The effects of an experimental freshwater cage aquaculture operation on Mysis diluviana

We utilized 7 years of data to determine the influence of decreased concentrations of hypolimnetic oxygen and increased densities of phytoplankton and fish on the mysid Mysis diluviana in a whole-lake experiment on the effects of cage aquaculture for rainbow trout (Oncorhyncus mykiss). Following establishment of the cage farm in 2003, densities of Mysis decreased from an annual average of 85 m(-2) (2002-2005) to 7 m(-2) in 2007-2008. This decline was strongly correlated with decreasing concentrations of hypolimnetic oxygen. Spatially, Mysis were restricted to stations overlying bottom waters with temperatures colder than 12 degrees C and oxygen concentrations > 1 mg L-1 so that Mysis occupied a progressively smaller fraction of the lake as O-2 declined in the hypolimnion. In a nearby reference lake, Mysis densities varied considerably among years (27-112 m(-2)), but never fell as low as in the experimental lake. Mysis growth rates did not vary between the reference lake and the experimental lake. Our results indicate that declines in hypolimnetic oxygen associated with nutrient enrichment can have severe impacts on populations of Mysis.