Interactions between native and nonnative species: Consequences of a brown trout introduction on a coldwater stream community

Although previous research has investigated competition between native and nonnative trout, few studies have examined effects of introduced trout on other species in coldwater stream communities. I designed a field experiment to examine interactions between native brook trout versus nonnative brown trout with native slimy sculpin to investigate whether introductions of nonnative trout affect food web relationships in small streams. I hypothesized that brown trout would compete with sculpin and reduce sculpin growth, whereas competition would not occur between brook trout and sculpin, based on the premise that brook tout and slimy sculpin have coevolved in Minnesota streams. Enclosures (1m2) were stocked with fish according to six treatments: combined-species treatments (juvenile brown trout with sculpin, juvenile brook trout with sculpin), single-species controls, and no fish. Fish were stocked at three densities to examine intraspecific versus interspecific competition. Replicates of each treatment were placed in riffles in Valley Creek, Minnesota, and six 38-day experiment trials were conducted over three summers (2002--2004). Enclosures did not reduce abundance of drifting invertebrate prey. Interactions between brown trout and sculpin led to reduced growth of both large sculpin and large brown trout in enclosures, whereas presence of brook trout was not associated with changes to sculpin growth. Presence of brown trout also led to indirect effects on food web dynamics that may have resulted in these observed changes in growth. Brown trout was the only species associated with a decrease in abundance of a benthic invertebrate taxon, suggesting that brown trout may feed in the benthos to a greater extent than brook trout. Brown trout were also associated with shifts in sculpin diets, indicating a change in sculpin feeding behavior when sculpin were with brown trout that was not evident when sculpin were alone or with brook trout. Feeding behavior and intraspecific interactions among brown trout, along with shifts in feeding behavior of sculpin when brown trout were present, likely was a mechanism underlying reduced growth of both species. My research shows that introduced brown trout have effects on the stream community in Valley Creek that are different than native brook trout.