8.2 Enhance fish and wildlife populations to increase nutrient uptake and biomass storage by animals

Key Message: Restoration of fish and wildlife through stocking programs and habitat creation or protection can directly and indirectly contribute to nutrient removal while providing other ecosystem benefits. Shellfish such as oysters and clams directly improve water quality by filtering nutrients and storing them in their bodies. Harvesting fish and shellfish, when appropriate, removes those nutrients from the environment. More research is needed to quantify these benefits. Regional partnerships could foster implementation of pilot projects.


Fish and wildlife play an important role in ecosystem nutrient cycling. Nitrogen absorbed by phytoplankton and other primary producers can be transferred up the food chain and stored in animal biomass (bioassimilation). Bivalves can filter and absorb nutrients from large quantities of water. A meta-analysis by the Chesapeake Bay Program found that oysters can bioaccumulate nitrogen in their soft tissue and shells (about 8.4% of dry weight). Although the effect is highly variable, oyster reefs are associated with denitrification rates of 1.5 to 14 times higher than sites without oysters (STAC, 2013). One acre of restored oyster reef was shown to remove up to 543 pounds of nitrogen pollution per year in Chesapeake Bay (Kellogg et al., 2013). Fish and birds that migrate as part of their life cycle can transport nutrients from nursery areas like tidal creeks and estuaries out of those systems (Figure 8.2.1). When fish and wildlife remove nutrients from the environment and store them in their bodies, they take those nutrients off the menu for algal blooms.

Figure 8.2.1. Nitrogen absorbed by plants is transferred up the food chain and stored in fish biomass, then can be exported via fish migration or harvest. Source: Janicki Environmental

Restoring populations of fish and wildlife also provides a host of other co-benefits to ecosystems and humans. Fishing provides recreational opportunities and food, while removing nutrients from the system. Conversely, excess nutrients fuel harmful algal blooms that can devastate fish and wildlife by depleting oxygen and/or releasing lethal toxins in the water. When aquatic organisms die in mass, more nutrients are suddenly released to the system, fueling more algal blooms. Swift cleanup of dead marine life in waterways and along shorelines during algal blooms would eliminate one of the larger nutrient sources for red tides.


Bivalve Stock Enhancement

Restoration of shellfish populations in Sarasota Bay is led by Sarasota Bay Watch (SBW), in partnership with Mote Marine Laboratory and Florida Fish and Wildlife Conservation Commission’s Fish and Wildlife Research Institute (FWC-FWRI). SBW organizes enhancement and citizen-science monitoring to restore shellfish stocks from the north end of Roberts Bay to Sister Keys. Initially, the restoration largely focused on scallops. However, poor survival rates exacerbated by red tides prompted a pivot to southern hardshell clams. In partnership with Gulf Shellfish Institute and Bay Shellfish Company, clam larvae are raised to a size of two inches before being released into Sarasota Bay. To date, roughly 217,000 clams have been released and demonstrated resilience to the 2018 red tide. About 160,000 large adult clams were released as part of the Bradenton Beach Pier Restoration Project. SBW plans to release an additional 1,000,000 clams in 2020-2021; monitoring of survival, growth, filtration, and overall impact will be conducted by New College of Florida, the Gulf Shellfish Institute, and Mote Marine Laboratory. SBW is exploring a submerged shellfish lease to grow clams in Sarasota Bay to reduce costs and labor associated with transporting farmed clams from Boca Grande.

Sarasota Bay Watch volunteers load clams onto vessels for release into Sarasota Bay. Source: Sarasota Bay Watch

The Sarasota Bay Estuary Program (SBEP) has restored and created several acres of oyster reefs by creating new substrates where oysters are likely to thrive. These reefs have attracted both oysters and fish, including jenny mojarras (Eucinostomus gula), scaled sardines (Harengula jaguana), Atlantic silversides (Menidia menidia), and Atlantic thread herring (Opisthonema oglinum).

A volunteer shovels shell to be bagged and stacked in shallow water to create oyster bars. Source: Tampa Bay Estuary Program

The non-profit Solutions To Avoid Red Tide partners with Manatee County Parks and Natural Resources and 11 local restaurants (including the Chiles Restaurant Group) to recycle oyster shells into new reefs. The Gulf Coast Oyster Recycling and Restoration program has collected 80 tons of leftover shells from partnering restaurants, building hundreds of feet of oyster reefs within Robinson and Perico Preserves. Another public-private partnership is working on legislation to add native clam restoration as an approved mitigation technique to offset impacts from dredging or coastal development projects.

As filter-feeders, oysters remove particles containing nitrogen from the water column and use them to build up their tissues and shells, or transfer the nitrogen to bottom sediments in their waste (Newell et al., 2005, Pollack et al., 2013). This bioassimilated nitrogen could, in theory, be harvested from Sarasota Bay. In practice, harvesting is not a goal of local shellfish enhancement programs, because populations are already depleted, and consumption of locally harvested shellfish is largely prohibited due to bacterial pollution. For harvesting to be considered as a nutrient removal option, sustainable oyster or clam populations must first be restored.

Fish stock enhancement and harvesting

Similarly, harvest of wild fish can remove the nutrients assimilated in their tissues. Scientists with the St. John’s River Water Management District demonstrated that the removal of 21.5 million pounds of gizzard shad (Dorosoma cepedianum) from Lake Apopka by anglers over a 24-year period eliminated an estimated 151,046 pounds of phosphorus and 454,845 pounds of nitrogen from the lake. The fish are processed and shipped to Louisiana as food for crayfish farms. Locally, commercial fishers harvest tilapia, shad, carp, catfish, and other fishes from stormwater ponds on behalf of HOAs. The harvested fish are sold for $0.54 per pound to processors who produce organic fertilizer (Sean Patton, pers com). Commercial fishers can net more than 1,000 pounds of fish per day, removing an estimated 25 pounds of nitrogen. Sarasota County Stormwater Environmental Utility has identified beneficial harvest of fish from stormwater ponds and canals as a low-cost nutrient reduction strategy. Exotic and invasive fish species should be targeted for removal rather than native species.

Prompt removal of dead marine life from waterways and along shorelines during algal blooms is a critical and significant opportunity to “harvest” biomass. Researchers at Mote Marine Laboratory and Florida Gulf Coast University are investigating the composting of dead fish for fertilizer as a potential mitigation strategy during red tides (see Chapter 4.7). Removal of the decaying fish would eliminate one of the larger nutrient sources for red tides and facilitate clean-up. A critical research question is the fate of brevetoxins during the decomposition process and whether compost accelerators can make the process more efficient.

Dead marine life piles up on beaches and clogs canals during severe red tide events. Source: Terry Ross

Mote Marine Laboratory’s aquaculture facility raises snook and mullet from eggs collected from wild populations and studies spawning, larval production and survival, and juvenile production. The Fisheries Stock Enhancement Program, a partnership between FWC-FWRI and Mote Marine Laboratory, has been tagging, releasing and tracking survival of hatchery-raised juvenile snook in Sarasota Bay since the late 1990s. Research demonstrates that hatchery-raised fish can survive into adulthood and contribute to wild populations.

Recently, Mote has begun releasing hatchery-raised juvenile snook into local tidal creeks and canals as part of an effort to understand habitat preference and survival. Findings are driving discussion between Mote researchers and the Sarasota County Stormwater Environmental Utility about stocking stormwater canals and ponds directly connected to tidal creeks with juvenile snook and mullet to increase nutrient removal via biomass growth. Snook and mullet use the upper reaches of tidal creeks and marshes as protected juvenile habitat where their weight can increase 1,000-fold (Locascio et al., 2017). Adults then migrate to bay and gulf waters where mullet are harvested by commercial net fishers and snook are popular catch and release targets for recreational anglers.

Fish Take Nutrients Off The Menu for Algae
As they grow, fish eat plants or small animals and store those nutrients. If the mature fish then migrates or is harvested those nutrients are exported out of the ecosystem (or at least take the long way around the nitrogen cycle), ultimately removing nutrients from the water and off the menu for algae.

Artificial reefs enhance fisheries in Sarasota Bay and nearshore Gulf waters where the sandy bottom is otherwise dominated by small benthic invertebrates. They can help compensate for historical losses of hard bottom habitat due to dredge and fill operations. A 2006 study of four reefs in Sarasota and Tampa bays showed that artificial reefs appeared to promote recruitment and retention of fish and invertebrates. Fish abundance was positively correlated to reef size (Blackburn et al., 2008). Artificial reefs are popular with anglers and may serve an important role in nutrient biomass removal. SBEP, FWC, and Mote Marine Laboratory partner to deploy and monitor artificial reefs in area waters. To date, 14 bay and 38 nearshore artificial reefs have been installed. New reef material is frequently added to these sites after initial deployment to enhance habitat.

Wildlife Protection Areas

Protected areas where fish and wildlife cannot be disturbed or harvested are effective management tools that increase population sizes within their boundaries, while generating an overflow effect that enhances regional populations. FWC has designated 34 Critical Wildlife Areas (CWAs) in Florida, two in Sarasota County and one in Manatee County. While their primary purpose is to protect nesting bird colonies, they also serve as de facto preserves.

  • Robert’s Bay CWA was established in 2016 in Sarasota County. A breakwater was installed around the mangrove islands in 2008 by SBEP, Sarasota County, SWFWMD, Audubon Florida, and Jacksonville Community Trust to keep boaters at a distance from the bird rookery and help restore habitat. The 3-acre project also provided substrate for oysters and other species.
  • Myakka River CWA was established in 1987 in Sarasota County. It protects one acre of small islands that serve as a wood stork rookery as well as habitat for herons, egrets, and anhingas.
  • Dot Dash Dit CWA was established in 2016 in Manatee County. Audubon Florida manages these three mangrove islands at the mouth of the Braden River, which host Tampa Bay’s only coastal colony of wood storks. The islands also support roseate spoonbills, great egrets, great blue herons, and other bird species. SBEP has prioritized installation of a breakwater of oyster shells or reef balls to stabilize the northwestern side of the islands and slow erosion.


More research is needed on the viability of stock enhancement of shellfish and fish for nitrogen storage and removal. Survival and growth data will help determine the best age for stocking and potential harvest, which will in turn determine methods and cost effectiveness. A regional collaborative on shellfish restoration could identify and expand effective methods and include FWC-FWRI, the Gulf Shellfish Institute, SBW, SBEP, CHNEP, The Nature Conservancy, and the Southwest Florida Oyster Working Group (see Chapter 9.2).

Local funding support could be directed to the Gulf Shellfish Institute and Sarasota Bay Watch to continue a two-year study on growth and survival of clams released to Sarasota Bay. Similarly, funding support could be directed to Mote’s Fisheries Stock Enhancement Program to determine growth, survival, and disposition of tagged snook and mullet released to tidal canals and connected ponds and marshes.

A fish harvesting pilot project could be conducted in Venice Gardens to determine the cost-effectiveness of harvesting non-native fish for nutrient removal. Fish harvest (cleanup and possible composting) from waterways and beaches during red tide events should be prioritized and included in emergency response protocols, potentially employing commercial fishing vessels that are otherwise out of operation.

Additional marine protected areas could be established in the region to protect juvenile fish and shellfish habitat, including seagrass and coastal wetlands. The Jim Neville Preserve in Roberts Bay and the Sister Keys in Sarasota Bay are potential candidates as some protections have already been secured by community stakeholders, and restoration and enhancement activities are underway.





Performance Measure

  • Research and data on the methods and cost effectiveness of local stock enhancement
  • Regional collaborative on methods for shellfish (clams and oysters) population enhancement

Experts or Leads

Jay Leverone, Staff Scientist, Sarasota Bay Estuary Program; Ernesto Lasso de la Vega, Sarasota Bay Watch; Sandy Gilbert, Solutions to Avoid Red Tide, Dr. Bruce Barber, Gulf Shellfish Institute; Laura Geselbracht, Senior Marine Scientist, The Nature Conservancy; Dr. Kevan Main, Manager for the Marine & Freshwater Aquaculture Research Program, Mote Marine Laboratory; Dr. Kenneth M. Leber, Associate Vice President for Research and Program Manager for Fisheries Ecology & Enhancement, Mote Marine Laboratory; Sean Patton, Stocking Savvy; Chris Young, Florida Fish and Wildlife Conservation Commission, Stock Enhancement Research Facility

Cost Estimate


Related Activities

Chapter 4.7, Chapter 9.2



Other Habitat and Wildlife Activities

8.3 Support large-scale land conservation programs

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8.4 Support urban forestation programs

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