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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
October 2012
ABSTRACT:
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.