Experimental Vegetation Control by Largemouth Bass-Tilapia Combinations

Abstract

Tilapia mossambica survived and reproduced in a central Illinois pond during the period May 15-September 30, 1962-65. In normal years, water temperatures are believed to have exceeded 60 F (15.6 C) during this period. This species died when exposed for extended periods to water temperatures of 55 F (12.8 C) and below, although it survived for short periods with temperatures in the 40's. A stocking of 58 tilapias weighing 1.6 lb/acre increased to 11,000 fish weighing 387 lb/acre when placed in a pond by themselves and left for a single growing season (May-October). When varying numbers of largemouth bass were added, the tilapia population lost ground with 826 bass/acre, increased to a limited extent with 91 bass/acre, and expanded 132 times with 37 bass/acre. Predator-prey relationships were probably influenced by the relative abundance of crayfish and tadpoles in the pond. When more than 1,000 tilapias/acre were present, they were able to eliminate algae and rooted submersed aquatic plants during 1962 and 1965. Vegetation density was only moderately affected during 1964 when about 164 tilapias/acre were present. Tilapias by themselves would be useful in artificial lakes and ponds where aquatic vegetation problems are common, aesthetic values are of primary importance, and summer water temperatures are suitable. Algae and rooted aquatic plants that create nuisances in lakes and ponds can be controlled by herbicides, but chemical treatment programs may be expensive, may require several applications during the plant growing season, and may produce accumulations of certain chemicals in the bottom muds. In some situations, aquatic herbicide applications may, indirectly, create oxygen deficiencies that suffocate aquatic gill breathers. Nutrient release from the decomposition of treated higher plants may produce algal blooms. These drawbacks have stimulated interest in biological means of controlling aquatic vegetation. Aquatic plants undoubtedly have diseases and organisms that parasitize them just as do land plants. However, at he present time our investigations are being directed toward discovering and measuring the usefulness of exotic fishes which actually consume aquatic plants. American interest in biological control of aquatic plants by herbivorous fishes was centered at the Fisheries Laboratory at Auburn University, Alabama, in the late 1950's (Swingle 1957, 1960). Several species of herbivorous fishes were imported from various parts of the world with the objective of measuring their usefulness. Among the first to be imported were several species of the genus Tilapia, all of which were known to be vegetation feeders. Tilapia mossambica (Fig. 1) had been widely studied in Africa and Asia as a food fish where it is raised in ponds and har-