Is predation by invasive brown trout a threat to native fish persistence?

Authors: Gary P. Thiedi, Phaedra Budy, Jeremiah Wood, Nora K. Burbank

Year: 2010 (xlii)

Abstract

In the USA, exotic brown trout (Salmo trutta)have been stocked widely, reproduce naturally, and represent a significant threat to native fishes. We examined the ecological interactions in a high-elevation stream containing exotic brown trout and native cutthroat trout (Oncorhynchus clarkii utah), mottled sculpin (Cottus bairdii), and mountain whitefish (Prosopium williamsoni) to determine the potential impact of an invasive trout on the native fish community. Brown trout become piscivorous at age-2 (200 mm total length) and sculpin, native trout, and whitefish represent 16.5 %, 0.1%, and 0% of their diet, respectively. Diets of brown trout overlap substantially with diets of cutthroat trout, and brown trout outcompete cutthroat trout where they occur sympatrically. Sculpin density was inversely related to trout density; where densities of trout were high, density of sculpin was low. There was no strong functional response between density of sculpin and proportion of sculpin in brown trout diets, likely because invertebrate prey are abundant in this system and piscivory may be simply opportunistic. While predation rates on native fishes appear low based on diet analyses, based on our predictions, brown trout theoretically experience little gape limitation with potential sculpin prey of all age classes, and native trout and whitefish less than 120 mm. As such, the combination of competition (firmly established in previous work), the strong allopatric distribution we observe between native and non-native fishes, and the predicted lack of substantial gape limitation clearly demonstrates that brown trout negatively impact native fishes in this system and likely elsewhere.