Vegetation Conversion in Pondfish Culture

A technique of vegetation conversion for the liberation of nutrients contained in filamentous algae and submerged rooted aquatic plants is presented as a practice which may prove useful in the management of warm-water hatchery ponds and possibly small farm ponds. An experiment in the production of bluegill fingerlings was conducted comparing three methods of fertilization with the vegetation converson technique. The rate of production in the conversion treatment compared favorably with that in two of the three fertilizer treatments and the cost of production was less than half that of the best fertilizer treatment. The density of phytoplankton as measured by the light penetration readings made during the growing season was best in the vegetation conversion treatment. Of the fifteen ponds receiving inorganic fertilizer, four of the ponds were dominated by the branched summer alga Pithophora, and four developed rooted aquatic vegetation. Only seven of the fifteen initially produced the type of vegetation which was desired. It was possible to convert the rooted vegetation by use of sodium arsenite and produce phytoplankton later in the season, but in the case of the Pithophora dominated ponds, conversion methods used were unsuccessful. In the unfertilized vegetation conversion treatment, all ponds developed rooted vegetation which was successfully converted to basic nutrients using sodium arsenite on one-fourth to one-eighth of the pond area. Phytoplankton blooms resulted from the sodium arsenite applications in all of the five ponds, with difficulty in liberating the nutrients tied up in the rooted vegetation being experienced in only one pond out of five.

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