Preface to the special drift foraging issue of Environmental Biology of Fishes

Abstract

“Imagine a fish feeding in swift water.” With this statement in their landmark foraging model paper, Nicholas Hughes asked the reader to visualize how a driftforaging fish captures its prey (Hughes and Dill 1990). It is a simple, elegant statement, but it captures the essence of stream fish ecology. A stream is defined, after all, as flowing water. Its inhabitants are the product of millennia of adaptations to the unique selective pressures created by this dynamic environment. Imagination is the key for many of us who have studied fish feeding in swift water. It was perhaps Nick’s greatest gift to us. Foraging adaptations, in fact, have for many years led community ecologists to categorize stream fishes by their foraging guilds (e.g. Schlosser 1982). Clearly, stream fish fitness is largely determined by their ability to forage effectively in flowing water. Like all animals, the life histories of stream fishes have been finely tuned by natural selection to capitalize on spatial and temporal availability of prey resources. This truth is so transparent that it is surprising to realize just how under-appreciated it has become in stream fish ecology and management. A survey of the stream fish literature shows howmuch attention is paid to quantifying where fish make their living, versus how little attention is paid to quantifying how they do so. The study of physical habitat in streams has become a sub-discipline of its own, whereas the study of drift foraging has until very recently remained mostly in the realm of ecological theory. Were this a simple academic question it might not be so critical to bring this issue forward. But stream fish management, with its multitude of social and economic implications, has yet to come to grips with the fact that the distribution, growth, and abundance of fish probably depend as much (or more) on food as on space (Chapman 1966). Evolutionary theory, in fact, says as much. Darwin himself suggested that species are shaped more by interactions among themselves (e.g. predators and their prey) than they are by interacting with the physical environment. Animals must acquire resources to grow and reproduce, and those that are most successful enjoy the highest fitness. Thus, restoring natural flow regimes (e.g. Poff et al. 1997) to maintain fish habitat is only half the battle. Without a better mechanistic understanding of how food and space influence fish distribution, growth and abundance, we lack the tools to predict the outcome of habitat change, be it degradation or restoration. We still need an answer to the question that Nicholas posed in the opening lines of Hughes and Dill (1990):