Stream Fish Responses to Grazing Exclosures

Abstract

Eight paired reaches of northeastern Oregon streams were selected such that one reach was an established livestock exclosure and a neighboring, geomorphologically similar reach was open to grazing. Exclosures varied in length from 123 to 436 m. Teams of snorkelers recorded fish species and size-groups in the exclosure and grazed reaches simultaneously so that diurnal changes in fish behavior did not confound results. Observed densities of age-0 redband trout Oncorhynchus mykiss in pools, corrected for snorkeler range of vision, were significantly different (P < 10⁻⁹), increasing by an average of 2.5-fold in exclosed reaches. Conversely, warmwater fishes (dominated by speckled dace Rhinichthys osculus) were negatively related to exclosures (P < 0.001). Densities of age-0 warmwater fish and age-0 O. mykiss were uncorrelated (P = 0.18). No first-order interactions existed among exclosure–control treatment, stream, and pool temperature variables in either age-0 analysis. Combined juvenile and adult O. mykiss results indicated significant interactions between several streams and temperature and only a marginal effect (P = 0.05) of exclosure. No significant exclosure effect existed on combined juvenile and adult warmwater fish (P > 0.90). The greater density of age-0 O. mykiss in exclosures was attributed to the potential food supply and cover due to increases in undercut banks, instream bank vegetation, width : depth ratio, and several riparian vegetation variables. Bioenergetic and scale arguments are presented that are consistent with the failure to detect a parallel effect with juvenile and adult O. mykiss. A marginally significant (P = 0.04) mean drop in pool water temperature of 0.7°C in exclosures was observed, but continual temperature monitoring indicated no significant difference (P > 0.50) between reaches in 7-d means of daily maxima or minima. These results are promising with respect to improvement of salmonid habitat through prevention of grazing, but the exclosures are too small and infrequent to be effective at the population or basin-wide level.