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The effect of food abundance on territory size and population density of juvenile steelhead trout (Oncorhynchus mykiss)


Optimal territory size models predict a decrease in territory size with increasing food abundance. However, most of these models may not be applicable to juvenile salmonids in streams, because they defend contiguous territories at high densities. The optimal size of a contiguous territory is predicted to (1) be independent of food abundance when food is rare and (2) decrease only when food abundance is high enough to induce a reduction in territory size below the contiguous optimum. To test these predictions, we raised equal densities of juvenile steelhead trout in outdoor stream channels over a 32-fold range of food abundance in the absence of emigration for 25 days. Increasing competition for scarce food resulted in increasing mortality, higher willingness to emigrate, higher variance in body mass, lower growth, lower population density and lower biomass. The size of territories decreased with increasing local population density, and increased with increasing body size. However, territory size did not change with food abundance, a result consistent with the prediction of a contiguous territory size model. On average, total salmonid biomass increased 5.7 times in response to the 32-fold increase in food abundance. Our data provide strong support for an earlier quantitative relationship between the abundance of stream salmonids and their food.

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We thank S.Ó. Steingrímsson, C.A. Johnson, I.L. Girard, L. Weir, C. Breau, and two anonymous referees for constructive comments on the manuscript, and Michelle Noël for help in conducting the experiment. We are grateful to D. Larson and K. Scheer (Fraser Valley Trout Hatchery) for supplying the fish, to B. Stanton (Chilliwack River Hatchery) for the loan of equipment, to E. Parkinson (B.C. Ministry of Fisheries) and Cultus Lake Laboratory (Department of Fisheries and Oceans) for their generous logistical support. The research was financially supported by an operating grant from the Natural Sciences and Engineering Research Council of Canada (NSERC) to J.W.A. Grant, and a Grant-in-Aid-of-Research from Sigma Xi to I. Imre. I. Imre was supported by an FCAR postgraduate fellowship and a Concordia graduate fellowship during this study. This experiment complies with the Animal Care Guidelines of Concordia University and the applicable laws of Canada.

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Imre, I., Grant, J.W.A. & Keeley, E.R. The effect of food abundance on territory size and population density of juvenile steelhead trout (Oncorhynchus mykiss). Oecologia 138, 371–378 (2004).

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  • Contiguous territory
  • Juvenile salmonids
  • Biomass