Preliminary analysis of nearshore food web changes in Lake Michigan inferred from isotopic analysis of yellow perch scales Craig A. Stricker 1, Brent A. Murry 2, David F. Clapp 3, Donald G. Uzarski 2 1. Background Disturbance in aquatic ecosystems can manifest from natural or anthropogenic changes in ambient chemical-physical conditions and species introductions leading to top-down or bottom-up impacts on food webs. The Great Lakes have undergone a variety of perturbations during the 20 th century. (1) US Geological Survey, Denver, CO, (2) Department of Biology, Central Michigan University, Mt. Pleasant, MI; (3) Charlevoix Fisheries Research Station, MI Department of Natural Resources, Charlevoix, MI 2. Statement of Problem Species introductions pose a significant challenge to the health and viability of fish populations endemic to the Great Lakes. Long-term records are desperately needed to better understand past and present fluctuations in species abundance and condition. Identification of factors influencing these populations will greatly aid resource managers in adaptively managing the Great Lakes to meet future challenges. We investigated the long-term trophic dynamics of Lake Michigan nearshore environments through the analysis of archived ( ) yellow perch (Perca flavescens) scales. Yellow perch are among the most popular 7. Conclusions A. Yellow perch in the southern basin of Lake Michigan appear to have undergone a recent diet shift, as evidenced by significant changes in  13 C. The shift is most likely toward a higher dependence on the non-native round goby, which provides a direct energetic link to zebra mussels as well as a probable indirect benefit from the nearshore shunt. B. The trophic level of yellow perch has fluctuated relatively little since the late 1970’s, despite the recent diet shift. This may be due to the mere substitution of secondary consumers in the diet, i.e. traditional prey fish to round goby. Diet likely remains subsidized by common invertebrates, though sphaerids and oligochaetes have declined. C. Growth and abundance are lower compared to pre-invasion of Dreissena and round goby, suggesting factors other than species invasions (beneficial or not) influence yellow perch in the southern basin of Lake Michigan. D. Fish scales represent excellent records of aquatic trophic dynamics and offer an efficient and inexpensive means of evaluating changes through time. 4. Study Area Yellow perch used in this study (n = 182; average age = 5) were collected from the southern basin of Lake Michigan using bottom gill nets ( , ) and creel surveys ( ). Most were caught within m water depth. 6. Results and Discussion A.  13 C ranged over 4 ‰, with highest values observed following 1996 and coincident with the establishment of zebra mussels and round goby. ANOVA indicated significant differences (F 4,176 = , p = 0.000) across time periods; samples from also had higher on average values. B. Marked changes in  15 N were observed over the 33- year period, with a significant (F 4,176 = , p = 0.000) decline following the early 1970s; a more subtle decline was observed in the samples (not shown). C. Elevated  13 C and  15 N values during the early 1970’s are consistent with elevated P loading; subsequent decreases are likely related in part to abatement practices, but also concomitant changes to the fish communities of Lake Michigan. D. The range in isotope values within individual time periods was fairly similar throughout the study, suggesting Lake Michigan yellow perch have a relatively static trophic niche breadth, but can adapt to changes in the nearshore forage base. 8. Future Research: Expand analysis of covariates influencing trophic ecology, fill in additional years, and include other important species from different habitats/trophic levels. 5. Methods A. Study Organism We chose to focus our initial efforts on yellow perch since it is a common near- B. Scales Scales are ideal structures for the study of trophic ecology owing to the record of age and growth and the ease of chemical and isotopic analyses. Scales D. Age and growth Scales from the same fish were used to age and back-calculate the last increment of growth using the Fraser-Lee method with a fixed intercept of 30 mm. shore species of the Great Lakes. Additionally, Lake Michigan yellow perch abundance has undergone major fluctuations, are hypothesized to be influenced by invasive species, and therefore represent an excellent sentinel species for the study of Great Lakes trophic dynamics. Adult perch consume aquatic invertebrates, such as amphipods isopods, chironomids, Mysis relicta, and fish, such as sculpin and round goby. 3. Objectives A. Describe historical changes in the trophic ecology of Lake Michigan yellow perch. B. Identify potential dietary shifts related to invasive species introductions. C. Estimate changes in growth related to nearshore food web perturbations. C. Isotopic analyses Scales were cleaned in DI water, residual skin excised, and the lower half trimmed. Approximately 1 mg of scale was weighed into tin capsules for  13 C and  15 N analysis measured by continuous flow techniques. record annual information throughout the life cycle of the fish, but grow allometrically and therefore largely integrate dietary information from the last season of growth. The isotopic composition of scale collagen is highly related to muscle. Recent invaders, such as dreissenid mussels, waterfleas, and round goby likely have had profound effects on food web energetics. Concurrent abiotic perturbations such as climate change, nutrient alterations, and changing water levels may interact synergistically with species invasions to alter food web relations, decrease system stability, and affect the productivity of desirable fish species. For example, reductions in phosphorus loading combined with dreissenid establishment have resulted in a nearshore energy and nutrient shunt (redirection from pelagic-profundal to nearshore benthos). The nearshore shunt has likely negatively influenced offshore fisheries, but effects on nearshore fish species remain less clear. nearshore sport fishes in the Great Lakes. The abundance of Lake Michigan yellow perch has undergone large fluctuations since at least the early 1970s, but for reasons largely unknown have remained at low numbers since establishment of dreissenids and round goby. E. ANOVA indicated significant differences in growth across time periods (F 4,176 = , p = 0.000); age was used as a covariate and was also significant (F 1,176 = , p = 0.000) owing largely to the older aged fish used in the late 1990s. Growth was highest in the early 1970s, probably owing to reduced abundance and competitive interactions; however, growth was lowest following the invasion and establishment of dreissenids and round goby. F. Abundance peaked in the late 1980’s and has since remained relatively low, with a few prominent year classes in the mid-2000s (not shown). G. Yellow perch in the southern basin of Lake Michigan have been known to shift diet in response to environmental and biotic perturbations. The most recent shift appears to be a significant increase in consumption of round goby. Round goby feed on zebra mussels and hence provide an energetic link between yellow perch and zebra mussels, with concomitant coupling to pelagic (nearshore and offshore) nutrients. 9. Acknowledgments: C. Gulbransen, M. Dreier