Depletion of turions and tubers of Hydrilla verticillata in the north New River canal, Florida

The North New River Canal provides a waterway for fresh water from Lake Okeechobee, Florida to the metropolitan area of Fort Lauderdale, and is a major drainage system for Broward County and the surrounding area. This canal has been infested with hydrilla (Hydrilla verticillata (L.f.) Royle) for the past three decades. A survey was conducted to estimate the number of turions, both axillary and subterranean (commonly known as tubers) of hydrilla in the North New River in relation to application of herbicides to manage this submersed aquatic weed. From October 1984 to January 1995, the numbers of turions and tubers found in cores of sediments at six sites during ten sampling periods were used to estimate their abundance in the North New River Canal. Differences in the number of propagules collected occurred due to sampling date, site, and location within the canal. Core samples collected from Site 1 in October 1984 on the north side of the canal contained a high of 918 turions m(-2). In January 1989, 887 tubers m(-2) were present in Site 4 on the north side of the canal. Only one axillary turion was collected in a single core during January 1991, and none after that. In January 1992, seven tubers were found in five cores and none for the remainder of the sampling periods. Herbicides, last application during 1989, was a major factor in controlling hydrilla. Once hydrilla was controlled with herbicides, no new turions or tubers were produced in the North New River Canal. Accidental escape of grass carp (Ctenopharygodon idella Val.), around 1988 or 1989 from an adjoining water management district provided additional control because hydrilla is a highly preferred food of this herbivorous fish. Grass carp feeding on sprouting turions and tubers was, in all likelihood, a major factor contributing to the depletion of hydrilla propagules within 3-4 years after removal of the parent plants with herbicides.