Food web perspectives on studies of bass populations in north-temperate lakes

There is growing recognition of the potential contributions of food web and ecosystem-level approaches to lake and fisheries management. Often, these approaches are data intensive and require information on predator-prey interactions and consumer diets. Gut content analysis adequately provides population-level diet information, but is more limited in ability to provide information at finer (intra-population) or coarser (whole food web) organizational scales. In this study, we review three applications of stable isotope techniques to studies of bass trophic ecology. In each case, the approach is used to provide quantitative information at organizational levels that is otherwise difficult to obtain using gut content analysis. In the first example, we examine intra-population trophic variation in young-of-the-year (YOY) smallmouth bass Micropterus dolomieu. delta(13)C indicated ontogenic trophic habitat shifts from pelagic to benthic prey during first summer. delta(15)N was correlated with body size on the final sample date, suggesting that dietary differences might explain intra-population variation in growth rates. The second example demonstrates an application of stable isotope techniques to examine impacts of species invasion at the whole-food web scale. This approach revealed that lake trout Salvelinus namaycush, typically viewed as a pelagic predator, can often rely heavily on littoral prey fish.-Reduction in littoral prey fish following the invasion of nonnative smallmouth bass and rock bass Ambloplites rupestris prompted lake trout to shift towards planktivory. The third example uses stable isotope information to classify lakes according to their sensitivity to bass invasion. Stable isotope evidence indicated that bass and lake trout subsist on separate food chains in lakes that contain pelagic prey fish (cisco, lake whitefish, rainbow smelt). Thus, bass introductions into these lakes is predicted not to affect lake trout populations. Identifying particular lakes and lake-types that are sensitive to bass introductions can guide efforts of lake managers to minimize negative impacts of bass introductions on lake trout.