Diets of Three Sunfishes in Lake Conroe, TX Before and After Grass Carp Introduction.

Abstract

Hydrilla (Hydrilla verticillata) is an invasive aquatic plant that grows quickly across shallow freshwater habitats. It is a problem for recreational users of lakes and landowners. Grass carp (Ctenopharynogodon idella) is an effective biological control agent that preferentially consumes and can control the spread of hydrilla. However, grass carp also will consume other vegetation, which influences aquatic communities via direct and indirect interactions that can change food and habitat availability and use by various species. Aquatic plants influence habitat and types of prey used by sunfish (Centrarchidae), which must also avoid their own predators. Prey use among sunfish species depends on density and taxonomic identity of both prey and vegetation. This was a one-year analysis of stomach contents from three common species of invertivorous sunfish: bluegill (Lepomis macrochirus), longear sunfish (Lepomis megalotis), and redear sunfish (Lepomis microlophus). Thirteen sampling stations were randomly selected using ArcGIS software. Percentage of water surface covered by vegetation was recorded at each station. A five-minute electrofishing sample was performed within the littoral zone early morning in late September. The stomach contents of all targeted sunfish (N=489) showed high percentages of diet overlap pre- (0.77-0.92) and post- (0.83-0.88) introduction of grass carp. Multivariate analysis showed total explained variation (15.5%) in sunfish diet composition was (P < 0.05) correlated significantly with sunfish species (6.67%), percent surface vegetation coverage (3.97%), and sampling periods pre- versus post-introduction of grass carp (2.13%). Prey-specific abundance showed that all sunfishes displayed a generalized feeding strategy in both sampling periods. Diets of each sunfish species showed differences in abundance (by volume) and occurrence (among individual fish) of prey items between sampling periods. Levin?s standardized index of diet breadth for all sunfish species decreased from pre- (0.12) to post-introduction (0.05). Results imply that vegetation control by grass carp influenced the diets and feeding strategies of three cohabitating sunfish species. These findings may help fisheries biologists to plan future management actions that influence assemblages of aquatic plants and macroinvertebrates, herbivorous fish, invertivorous prey-fish, and piscivorous game fish, to promote a healthy and balanced ecosystem for Lake Conroe stakeholders.