Observations of Silver Carp(Hypophthalmichthys molitrix) Planktivory in a Floodplain Lake of the Lower Mississippi River Basin

The invasive silver carp (Hypophthalmichthys molitrix) has become pervasive in much of the Mississippi River, its tributaries, and in connected lakes and wetlands. As an increasingly abundant planktivore, it competes directly for food with native fishes. Its greatest impact may be in connected backwater lakes and wetlands, which due to their high primary production serve as critical sites for feeding and growth of many fishes. To assess the impact that silver carp may have on one such system, we examined the composition of plankton samples and of alimentary tract (gut) contents of collected from an oxbow lake in Mississippi, Forest Home Chute. Through an occasional connection to the Mississippi River, Forest Home Chute was invaded by silver in winter 2005, after which the river and lake became disconnected for about two years. In the water-column, the most common types of phytoplankton were euglenoid algae, cyanobacteria, and diatoms. The vast majority of zooplankton was rotifers with densities sometimes exceeding 7,000 organisms per liter. Very high concentrations of phytoplankton in the gut, relative to in the water-column, indicate substantial consumption of phytoplankton production. In October 2006, euglenoid phytoplankters were a much greater, and cyanobacteria a much lesser, proportion of prey in the fish gut compared to their proportions in the water-column. In December, however, there was no evidence of selective consumption by the silver carp population. Some of the phytoplankters observed in the lowest portion of the gut, including pinnate diatoms and euglenoid algae, were motile, indicating they had survived transit through the 5 to 7-m long gut tract. There was no evidence of rotifer survival of gut passage. By its high consumption of plankton, possible selective planktivory, and differential digestion of consumed phytoplankton and zooplankton, the silver may be altering the food web structure of these important connected lakes.