Determining the Role of Fish Planktivory on Spiny Water Flea (Bythotrephes Longimanus) Production in Upper Great Lakes

Abstract

Aquatic invasive species pose a threat to the stability of food webs. The Great Lakes invasion of the spiny water flea Bythotrephes longimanus has reduced densities of more easily captured zooplankton (cyclopoid copepods and cladoceran species) for prey fishes. As a readily available prey item, Bythotrephes has been incorporated into fish diets. Therefore the ability of fish to effectively control Bythotrephes biomass by high consumption rates could potentially impact fish and zooplankton communities. I compared estimates of Bythotrephes production to consumption by fish in lakes Michigan and Superior. Comparisons were made in one day in April, July and September in the northern basin of Lake Michigan at near (18-m), intermediate (46-m), and offshore depths (110-m). Similar comparisons were made between September and November at similar depths in the Apostle Islands area of Lake Superior. Alewife and cisco were the dominant consumers respectively in Lake Michigan and Superior. In September, consumption by fishes in Lake Michigan exceeded production (up to 178 %) at nearshore and intermediate sites, while at offshore sites, consumption was less than 15 % of Bythotrephes production. In Lake Superior, consumption exceeded production (up to 842%) at all offshore sites each month but only in November at the intermediate site. Contrasting Lake Michigan, nearshore consumption of Bythotrephes by fishes was nonexistent in Lake Superior. Although consumption exceeded production on multiple occasions in Lake Michigan, Bythotrephes never declined following excessive consumption indicating a lack of control. However, control occurred twice at the offshore Lake Superior site in September and October. To explore factors other than fish consumption, a generalized additive model was employed for data from both lakes. Only epilimnetic temperature was included in the most parsimonious model explaining biomass changes of Bythotrephes. Overall, control by biotic (fish consumption) processes was limited, and abiotic (temperature) processes appeared to have a stronger influence on invasive Bythotrephes dynamics. These analyses demonstrate the need to identify not only the interactions between invasive species and other biota, but also the physical parameters of lakes that could regulate their populations.