Size and growth relationships in juvenile steelhead: The advantage of large relative size diminishes with increasing water temperatures

Abstract

In territorial stream salmonids, asymmetric competition can perpetuate individual size differences over time, but the extent to which this is manifested can be environmentally mediated. Here we study the variation in juvenile steelhead (Oncorhynchus mykiss) growth rates to identify the conditions (population density and water temperature) under which an individual’s size relative to its conspecifics conferred an advantage. Among steelhead rearing in the same stream section we found that relatively larger individuals on average grew faster than smaller conspecifics. However, comparing across stream sections there was a negative interaction between relative size and water temperature. The effect of an individual’s relative size on its growth rate decreased as temperatures were increasing, indicating that the advantages of being large diminished during periods of high temperatures or in locations with relatively higher temperatures. Compared to temperature, the effects of population density on the growth rate were less substantial. The results suggest that larger individuals on average acquire more resources than smaller individuals, and demonstrate that water temperature exerts an important, modulating control over growth performance in heterogeneous environments.