An Historical Perspective on Drift Foraging Models for Stream Salmonids

More than 25 years ago, Fausch (1984) proposed a simple model of optimal foraging positions for salmonids in streams, whereby fish maximize their net energy intake (NEI) by selecting focal positions in low water velocity near faster currents that deliver abundant drifting invertebrates.  The theory was based on earlier observations in artificial and natural streams describing characteristics of salmonid positions (e.g., Kalleberg 1958, Chapman and Bjornn 1969; Everest and Chapman 1972; Griffith 1972; Fausch and White 1981), and a conceptual model by Chapman (1966).  A test of this simple drift foraging model showed that the growth rate of juvenile trout and salmon in an artificial stream was related to NEI, and that the rank of NEI at positions held by coho salmon (Oncorhynchus kisutch) correlated nearly perfectly with their rank in the dominance hierarchy.  Fausch (1984) inferred from these findings that positions that optimize NEI, within the constraints of the dominance hierarchy, were the resource for which these stream salmonids competed.  In turn, the model was used to test the effects of interspecific competition by coho salmon on the profitability of foraging positions held by brook trout (Salvelinus fontinalis) and brown trout (Salmo trutta), and these results were used to infer potential effects of the introduced salmon on resident trout in Lake Michigan tributaries (Fausch and White 1986).  Though the goals for this model were originally modest, it was tested in the field and further refined by Hughes and Dill (1990), and was recently tested by Urabe et al. (2010).  During the last 20 years, the general theory has been extended and widely applied to describe salmonid foraging positions in streams and rivers, and used for management and restoration of habitat and flow regimes to benefit these fishes.