Improvements in water quality following biomanipulation of gizzard shad ( Dorosoma cepedianum ) in Lake Denham, Florida

We removed gizzard shad (GS; Dorosoma cepedianum) from Lake Denham, a 104 ha hypereutrophic lake in the Ocklawaha chain of lakes in central Florida, to evaluate biomanipulation for restoration of shallow hypereutrophic Florida lakes. A commercial haul seine removed GS in winters 1990, 1991, and 1992. The 3-year total removal was 51,738 kg wet weight or 498 kg/ha, which reduced the stock of harvestable adult GS to less than 3% of the initial level. Water quality in Lake Denham improved progressively during the biomanipulation, and some improvements persisted through 2007 despite removal of a fish barrier in 1993. From 1989 until 1993, mean annual Secchi disk visibility in Lake Denham increased from 23 to 57 cm, mean annual chlorophyll a decreased from 143 to 40 ug/L, and mean annual total phosphorus declined from 194 to 93 ug/L. Neither land use nor rainfall changed significantly during the experiment, and similar improvements were not evident in other basin lakes during the same period. These findings and work elsewhere support our conclusion that removal of GS can be an effective component in the restoration of shallow hypereutrophic lakes. We postulate that the improvements were associated with reduced nutrient cycling and bioturbation rather than increased zooplankton predation on phytoplankton. Long-term improvements in water quality in Lake Denham indicate that effects of biomanipulation can persist when the benthivorous fish populations remain depressed.