**The title, authors, and abstract for this completion report are provided below. For a copy of the completion report, please contact the GLFC via e-mail or via telephone at 734-662-3209**
Pelagic and benthic food web shifts affect availability of polyunsaturated fatty acids to lake trout, implications for early life stages survival
Sergiusz Czesny1, Jacques Rinchard2, John M. Dettmers3, and Konrad Dabrowski4
1 Lake Michigan Biological Station, Illinois Natural History Survey, 400 17th Street, Zion, IL 60099
2 Department of Environmental Science and Biology, The College at Brockport State University of New York 350 New Campus Drive Brockport, NY 14420
3 Great Lakes Fishery Commission 2100 Commonwealth Blvd, Ste 100, Ann Arbor, MI 48105
4 School of Environment and Natural Resources, The Ohio State University, 2021 Coffey Road, Columbus, OH 43210
Natural reproduction of lake trout (Salvelinus namaycush) in Lake Michigan is thought to be compromised through nutritional deficiency associated with inadequate levels of thiamine (vitamin B1) in their eggs. Mortality caused by thiamine deficiency, frequently referred to as early mortality syndrome (EMS), however, is not the only significant cause of low lake trout survival at early life stages. Through a combination of extensive field sampling and experimental study, we sought to link temporally and spatially heterogeneous prey resources in the Lake Michigan food web with variable mortality rates among families of lake trout obtained from two distinct spawning regions (north and south). Firstly, we described among- and within-species variability in lipid content and fatty acid profiles of Lake Michigan forage fish and invertebrates. Secondly, we determined individual variation in fatty acid and thiamine concentrations in lake trout eggs collected at two spawning regions in Lake Michigan. Lastly, we attempted to explain cause-and-effect in early-life-stage mortality among lake trout families with a suite of predictor variables including spawning location, egg fatty acid content and thiamine concentration. Despite spatial and temporal variation in fatty acid signatures among common forage species, within-species variability was relatively small compared to among-species variation. Our analyses indicated that fatty acid signatures can be used in freshwater systems to study food web interactions and delineate spatio-temporal changes in food web structure. Lipid and fatty acid composition of lake trout eggs differed between spawning locations. Lake trout offspring from the south were affected by a high occurrence of yolk edema, whereas a higher frequency of EMS was observed among offspring from the north. Random Forest regression analysis revealed location as the most influential predictor of yolk edema mortality whereas thiamine level in eggs was the strongest predictor of EMS-related mortality. Several polyunsaturated fatty acids were also predictors of both mortalities. There is evidence of variable spatio-temporal fatty acid availability among lake trout prey in Lake Michigan that, together with thiamine shortage, may contribute to low survival of lake trout progeny.