Changes in Native Aquatic Vegetation, Associated Fish Assemblages, and Food Habits of Largemouth Bass (Micropterus salmoides) Following the Addition of Triploid Grass Carp to Manage Hydrilla (Hydrilla Verticillata) in Lake Conroe, TX

Nuisance aquatic vegetation (mainly Hydrilla Verticillata ) has become problematic in Lake Conroe, TX. Consequently, triploid grass carp (Ctenopharynogodon idella) were stocked at densities sufficient to completely denude the reservoir of all vegetation (invasive and native plants) within one year. As a result, an assessment was designed to investigate the changes (before and after carp stocking) in the plant assemblage among sampling stations, changes in water quality parameters, length frequency and condition changes of Centrachid species, largemouth diet changes, and changes in the fish assemblages among randomly selected sampling stations between early fall 2007, when grass carp were stocked, and one year later in early fall of 2008. The areas for sampling were based upon aquatic vegetation surveys by Texas Parks and Wildlife during 2007 and 2008, thirteen sampling stations were randomly selected using ArcGIS software and the percentage of water surface covered by vegetation was recorded at each station. Within each station, fish were collected by electrofishing the entire station for five minutes; water samples were also collected. Largemouth bass diet did significantly change for mature (<200 mm-TL) bass as indicated by a chi-square test. Largemouth bass from the samples were shown to consume less sunfish and more shad by the second (post-carp) sample. This is consistent with expected results due to the removal of vegetation consequently eliminating small sunfish habitat. In similar fashion, significant length-frequency changes were seen in the second year as there were fewer smaller (juvenile) Centrachid species found in the sampling sites. Contrary to the Centrachids, length-frequency of gizzard shad significantly decreased in size by the second sampling year. Based upon the aquatic vegetation surveys within the sampling sites of 2007 and 2008, there was an almost complete elimination of all aquatic plants following carp introduction. This result was consistent with what was expected from the carp introductions. Changes in water quality parameters (phosphorous, nitrate, nitrites, orthophosphate, chlorophyll (a)), were generally inconclusive, with the exception of nitrate which significantly increased by the second year. The water quality parameters along with other measured habitat parameters were used in the multivariate analysis.