The role of diet in regulating reproductive success and contaminant accumulation in Great Lakes herring gulls (Larus argentatus)

grantor: University of TorontoThe Canadian Wildlife Service's Great Lakes Herring Gull Monitoring Program has been extremely successful in documenting contaminant sources, effects, and temporal trends. However, it has also allowed us to evaluate the impact of other environmental stressors on the Great Lakes ecosystem. In this study, the role of diet quality in regulating the success of Great Lakes herring gull populations was examined. Lake Superior birds were found to have a lower proportion of fish in their diet than birds on Lakes Erie and Ontario. Stable nitrogen and carbon isotope analysis verified inter-colony dietary differences and identified temporal changes in diet composition. These spatial and temporal differences in diet likely reflected differences in fish availability to the gull. When fish were available, gulls consumed them preferentially, when they were not, other food items were substituted. Amino acid analysis of herring gull blood plasma revealed that protein was less available on the Lake Superior colonies than on colonies located on the lower Great Lakes. An index of plasma amino acid concentrations was correlated with colony-wide estimates of reproductive endpoints indicating that foods replacing fish may have been of inferior quality. Productivity in Great Lakes herring gulls is an important factor regulating the viability of populations. A number of diet-related endpoints (adult body condition, clutch volume and intra-clutch variation in egg volume) were examined as indicators of productivity. Body condition was found to be relatively insensitive while both egg volume endpoints were correlated with colony-wide estimates of reproductive success. Herring gull productivity at some Great Lakes colonies appears to be limited by the availability of high-quality food. The Herring Gull Monitoring Program has been instrumental in evaluating temporal declines in contaminant levels. However, further research is required to improve our understanding of the factors that regulate annual perturbations in contaminant concentrations so that we can accurately assess our progress in eliminating environmental contaminants from the Great Lakes. In this study, biotic (diet, migration) and abiotic (weather) factors were found to be important in regulating contaminant concentrations in Great Lakes herring gull eggs.Ph.D