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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
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.