708 Transactions of the American Fisheries Society 127:708–717, 1998 q Copyright by the American Fisheries Society 1998 ARTICLES Effect of Temperature and Macrohabitat on Interspecific Aggression, Foraging Success, and Growth of Brook Trout and Rainbow Trout Pairs in Laboratory Streams DANIEL D. MAGOULICK* 1 AND MARGARET A. WILZBACH 2 Department of Biological Sciences, University of Pittsburgh Pittsburgh, Pennsylvania 15260, USA Abstract.—Researchers have often suggested that abiotic conditions influence interactions among salmonids in general and between brook trout Salvelinus fontinalis and rainbow trout Oncorhynchus mykiss in particular with brook trout having an advantage at lower temperatures and slower, deeper habitats (i.e., pools). In artificial stream channels, we tested whether temperature (138C and 188C) and macrohabitat (riffle and pool) affected aggressive interactions, foraging success, and growth rates of interspecific pairs of juvenile brook trout and rainbow trout. Brook trout were significantly more aggressive, captured significantly more prey, and had significantly greater growth rates than rainbow trout at both 138C and 188C. The differential effect of temperature on aggressive inter- actions between the species was significant; a greater difference was found at 138C than at 188C. However, this did not lead to a temperature-based shift in behavioral dominance because brook trout dominated rainbow trout at both temperatures. Overall, we recorded significantly greater growth rates and significantly fewer aggressive interactions and prey captures at 138C than at 188C. Macrohabitat did not significantly affect aggressive interactions or foraging success, but growth rates were significantly greater in riffles than in pools for both species. Macrohabitat had no differential effect on brook trout and rainbow trout. Our results suggest that differential effects of temperature and macrohabitat are not sufficient explanations of observed brook trout and rainbow trout distributions in streams. Due to widespread introductions, brook trout Salvelinus fontinalis, native to eastern North Amer- ica, often occur in streams with rainbow trout On- corhynchus mykiss and brown trout Salmo trutta (Larson and Moore 1985; Rose 1986; Lohr and West 1992). In these streams, brook trout are typ- ically more abundant upstream and rainbow and brown trout are more abundant downstream (Bur- ton and Odum 1945; Vincent and Miller 1969; Gard and Seegrist 1972; Larson and Moore 1985; Fausch 1989; Magoulick and Wilzbach, in press). Major hypotheses to explain this pattern are (1) that rainbow trout and brown trout are superior competitors and displace brook trout to headwaters (Larson and Moore 1985; Rose 1986; Lohr and West 1992) and (2) that brook trout select different habitats or are adapted to different environmental conditions than brown and rainbow trout (Platts 1976; Cunjak and Green 1983). Because natural streams often have distinct gra- * Corresponding author: danm@mail.uca.edu 1 Present address: University of Central Arkansas, De- partment of Biology, Conway, Arkansas 72035, USA. 2 Present address: Tarpon Bay Environmental Lab, 900A Tarpon Bay Road, Sanibel, Florida 33957, USA. dients of temperature and other environmental conditions, abiotic conditions have been suggested as affecting competitive interactions among sal- monid fishes in general (Fausch 1989), and inter- actions between brook trout and rainbow trout in particular (Gard and Flittner 1974; Larson and Moore 1985; Cunjak and Green 1986; Fausch 1988; Larson et al. 1995). Brook trout have been shown to prefer low-velocity areas corresponding to pool habitat (Griffith 1972; Cunjak and Green 1983), whereas rainbow trout have been shown to prefer high velocities corresponding to riffle hab- itat (Cunjak and Green 1983; Smith and Li 1983; Moyle and Baltz 1985; Grossman and Freeman 1987). Brook trout are also thought to prefer lower temperatures than rainbow trout (Gard and Flittner 1974; Cunjak and Green 1986), and because tem- perature and pool habitat typically increase down- stream, brook trout are presumed to have an ad- vantage in headwater reaches (Cunjak and Green 1986; Fausch 1988). Fausch (1989) hypothesized that small, cold headwater reaches act as refuges for brook trout because potential competitors can- not survive there or because brook trout are com- petitively superior in these upper reaches. Both types of coexistence among competitors can be