dependent growth and survival on population dynamics are well-known, mechanisms that give rise to density dependence in animal populations are not well understood. We tested the hypothesis that the trade-off between growth and mortality rates is mediated by foraging activity and habitat use. Thus, if depletion of food by prey is density-dependent, and leads to greater foraging activity and risky habitat use, then visibility and encounter rates with predators must also increase.
  • 2We tested this hypothesis by experimentally manipulating the density of young rainbow trout (Oncorhynchus mykiss) at risk of cannibalism, in a replicated single-factor experiment using eight small lakes, during an entire growing season.
  • 3We found no evidence for density-dependent depletion of daphnid food in the nearshore refuge where most age-0 trout resided. Nonetheless, the proportion of time spent moving by individual age-0 trout, the proportion of individuals continuously active, and use of deeper habitats was greater in high density populations than in low density populations. Differences in food abundance among lakes had no effect on measures of activity or habitat use.
  • 4Mortality of age-0 trout over the growing season was higher in high density populations, and in lakes with lower daphnid food abundance. Therefore, population-level mortality of age-0 trout is linked to greater activity and use of risky habitats by individuals at high densities. We suspect that food resources were depleted at small spatial and temporal scales not detected by our plankton sampling in the high density treatment, because food-dependent activity and habitat use by age-0 trout occurs in our lakes when food abundance is experimentally manipulated (Biro, Post & Parkinson, in press).

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