Substrate Alteration by Spawning Brook Trout in a Southeastern Wyoming Stream

-To measure the changes in substrate composition produced by spawning brook trout Salvefinus fontinalis, we collected 187 freeze-core samples, both before and after spawning, from egg pockets, inside redds but excluding egg pockets, and outside redds. In the upper strata of reddassociated samples, we found that the proportions of particles 0.85-1.70,0.425-0.85,0.2 12-0.425, and less than 0.212 mm in diameter were significantly lower in egg pockets than outside redds, and the proportions of particles 0.425-0.85, 0.212-0.425, and less than 0.212 mm in diameter were also lower in egg pockets than in other areas inside redds. Samples collected outside redds were similar to samples collected before spawning. We observed no significant correlations between the proportion of fine sediment in samples from outside redds (and presumably the proportion present before spawning) and the proportion of fine sediment in egg pockets. In vertically stratified samples, the lower strata contained a greater proportion of the smaller substrate particles than the upper strata. Spawning brook trout removed fine sediment from the substrate during redd construction, and this removal was most apparent in the egg pocket. Thus, egg pockets, rather than entire redds, should be the focus of studies of survival to emergence. sediment often affects the survival of embryonic trout and salmon (Cordone and Kelly 1961; Iwamoto et al. 1978; Everest et al. 1987). Survival to emergence of salmonid embryos decreases as the quantity of fine sediment in the incubation substrate increases (Phillips et al. 1975; Witzel and MacCrimmon 1983a). To determine the consequences of human-induced sediment introduction into spawning streams, researchers have linked models that predict sediment delivery (Cline et al. 198 1) with sediment-related predictions of survival to emergence (Tappel and Bjornn 1983). Stowell et al. (1983), who used this approach, suggested that predictions of survival to emergence might be based on substrate samples collected before the spawning season. This implies that increases in fine sediment deposition before spawning will be reflected in the substrate composition of redds, that is, that there is a direct relationship between prespawning and postspawning substrates. Everest et al. (1987) stated that salmonids are not passive users of substrates, but that they distinctively modify substrate composition during I The unit is jointly supported by the University of Wyoming, the Wyoming Game and Fish Department, and the U.S. Fish and Wildlife Service. redd construction. For example, McNeil and Ahnell(1964) suggested that pink salmon Oncorhynchus gorbuscha removed 3,600 kg of sediment particles less than 0.1 mm in diameter from a single spawning riffle. Helle (1970) reported a 3.6% decrease in sediment particles less than 0.8 mm in diameter after the spawning of pink salmon. Compared to 1 -year-old redds, new redds of coho salmon Oncorhynchus kisutch contained 25% fewer sediment particles less than 3.33 mm in diameter (Ringler and Hall 1988). Thus, predicting the survival to emergence of embryos based on the sediment content of undisturbed prespawning substrates may be unrealistic. Most research on substrate modification has focused on large, anadromous salmonids in the Pacific Northwest (Everest et al. 1987). To determine if relatively small resident salmonids could measurably alter substrate composition during spawning in small, high-elevation streams and if a correlation existed between undisturbed prespawning substrates and disturbed postspawning substrates, we examined substrate modification by brook trout Salvelinus fontinalis in a second-order mountain stream. Besides identifying substrate changes caused by spawning brook trout, we hoped to collect information on the substrate composition of redds and