ASSESSING POPULATION RESPONSES TO MULTIPLE ANTHROPOGENIC EFFECTS - A CASE-STUDY WITH BROOK TROUT

Population declines are often caused by multiple factors, including an thropogenic ones that can be mitigated or reversed to enhance populati on recovery. We used a size-classified matrix population model to exam ine multiple anthropogenic effects on a population and determine which factors are most (or least) important to population dynamics. We mode led brook trout (Salvelinus fontinalis) in southern Appalachian mounta in streams responding to multiple anthropogenic effects including the introduction of an exotic salmonid species (rainbow trout, Oncorhynchu s mykiss), a decrease in pH (through acidic deposition), an increase i n siltation (from roadbuilding and logging), and an increase in fishin g pressure. Potential brook trout responses to rainbow trout include a decrease in survival rate of small fish, a change in density dependen ce in survival of small fish, and a decrease in growth rates of all si zes. When we included these responses in the population model, we foun d that population size tended to decrease with an increase in small-fi sh growth rate (producing a population with fewer, but larger, fish). In addition, changes in patterns of density-dependent survival also ha d a strong impact on both population size and size structure. Brook tr out respond to decreases in pH with decreased growth rate in all size classes, decreased survival rates of small fish, and decreased egg-to- larva survival rates. This combination of effects, at magnitudes docum ented in laboratory experiments. had severe negative impacts on the mo deled population. If siltation effects were severe, the extreme increa se in egg-to-larva mortality could have strong negative effects on the population. However, even very strong increases in large fish mortali ty associated with sport harvesting were not likely to cause a local e xtinction. In all of these cases, the interaction of drastic changes i n population size structure with randomly occurring floods or droughts may lead to even stronger negative impacts than those predicted from the deterministic model. Because these fish can reproduce at a small s ize, negative impacts on survival of the largest fish were not detrime ntal to the persistence of the population. Because survival of small j uveniles is density dependent, even moderate decreases in survival in this stage had little effect on the ultimate population size. In gener al, a brook trout population will respond most negatively to factors t hat decrease survival of large juveniles and small adults, and growth rates of small juveniles.