Linking silver carp habitat selection to flow and phytoplankton in the Mississippi River

Invasive silver carp (Hypothalmichthys molitrix) occurs throughout much of the Mississippi River and threatens the Laurentian Great Lakes. To quantify habitat selection relative to river flow and potential phytoplankton food, 77 adult silver carp were implanted with ultrasonic transmitters during spring 2008 through spring 2009 in adjacent upstream dammed and downstream undammed reaches (48 km total) of the Mississippi River. Sixty-seven percent of the fish were located. Selection of major river habitat features (dammed vs. undammed, backwaters, channel border, wing dikes, island side channels, and the main channel) was quantified. Flow rates and chlorophyll a concentration were compared between silver carp locations and random sites. Foregut chlorophyll a concentrations plus presence of macrozooplankton and detritus of 240 non-tagged silver carp were quantified. About 30% of silver carp moved upstream into the dammed reach, where average flow was slower and chlorophyll a concentration was higher. Silver carp selected wing dike areas of moderate flow (about 0.3 m/s) and elevated chlorophyll a (about 7 μg/L) relative to random sites. No silver carp occurred in areas where flow was absent. Wing dikes were preferred while the main channel was avoided. Chlorophyll a concentrations in guts were positively related to temperature and were unrelated to flow or river chlorophyll a concentration. Macrozooplankton and detritus were rare in guts. Silver carp seek areas of low flow and successfully forage across a range of temperatures, flows, and chlorophyll a concentrations that occur in rivers and large lakes.