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Mysids are an important component of Great Lakes foodwebs, both as a prey for fish and as a predator on zooplankton. We monitored mysid abundance in Lake Ontario using lake-wide hydroacoustics data and vertical net hauls collected 1–5 August 2008 during the Ontario Ministry of Natural Resources and New York State Department of Conservation forage fish survey. Acoustic volume backscattering strength was highly correlated with both density and biomass of mysids although the correlation with biomass was stronger. The slopes of these relationships were not significantly different from theory (0.10) indicating a linear relationship between abundance and backscattering strength. Size structure significantly affected the relationship between backscattering strength and density but not between backscattering strength and biomass. Average target strength for areas with small mysids was −91.9 dB per mysid, and in areas with both small and large mysids it was −88.2 dB per animal. Acoustic estimates for Lake Ontario calculated with these regressions provided a lake-wide estimate weighted by lake area within depth intervals, of 228 mysids m−2(CV 17.6%) and 0.93 g dw m−2(CV 12.9%). Density and biomass in 14 net hauls averaged 316 mysids m−2(range 0 to 1113) and 1.35 g dw m−2(range 0 to 3.89). Mysid biomass was 33% of the biomass of other crustacean zooplankton in the summer of 2008. As in 2005, areas of low mysid abundance were detected in the middle of the transects and along the north shore. Lakewide mysid abundance in the summer of 2008 was higher than in a similar acoustic survey in the summer of 2005 and in whole-lake autumn 2000–2007 net surveys, suggesting that mysids are persisting in Lake Ontario and abundant relative to other Great Lakes. [ABSTRACT FROM AUTHOR]

We investigated population responses of to ecosystem changes induced by invasion of dreissenids and predatory cladocerans, and . Lake productivity declined as dreissenids invaded the offshore region. Whole-lake mysid biomass was compared before (early 1990s) and after (2002-2007) the invasion period; it declined 40%-45%. Abundance of young mysids and presence of a summer cohort increased with summer, epilmnetic, nighttime zooplankton biomass (i.e., food biomass index). Cercopagis + Bythotrephes biomass was negatively correlated with this index, implicating them in the mysid decline. Eggs per gravid female increased with autumn, total-water-column zooplankton biomass, reflecting the greater use of hypolimnetic waters by adults. Reproductive success was below replacement during the period 2002-2005. First-year mysid growth rate was maintained while population abundance declined, suggesting selection for individuals that feed effectively at low food concentrations. Mortality rates in the first and second years were dependent on cohort density, indicating that competition for food limited abundance in the first 2 years. Fish predation indices (smelt and alewife combined) were correlated positively with mortality rates and negatively with abundance in the third year. Thus, mysids cannot support as many fish in invaded compared with non-invaded lakes. They may also not be a stable food resource; unusual cohort losses occurred in some years. [ABSTRACT FROM AUTHOR]