Competition between Hatchery‐Raised Rio Grande Cutthroat Trout and Wild Brown Trout

Abstract

AbstractAn important problem for conservation of cutthroat trout Oncorhynchus clarkii is the presence of nonnative trout. We used eight 20‐m instream enclosures on the Rio Cebolla, New Mexico, to evaluate competition between hatchery‐raised Rio Grande cutthroat trout O. clarkii virginalis and similar‐sized wild, nonnative brown trout Salmo trutta. Four control enclosures each contained 20 fish of the same species; four treatment enclosures each contained equal numbers of like‐sized fish of the two species. Results indicated that wild brown trout were superior competitors to hatchery‐raised Rio Grande cutthroat trout. Significant interspecific competition was inferred from comparisons of treatment fish with their controls. Among Rio Grande cutthroat trout, control fish grew in length and body weight, whereas growth of fish confined in enclosures with brown trout was significantly lower than that of controls (P < 0.001). In contrast, brown trout enclosed with Rio Grande cutthroat trout experienced significantly greater growth than did control brown trout (P < 0.005). Rio Grande cutthroat trout in enclosures with brown trout had lower postmortem fat content and an elevated incidence of caudal fin damage than did control fish. Gut contents showed that control fish of both species fed most frequently on small larval caddisflies Lepidostoma spp. However, when the two species were confined together, more brown trout had food in the gut and the food items consumed by Rio Grande cutthroat trout shifted from Lepidostoma spp. to surface‐occurring invertebrates (terrestrial infall, water striders, water mites, and phaerate adult aquatic insects) and benthic‐associated invertebrates (oligochaetes, caddisflies (Limnephilidae and Helicopsyche spp.), and snails). Collectively, the results indicated interspecific competition that was mediated through elevated aggression by brown trout that altered foraging activity by both species. The results underscore the importance of minimizing contact with nonnative brown trout when using hatchery fish to restore cutthroat trout to suitable habitats.