Use of an Incremental Triploid Grass Carp Stocking Strategy for Maintaining Vegetation Coverage in a Riverine Texas Reservoir

摘要

Lake Austin is a 648-ha riverine impoundment of the Colorado River located in central Texas and supports a trophy (>=5.9 kg minimum weight) largemouth bass Micropterus salmoides fishery. Much of the reservoir is bordered by waterfront homes and, in addition to largemouth bass anglers, also is used by recreational boaters. Aquatic vegetation management on this reservoir has been an issue of concern since the 1950s. Parrot feather Myriophyllum aquaticum and Eurasian watermilfoil M. spicatum were the traditional species of concern but were successfully controlled with winter drawdowns. In 1999, hydrilla Hydrilla verticillata (9 ha) was documented for the first time, and by 2002, coverage had expanded to 130 ha. A hydrilla management plan was developedby a multistakeholder group that included biological (including triploid grass carp Ctenopharyngodon idella], mechanical, physical (reservoir drawdowns), and chemical management options. Of these options, most attention was given to the use of triploid grass carp. Many waterfront property owners viewed grass carp as the only reasonable solution, while largemouth bass angling groups vehemently opposed this management option. After much discussion, the stakeholder group decided that a slow methodical approach to triploid grass carp use, with a number of incremental stockings, might have a chance of successfully reducing hydrilla coverage without denuding the reservoir of all aquatic vegetation. The strategy was to increase the number of grass carp slowlyuntil there were just enough in the reservoir to control hydrilla but not so many as to eliminate less preferred plant species. After an initial stocking in February 2003, the number of additional grass carp stocked was based on results of hydrilla coverage surveys conducted at 2—4-month intervals during the growing season. From February 2003 through November 2004, a total of 8,125 triploid grass carp were stocked in six stockings. The number of grass carp per hectare of hydrilla ranged from 29.2 to 68.3 until coverage declined in December 2004 to 34 ha. From February 2005 to March 2007, hydrilla coverage ranged from only l to 27 ha (0.1—4.2% coverage) while other plant species less preferred by grass carp, primarily Eurasian watermilfoil expanded.As a result, mean total vegetation coverage from February 2005 to March 2007 was 17% and continued to provide habitat for largemouth bass. Grass carp stockings were reinitiated in October 2006 when hydrilla coverage increased to 27 ha. In 2005 and 2006,the incremental stocking strategy implemented at Lake Austin appeared to help control hydrilla without denuding the reservoir of all aquatic vegetation. Cooperators and interest groups seemed satisfied with the strategy and results.