Contribution of terrestrial invertebrates to yearly brook trout prey consumption and growth

Terrestrial invertebrates are an important component of stream fish diets, especially during the summer months, when aquatic invertebrates are limited. The seasonal diet composition of the brook trout Salvelinus fontinalis inhabiting two West Virginia streams was used in a bioenergetics model to simulate yearly prey consumption for two cohorts of age-1 brook trout in each stream. Baseline simulations were run by fitting the model to observed growth from June 2000 to June 200 1 for the first cohort and from June 2001 to June 2002 for the second cohort. Terrestrial invertebrate inputs can be negatively impacted by anthropogenic effects (e.g., timber harvest and pesticide application). To determine the degree to which growth would be suppressed by a reduction in terrestrial invertebrate consumption, we ran simulations in which the amount of terrestrial invertebrate consumption was reduced by 25, 50, 75, and 100%. Simulations were then run to determine the consumption of aquatic invertebrates that would be necessary to compensate for a reduction in terrestrial invertebrate consumption. The baseline simulations showed that terrestrial invertebrates accounted for 38-47% of the biomass consumed annually and 51-63% of the energy consumed. Reductions in the amount of terrestrial invertebrates in the diet of brook trout reduced growth; to maintain the same growth, brook trout would need to increase yearly consumption of aquatic invertebrates by more than 100% in the absence of terrestrial invertebrate input. These modeling results illustrate the importance of terrestrial invertebrates to headwater stream fish production, and managers need to recognize this energy source in assessing a stream's potential to support fish and in managing riparian forests.