Acoustically derived habitat associations of sympatric invasive bigheaded carps in a large river ecosystem

Abstract

AbstractBigheaded carp (Hypophthalmichthys spp.) occur throughout much of the Mississippi Basin, USA. Efforts to control the spread of these invasive species require information on their spatial ecology, though sampling is hindered by their broad extent, habitat tolerances, and species‐specific behaviour. Mobile hydroacoustics was used to quantify habitat and depth use of bigheaded carp over four years in the heavily invaded Lower Illinois River, a major Mississippi tributary and potential dispersal pathway to the Great Lakes. Horizontally oriented transducers (combined with capture gear for species designation) enabled sampling of the main habitat features in this large flood plain river. Silver carp (Hypophthalmichthys molitrix) were dominant over bighead carp (Hypophthalmichthys nobilis) at all but one site, although habitat use was similar for both species. Densities were highest in lotic backwaters, followed by lentic backwaters and nearshore main channel, with lowest densities in the mid main channel. Bigheaded carp size and species composition were independent of habitat type. Depth associations were similar for both species, with average occurrence at 2.5–3.5 m in the main channel and 1–2 m in backwaters. However, depth relative to the river bed was largely similar across habitat types. Bigheaded carp density and depth use in the main channel were linked non‐linearly to river discharge and water temperature, respectively; densities were reduced during high discharge, whereas depth use became shallower at higher temperatures. Density–hydrology trends were less apparent in backwaters. These findings highlight critical aspects of bigheaded carp spatial ecology that will facilitate effective management in invaded and at‐risk ecosystems.