Foraging and growth responses of stream-dwelling fishes to inter-annual variation in a pulsed resource subsidy

Pulsed resource subsidies generate ephemeral fluxes of nutrients and energy among ecosystems. The effects of pulsed subsidies should depend on the magnitude of the pulse, the in situ productivity of the recipient system, and the ability of consumers to capitalize on the resources, yet empirical data on these relationships are limited. We assessed the ecological consequences of variation in resource pulse magnitude, as represented by anadromous salmon returns, for the foraging and growth responses of two species of stream-dwelling salmonids in two streams that vary in productivity in southwestern Alaska. Over 11 years and across a greater than 10-fold variation in sockeye salmon (Oncorhynchus nerka) density, both rainbow trout (O. mykiss) and Arctic grayling (Thymallus arcticus) exhibited a relatively similar, but mechanistically different, non-linear saturating growth response to increasing salmon density. This growth response was driven by both an increase in consumption of salmon eggs and also a decrease in dietary overlap between the two species. However, the relative change in growth from low to high salmon densities was different between streams and depended on in situ stream productivity. In low salmon density years the growth of resident consumers fell 46-68% relative to high years in the low productivity stream, but only by 26-34% in the high productivity stream. Our study provides strong evidence that understanding of both the foraging ecology of consumers and the in situ productivity of recipient ecosystems, which together regulate the ecological consequences of variation in resource subsidies, is required for successful implementation of ecosystem-based management in systems dependent on pulsed resource subsidies.