The penalty for arriving late in emerging salmonid juveniles: differences between species correspond to their interspecific competitive ability

Abstract

Summary1. Timing of offspring arrival (i.e. hatching, birth or emergence from nests) is commonly shown to have strong effects on their performance through body size and prior residency effects, but less is known about how such effects differ among species. The strength of such effects tends to be related to competitive intensity (e.g. population density). Variation in timing effects among species may therefore be expected to be related to their competitive ability. 2. Here, we test this hypothesis by conducting sympatric and allopatric competition experiments in two ecologically similar salmonid species, Atlantic salmon and brown trout, at the onset of exogenous feeding. We first test the competitive ability of the two species in sympatric conditions in the absence of timing effects. We then test the strength of timing effects by manipulating this for the two species in allopatric conditions, and evaluate whether any difference in response to timing between the two species corresponds to species-specific competitive abilities. 3. In sympatry, trout outperformed salmon emerging at the same time, despite salmon being 32% larger at the time of emergence. Thus, trout was the stronger competitor. 4. In allopatric treatments, late emerging trout performed significantly poorer in terms of survival and growth than late emerging salmon. Further, dominant trout appeared better able to monopolise resources at higher competitive intensities than dominant salmon. There were also indications that population regulation was stronger in brown trout. Replicates with a high initial density of salmon ended up with twice the final density of low-density replicates, whereas trout ended up with similar final densities across treatments.5. The results demonstrate that species-level differences in interspecific competitive ability corresponded with a higher intraspecific asymmetric competition among timing phenotypes, resulting in a higher penalty of emerging late in the more aggressive, dominant trout. Further, it suggests that intrinsic competitive abilities may affect the intensity of selection on offspring arrival, and thus likely affect evolution of phenological traits such as breeding time.

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