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Toxicokinetic and food web models to quantify the effects of Hemimysis anomala on Great Lakes food webs
1 Ontario Ministry of Natural Resources, Glenora Fisheries Station, 41 Hatchery Lane, Picton, Ontario, K0K 2T0
2 Queen’s University, Department of Biology, Kingston, Ontario, K7L 3N6
3 Current address: Saint Mary’s University, Department of Environmental Science, Halifax, Nova Scotia, B3H 3C3
Hemimysis anomala, the bloody red shrimp is a small, diel migratory zooplankton that invaded the Great Lakes in 2006 and has since spread through the basin and inland lakes, reaching densities in excess of 20,000 Hemimysis·m-3 in the Port of Montreal. In invaded European waters, Hemimysis establishment has been associated with changes in phytoplankton and zooplankton production and species composition, and in some cases fish growth and condition. To better understand their potential impacts on Great Lakes aquatic ecosystems, we sampled four sites in Lake Ontario, across a gradient of Hemimysis density, in the spring, summer, and fall of 2009-2011. We combined abundance and condition metrics, gut contents and stable isotope analyses, and bioenergetic and contaminant modelling to describe differences in food web structure and production across this Hemimysis gradient. While seasonal densities exceeded 1,800 Hemimysis·m-3, few Hemimysis were visually detected in fish stomachs. Prey preference studies suggested Hemimysis would be preferred over another abundant benthic prey item, while digestion rate experiments indicate Hemimysis will digest beyond recognition within 2 hours at typical lake temperatures. For these reasons, we felt stomachs may not provide a reliable means to assess predation rate on Hemimysis. Preliminary testing with a DNA molecular probe suggests a higher frequency of occurrence of Hemimysis in fish stomachs, and stable isotope analyses of dominant fish species suggests Hemimysis are consumed with increasing frequency across the gradient of Hemimysis density. However, current predation rates are low, which may, in part, explain why we were unable to detect differences in fish condition or growth across the gradient. Bioenergetic models suggest yellow perch growth rate will actually decline if fish preferentially consume Hemimysis over traditional prey, unless Hemimysis reach extreme densities that have only infrequently been observed in the Great Lakes to date. While Hemimysis may be contributing to a lengthening of the food chain, reducing energy transfer efficiency to higher trophic levels, more work needs to be done to determine if Hemimysis are substantially altering contaminant biomagnification rates. Overall, our study suggests Hemimysis can reach high densities in the nearshore, and while they are consumed by at least 10 fish species, Hemimysis do not contribute a large portion to the diet. As such current fish growth and condition is unaffected, but if Hemimysis impact the production and composition of lower trophic levels as seen in Europe, fish growth rates may decline. We anticipate any effects of Hemimysis on Great Lakes food web and fisheries to be localised to nearshore areas where Hemimysis reach peak density.