During the past few months, the habitat of an important bivalve species, the Southern hard clam (Mercenaria campechiensis), has been evaluated throughout Charlotte Harbor and preparations are underway to begin a small-scale hard clam restoration pilot study.

 

Hard clams are large (up to six inches), round, white bivalves with prominent concentric ridges on the outer surface. “They play a vital role in estuarine ecosystems as filter feeders and in transferring energy throughout the food web,” said SCCF Research and Policy Associate Leah Reidenbach. “With the filter-feeding activity of clams, large populations can reduce phytoplankton and increase water clarity, which benefits seagrass leading to more habitat for invertebrates and fishes.” They also permanently remove nitrogen through excretion and benthic microbial denitrification in the sediment and through accumulation in the shells during growth. 

Prior to the 1960s, hard clams were abundant in Southwest Florida and were part of a healthy estuarine ecosystem, but populations were reduced through overharvesting. As broadcast spawners, low densities of hard clams make it difficult for populations to rebound to sustainable levels. Through restoration, hard clam densities may return to a level that allows populations to become established and self-recruit for population growth. 

 

Very little is known about the abundance and distribution of the natural population of hard clams throughout Charlotte Harbor, so a habitat suitability model was developed to map optimal hard clam habitat. Ultimately, the goal is to create harvest-free restoration areas or “spawner sanctuaries.” 

 

Research Associate Mark Thompson used long-term data on water quality in the Charlotte Harbor Estuary to create a habitat suitability map using a model based on multiple environmental factors that are critical for optimizing hard clam growth, survival, and reproduction. The map shows optimal habitat for spawner sanctuaries (average salinity greater than 25, a sandy bottom, and high food availability) and unsuitable habitats (anoxic, muddy areas, with an average salinity less than 12.5)

 

Based on the map, logistical factors, and discussions with local stakeholders, 10 potential restoration sites were selected and surveyed for the presence of hard clams. Adult hard clams were found at three of the sites at low densities. Now, SCCF is in the process of collaborating with local conservation groups to share our research and develop a statewide plan guiding restoration practices and projects.

 

In a separate but parallel project supported by the Sanibel Captiva Shell Club, SCCF will complete a small-scale pilot study to evaluate the effectiveness of hard clam restoration in Pine Island Sound. Twelve thousand hard clams will be planted at densities of approximately 13 clams m-2. The restoration site will be monitored for one year for changes in water quality, phytoplankton community structure, and hard clam survival, growth, and reproduction. The results from this project (starting March 2021) will provide important insight for future large-scale restoration projects.