Educational resources

Harvesting gizzard shad to improve water quality

As part of a multifaceted approach to restoring water quality in waterways, the St. Johns River Water Management District has been reducing the “rough fish” population in various waterways. Such work has proven beneficial in lakes Griffin, Denham and Apopka in central Florida, Lake George in the St. Johns River, and at Newnans Lake east of Gainesville.

In the early 1990s, district scientists discovered that removal of gizzard shad (Dorosoma cepedianum) helps improve the health of some waterways that are overrun by algae where the algae cloud the water and cause a sickly green color. Algae are present in all water bodies, 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 waterbody’s health and the fish habitats.

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

How nutrients affect Florida waters

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

How shad harvests may improve a waterway

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

Removing large numbers of bottom-feeding gizzard shad may improve water quality by reducing resuspension of sediments and recycling of nutrients from the bottom of the waterbody. Removing large numbers of shad from a waterbody also removes the nutrients in the fishes’ 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.

How harvests are conducted

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 temporarily constructed landing site on the water body’s shoreline. 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.

When the harvest is over, the fishery and the district dismantle the site and the district restores the area to its previous condition.