PRESENTATION
R. Glenn Thomas(1) and Rachel Walley(2) gthomas@lsu.edu
(1) School of Renewable Natural Resources, LSU AgCenter, Baton Rouge, Louisiana 70803
(2) Louisiana Department of Wildlife and Fisheries, Inland Fisheries Division, Baton Rouge
Abstract
Recent invasion of Basin waters by fishes and plants portend major ecological changes. Grass carp Ctenopharyngodon idella populations have increased since the 1970’s, and beginning in the early 1980s, commercial fishermen noted increasing populations of bighead carp Hypophthalmichthys nobilis and silver carp Hypophthalmichthys molitrix (together, the bigheaded carps). However, populations of the big‐headed carps appear to be increasing at a much slower rate than that occurring in the Midwest¸ possibly due to intermittent reduced seasonal availability and limited suitability of backwater habitat for juvenile fish. In 2002, harvester reports of sporadic captures of “different‐looking” grass carp indicated the possible presence of black carp Mylopharyngodon piceus. Since then, eight black carp have been collected from sites connected to the Basin. As fecund diploid specimens, these fish indicate a likely reproducing population that threatens molluscan species‐of‐concern. Although statewide reported commercial landings of grass and bigheaded carps have been as high as 80,000 lbs/yr, limited market demand results in many captures being discarded. Infestation of Basin sites with the floating aquatic fern common salvinia Salvinia minima was reported in 2005, and by May 2006 hundreds of acres of backwater sites experienced 100% coverage. A study conducted in 2005 and 2006, suggested that S. minima had the greatest impact on the native plant community, exhibiting inverse abundance relationships with many native species. The study also showed that the percent coverage of invasive plant species was greater than native plant coverage over all aquatic habitat types in the Basin. Such sites invariably exhibit reduced water quality, decreased vessel accessibility, and non‐flowing areas are often anoxic. During summer 2006 specimens of giant salvinia Salvinia molesta, usually considered more noxious than S. minima, were discovered within habitats of the Basin. Like it has in other infested water bodies worldwide, S. molesta is expected to exhibit large impacts on the water quality and native plant community with subsequent expansion in the basin. A number of other aquatic invasive species have been identified at sites in and near the Basin, including water hyacinth Eichhornia crassipes, hydrilla Hydrilla verticillata, alligatorweed Alternanthera philoxeroides, paragrass Brachiaria mutica, and apple snail Pomacea canaliculata. |