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 delivering other ecosystem benefits. Shellfish such as oysters and clams improve water quality by filtering nutrients and storing them in their tissues. Sustainable harvesting of fish and shellfish removes those nutrients from the environment. Further research is needed to quantify these benefits. Regional partnerships could support the implementation of pilot projects.

Importance

Fish and wildlife play a vital role in nutrient cycling. Nitrogen absorbed by phytoplankton and other primary producers is transferred up the food chain and stored in animal biomass (bioassimilation). Bivalves can filter and absorb nutrients from large volumes of water. A Chesapeake Bay Program meta-analysis found that oysters bioaccumulate nitrogen in their tissues and shells (about 8.4% of dry weight). Although variable, oyster reefs have been linked with denitrification rates 1.5 to 14 times higher than sites without oysters (STAC, 2013). One acre of restored oyster reef in Chesapeake Bay removed up to 543 pounds of nitrogen annually (Kellogg et al., 2013). When fish and wildlife bioassimilate nutrients, those nutrients are no longer available in the water to fuel algal blooms. Migratory species, such as fish and birds, can also export nutrients out of tidal creeks and estuaries (Figure 8.2.1). Likewise, harvesting fish removes nutrients from the system.

Conversely, excess nutrients fuel harmful algal blooms that deplete oxygen and release lethal toxins, harming marine life. Mass die-offs of aquatic organisms release additional nutrients, fueling subsequent blooms. Swift cleanup of dead marine life during algal events could eliminate one of the largest nutrient sources for red tides (Shafer and Shafer 2020).

Overview

Bivalve Stock Enhancement

Clam restoration in Sarasota Bay is led by Sarasota Bay Watch (SBW) in partnership with Mote Marine Laboratory and FWC’s Fish and Wildlife Research Institute. SBW organizes citizen-science monitoring and enhancement efforts from the north end of Roberts Bay to Sister Keys. While early restoration focused on scallops, red tide events prompted a pivot to red tide-resistant southern hardshell clams. In collaboration with Gulf Shellfish Institute and Bay Shellfish Company, clam larvae are raised to 2 inches before release. From 2018–2022, about 1.5 million clams were released and demonstrated resilience to the 2018 red tide. Monitoring by New College of Florida, Gulf Shellfish Institute, and Mote Marine Laboratory will evaluate survival, growth, and ecological impact. In 2022, SBW established a five-year submerged shellfish lease to reduce transport costs and labor.

In 2022, HB 9161, the Clams and Seagrass Restoration Bill, was passed thanks to advocacy by the Gulf Shellfish Institute and its “All Clams On Deck” program. The bill allocated $2.5 million for large-scale clam and seagrass restoration projects in Sarasota Bay, Tampa Bay, and Charlotte Harbor.

Solutions To Avoid Red Tide (START) partners with Manatee County and 11 restaurants (including the Chiles Restaurant Group) to recycle oyster shells into new reefs. Their Gulf Coast Oyster Recycling and Restoration Program has collected 80 tons of shells, creating hundreds of feet of oyster reef in Robinson and Perico Preserves.

SBEP has restored several acres of oyster reef by adding suitable substrates, attracting both oysters and fish species. Oyster River Ecology (ORE) has secured approval for a 10-acre oyster restoration aquaculture site in the Manatee River and helped Manatee County acquire an additional 6.2-acre site. Once cultch is placed, these areas could support 7 million oysters capable of filtering 175 million gallons of water daily—the first approved aquaculture-based oyster restoration sites in Florida.

Seawalls, once important oyster habitats, are increasingly made of vinyl or coated materials unsuitable for settlement. SBEP and Longboat Key are piloting a project to install concrete mangrove-like panels over vinyl seawalls to provide better oyster substrate.

Vertical Oyster Gardens (VOGs)—ropes of recycled oyster shells hung from docks—offer another innovative method. SBEP, Gulf Shellfish Institute, TBEP, and Manatee County supply VOGs to dock owners, while Sarasota County installs them at public facilities. Oyster Boys Conservation (OBC) has recycled over 20,000 lbs of shell since 2022, installing over 660 VOGs. In 2024 alone, OBC built 1,000 VOGs.

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

Wild fish harvest can also remove nutrients. In Lake Apopka, the St. Johns River Water Management District reported that removal of 21.5 million pounds of gizzard shad over 24 years eliminated 151,046 pounds of phosphorus and 454,845 pounds of nitrogen. Locally, commercial fishers harvest tilapia, shad, carp, and catfish from stormwater ponds for HOA clients. Fish are sold at $0.54 per pound for organic fertilizer (Sean Patton, pers com). One day’s harvest can remove 25 pounds of nitrogen. Sarasota County considers this a low-cost nutrient reduction strategy, particularly for targeting invasive species.

Fish removal during red tide events also offers nutrient reduction potential. Mote Marine Laboratory and Florida Gulf Coast University are studying composting of dead fish for fertilizer (see Chapter 4.7). Preliminary findings show this approach is economically feasible and effective at both eliminating one of the larger nutrient sources for red tides and facilitating rapid clean-up (Shafer and Shafer 2020). However, more research is needed—especially on brevetoxins, compost accelerators, and large-scale feasibility.

Mote’s aquaculture facility raises snook and mullet and monitors hatchery performance. Since the late 1990s, the FWC-FWRI and Mote partnership has tagged, released, and tracked snook in Sarasota Bay. Since 2021, thousands of juvenile snook have been released to assess habitat preference and survival. 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) before migrating to the bay and Gulf. Mullet and snook contribute to nutrient cycling and are valued by recreational and commercial fishers.

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). SBEP, FWC, and Mote manage 14 bay and 38 nearshore reefs. From 2021–2024, 330 reef balls were deployed to five reefs. In November 2022, 550 tons of limestone were added to the Bridge Reef site, with more planned in 2025.

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.

• Robert’s Bay CWA (est. 2016): Includes breakwater and mangrove islands that support nesting birds and oyster habitat.
• Myakka River CWA Protects islands used for nesting by threatened wood storks, herons, egrets, and anhingas.
• Dot Dash Dit CWA (est. 2016): Protects mangrove islands at the Braden River mouth. Hosts 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 plans to stabilize eroding shoreline with reef balls or oyster shells.

While their primary purpose is to protect nesting bird colonies, they also serve as de facto preserves. For example, the Robert’s Bay CWA is surrounded by seagrass meadow, which is indispensable for its role in securing sediments, cycling nutrients, and providing food and habitat for wildlife. Preserving recent gains in seagrass acreage (see Chapter 10.1), such as by maintaining protected areas, is vital to the health and function of our local ecosystems.

Approach

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, All Clams on Deck, 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 Oyster River Ecology for oyster reef restoration and Mote’s Fisheries Stock Enhancement Program for fish stocking and monitoring of growth, survival, and disposition of tagged snook and mullet released to tidal canals and connected ponds and marshes.

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.

Resources

Status

Implementation

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

Ryan Gandy, Staff Scientist, Sarasota Bay Estuary Program; Ernesto Lasso de la Vega, Sarasota Bay Watch;Dr. Stephen Hesterberg, Gulf Shellfish Institute; Dr. Angela Collins, Florida Sea Grant; Dr. Nicole Rhody, Manager for the Marine & Freshwater Aquaculture Research Program, Mote Marine Laboratory; Dr. Nate Brennan, 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; Damon Moore, Oyster River Ecology

Cost Estimate

$100,000-$1,000,000

Related Activities

Chapter 4.7, Chapter 9.2