Native brook trout and naturalized brown trout effects on two Pennsylvania headwater stream food chains
Open Access
- Author:
- Tzilkowski, Caleb J
- Graduate Program:
- Wildlife and Fisheries Science
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 07, 2005
- Committee Members:
- Jay Richard Stauffer Jr., Committee Chair/Co-Chair
C Paola Ferreri, Committee Member
Hunter J Carrick, Committee Member
Eric S Post, Committee Member - Keywords:
- food chains
indigenous species
introduced species
streams
predator-prey interactions
trophic cascade - Abstract:
- Introduced trout have had direct and indirect effects on native fishes, amphibians, and invertebrates that have in some cases, induced detrimental community- and ecosystem-level effects. Recently, some North American agencies have invested substantial time, effort, and resources to eradicate exotic salmonids due to their documented or presumed negative effects on native species. Naturalized Brown Trout Salmo trutta populations have been established by historical and ongoing intentional introductions throughout Pennsylvania. Beyond potentially displacing Brook Trout Salvelinus fontinalis, it was unknown if naturalized Brown Trout detrimentally affect other Pennsylvania aquatic organisms and receiving food chains prior to this study. Sympatric Brook and Brown Trout effects on benthic food chains were compared in two Pennsylvania headwater streams (Lingle Run and Tomtit Run) during 2003 and 2004 summers with enclosure experiments and analysis of ambient trout stomach contents. Brook and Brown Trout effects on densities of two predaceous insect families (Cordulegastridae; Rhyachophilidae) were marginally different in experimental enclosures as inferred with a predator impact index: the remaining 26 benthic families responded similarly between the trout species. Furthermore, analysis of captured Brook and Brown Trout stomach contents suggested that Brown Trout had modest effects on ambient headwater communities relative to the sympatric Brook Trout populations. Brook and Brown Trout were top-level keystone species that caused similar precipitous changes in experimental channel food chains in both streams. Most previous stream predator studies reported predator effects on tri-level food chains with algae as a basal resource: predators in these studies usually indirectly increased algal biomass by suppressing herbivore abundance or feeding activities. Trout effects on Lingle and Tomtit Run food chains were different than those reported from other stream predator studies, possibly because these food chains were characterized by five trophic levels and driven by allochthonous inputs. Results of this study suggested that trout can have strong negative effects on tertiary consumers (e.g., crayfish Cambarus bartonii, northern two-lined salamander Eurycea bislineata) that indirectly increase predaceous insect abundance and/or biomass. Unexpectedly, the increase of predaceous insects did not suppress primary consumer abundances; conversely, detritivorous and herbivorous insects were more abundant in trout treatments than in controls. These results suggested that primary consumers responded more positively to the lack of tertiary consumers as compared to any negative response to comparatively abundant predaceous insects. Primary consumers were more abundant in trout treatments in both streams, and in one stream (Lingle Run), abundant detritivores increased leaf processing rates relative to controls: all other published studies regarding predatory fish in streams have reported the opposite effect of increased algae or detritus in the presence of top-level predatory fish. Mechanisms that produced these counterintuitive results were not tested and thus deserve further study. Large Brown Trout are reportedly primarily piscivorous whereas Brook Trout feed primarily on relatively small invertebrates. Taxa consumed by free-roaming Brook and Brown Trout were similar in both streams throughout both years. Although Brown Trout were on average, significantly larger than Brook Trout in both streams, the average prey size and average total mass of stomach contents were significantly greater (P < 0.001) in Brook than Brown Trout stomachs. Based on stomach contents analysis and Brook and Brown trout densities, the sympatric trout populations had similar predatory effects on benthic food chains in Lingle Run. Brook Trout were more abundant than Brown Trout in Tomtit Run, which at the population level, yielded a substantially greater impact of Brook than Brown Trout on benthic organisms. Introduced trout have had dramatic negative effects on receiving communities and ecosystems. This study did not provide evidence that naturalized Brown Trout had negative predatory effects on Lingle and Tomtit Run benthic food chains relative to native Brook Trout; however, interactions between the trout species likely have mediating influences on their roles as top-level predators and deserve further study.