**ABSTRACT NOT
FOR CITATION WITHOUT AUTHOR PERMISSION. The title, authors, and abstract
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For a copy of the full completion report, please contact the author via
e-mail at mwalsh@usgs.gov or via telephone
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Forecasting ecosystem effects of Hemimysis anomala in
Lake Ontario
1 USGS Great Lakes Science Center, Lake Ontario Biological
Station, 17 Lake Street, Oswego, New York, 13126
2 Cornell University, Cornell Biological Field Station, 900 Shackelton Point Road, Bridgeport, NY 13030
August 2012
ABSTRACT:
Hemimysis anomala, a Ponto-Caspian near shore mysid
species, was discovered in the Great Lakes Basin in 2006, and has since
expanded its range within the Basin. To
better understand the potential for this non-native species to affect near
shore food webs, we used a combined field and laboratory approach in an invaded
area of southeastern Lake Ontario. Overall,
our results indicate that Hemimysis could have an important role as both a prey item
and a predator in areas where it becomes established and achieves a high
density. However, behavior and habitat
use appear to be influenced by predation risk, so the ability of the species to
expand beyond areas that offer refuge from predation may be limited. The results of our feeding rate, preference
and tolerance experiments indicate a wide temperature tolerance,
and an optimal temperature range for Hemimysis between 22 and 27 °C. Lethal temperature limits observed indicate
that water temperatures in Lake Ontario will not limit population growth of Hemimysis. Predation risk appeared to influence
migration and substrate use and could limit expansion of Hemimysis populations if
distances between suitable habitats providing refuge are large. Difference in vertical migration was observed
between juvenile and adult Hemimysis, with juveniles migrating at higher light levels,
which may be linked to risk of predation by cannibalism. Both age classes of Hemimysis
preferred, and were less sensitive to, brighter light conditions than their Mysis counterparts, which is
likely due to different ecology of the two species and reflects the importance
of twilight to the more near shore mysid. In light conditions and when predator kairomones were present (both conditions of increased
predation risk), a significant proportion of Hemimysis used rocky substrate
rather than sandy. Hemimysis
consumption by nearshore fish can be high, but it
varied across seasons and years, and may be most prevalent in fish that feed up
in the water column, at or near dark, and have the ability to consume swift
moving prey like Mysis diluviana or
small fish. As predators themselves, Hemimysis have
potential to impact zooplankton populations and feed opportunistically. Gut content and stable isotope analyses
indicate omnivorous and opportunistic food habits of Hemimysis in the field, including
evidence of consuming other zooplankton predators (Leptodora, Bythotrephes). Prey size and/or motility may affect prey
selection, but the species was able to use many different prey sources in the
Lake, making it unlikely that food limitation would inhibit population growth. Density
appears variable among months and years, and can be variable among
replicates. In general, Hemimysis were
more abundant at shallower sites, but were collected from deeper areas on at
least one sampling occasion in both years. Peak densities were found in
mid-summer in both years. We documented
high densities of Hemimysis
with peak densities (800-900/m3)at depths greater than those at
which Hemimysis is generally thought to inhabit or
where samples are targeted (8 m in August 2010 and 20 m in November 2010). This sampling also documented presence to
depths of 32 m, well beyond the depth range anticipated.