Water bodies, watersheds and storm water

Shad harvesting

As part of multifaceted waterway cleanup efforts, the St. Johns River Water Management District has been reducing the “rough” fish population of several central Florida lakes. Such work has proven beneficial in lakes Griffin, Denham and Apopka, and during a one-year experimental project at Newnans Lake east of Gainesville.

More than a decade ago, District scientists discovered that removal of gizzard shad (Dorosoma cepedianum) helps improve the health of some lakes that are overrun by algae, which cloud the water and cause a sickly green color. Algae are present in all lakes, but too many nutrients in the water can result in excessive algal growth. The rough fish harvesting process helps cleanse nutrient-rich water and restores the water body’s health and the fish habitats.

Fishers haul in a net with gizzard shad.

Gizzard shad are a native fish found in most Florida lakes and account for 5 to 20 percent of the total fish population in healthy central Florida lakes. However, in nutrient-rich, algae-dominated lakes such as Apopka, Griffin and Denham, shad proliferate and can account for more than 90 percent of the total fish population. Shad become abundant in lakes with high levels of nutrients and large numbers of gizzard shad may contribute to nutrient recycling by stirring up bottom sediments during feeding.

How nutrients affect Florida lakes

Water quality in most Florida lakes is affected by the amount of nutrients entering the lake from surrounding lands. Human activities can increase nutrients entering the lake, degrading water quality. Low nutrient lakes have clear tea-colored water and lots of underwater plants. Excess nutrients feed algae that turn lake water murky green, which blocks sunlight to underwater plants. Lakes with murky green water and fewer underwater plants have fewer sport fish and more gizzard shad.

A day’s catch is unloaded at Herlong Park.

How shad harvests may improve lake water

Gizzard shad feed on algae on the lake bottom, stirring up sediments and clouding the water. Shad excrete nutrients back into the lake water, recycling nutrients from the lake bottom that feed more algae, keeping the lake murky green.

Removing large numbers of bottom-feeding gizzard shad may improve water quality in degraded lakes by reducing resuspension of lake sediments and recycling of nutrients from the lake bottom. Removing large numbers of shad from a lake also removes the nutrients in their bodies.

Also, the shad eat microscopic zooplankton. Because zooplankton feed on algae, removal of gizzard shad may result in a larger population of zooplankton, which will consume greater amounts of algae and provide increased food resources for juvenile game fish species.

Fish are loaded in boxes for weighing.

Previous shad harvests

Harvest of large numbers of gizzard shad from three nutrient-rich lakes in central Florida may have helped improve water quality in those lakes. At Lake Denham, a smaller lake with problems nearly identical to Lake Griffin, two years of experimental shad harvesting greatly reduced the shad population. During the experimental period, which ended in 1994, nutrient levels decreased by more than one half and water clarity increased four-fold, from about nine inches to more than three feet. In addition, the amount of underwater vegetation — habitat for juvenile game fish — also increased. Local fishermen reported substantial increases in game fish caught in Lake Denham.

Gizzard shad move up a conveyor belt for processing.

Annual large-scale shad harvests began at Lake Apopka in 1993 and Lake Griffin in 2002. The shad harvests occurred at the same time as runoff of nutrients into the lakes from surrounding lands was being reduced. In Lake Apopka, the nutrient phosphorus declined 55 percent and water clarity improved 47 percent. In Lake Griffin, phosphorus declined 32 percent, and water clarity improved 41 percent. Some of these improvements in water quality may be due to shad harvesting and some may be due to reduction in nutrients entering the lakes.

How harvests are conducted

Iced fish are loaded for shipment to a processing plant.

The District hires commercial fishing vendors to net shad. Other fish such as bowfin, gar and tilapia are also harvested when caught in the nets, while sport fish are immediately released. Harvested fish are unloaded from boats at a landing site temporarily constructed on the lake shore. Each boat’s catch is weighed, iced and transferred to a refrigerated trailer truck for transport to a processing plant. The fish are sold as bait for crayfish and crab traps.

The harvest season runs from December to April, when the fish are easiest to catch because they school more. When the season is over, the fishery and the District dismantle the site and the District restores the area to its previous condition.

Updated on 11-3-2011