Florida has become a leading producer of hard clams in the nation with Cedar Key providing over 90% of the state’s production. Recently, clam growers have reported below average survivals or total losses during the prolonged hot summers. A variety of factors and stressors may contribute to these mortalities. However, diversity in farm location and operational scale has made it difficult to determine whether losses incurred can be traced to one or two over-arching causes, such as water temperature, or are the result of related or unrelated issues. In partnership with the clam aquaculture industry, a multi-step process is being implemented to begin to identify contributing factors associated with mortality events experienced by growers and develop a working relationship with research and extension faculty in conducting preliminary research and monitoring activities over the summer of 2024.
A survey of members of the clam aquaculture industry in Cedar Key was conducted to gain an understanding of crop losses, contributing factors, and research and monitoring efforts needed.
To facilitate a direct response to concerns of the clam aquaculture industry, researchers, extension specialists and agents with diverse expertise were brought together through a round table session to identify resources available or needed to begin to address clam mortalities.
Based on results of the growers’ survey and round table session, the following projects are being implemented during the summer in 2024 to gain preliminary information or augment existing activities with funding from Florida Sea Grant.
Investigator: Shirley Baker, University of Florida (UF)/IFAS SFFGS Fisheries and Aquatic Sciences Program
Objectives: This project aims to delve into the adaptive mechanisms of clams when exposed to acute thermal stress. The behavioral and physiological responses of hard clams to thermal stress will be investigated. A series of short-term experiments simulating field thermal stress will be conducted. By incrementally raising water temperatures and monitoring through respirometry, physiological reactions will be measured. From these observations, metabolic peak temperatures, critical thermal maxima, and thresholds for mortality will be established. The capacity for recovery from non-thermal stress will also be evaluated.
Investigator: Sarah Hutchins and Stephen Hesterberg, Gulf Shellfish Institute
Objectives: Hard clams that survived high thermal stress (>37°C) were spawned and are currently in the nursery stage. This project will monitor performance of the thermally exposed F1 progeny relative to conventional lines on commercial leases during the field nursery stage. Two clam farmers with leases in different environmental contexts will be provided with seed, supplies, and resources needed to monitor thermally selected clam performance compared to conventional stocks. Differences in performance will be evaluated on both low salinity and high salinity leases.
Investigator: Leslie Sturmer and Natalie Anderson, UF/IFAS Shellfish Aquaculture Extension
Objectives: By monitoring food availability, using chlorophyll as a proxy, in addition to increasing monitoring of water temperature and salinity at Cedar Key leases, insight on basic but key relationships between environmental factors and survival and health of cultured clams may begin. Chlorophyll sensors will be added to existing monitoring stations to assess phytoplankton. Additional conductivity loggers will be deployed at the Gulf Jackson and Pelican Reef AUZs expanding thermal and salinity surveillance. HOBO temperature loggers will also be provided to growers to record bottom temperatures. Growers can access real-time and archived data via the website, https://shellfish.ifas.ufl.edu/water-quality-monitoring/.
Investigator: Todd Osborne, UF/IFAS Soil, Water and Ecosystem Sciences Department
Objectives: Organic matter in estuarine sediments is highly variable across space and time and exerts significant influence over major biogeochemical processes, such as oxygen consumption and sulfide production. Increased temperature can exacerbate already low oxygen conditions (hypoxia) and buildup of toxic sulfide gases in sediments, ultimately controlling growth and survival of hard clams. In this study, sediment organic matter (SOM) will be measured over time at participating leases to enable the correlation of SOM and hypoxia or sulfide stress induced mortality during summer months. Six participating growers will sample sediments monthly on leases at harvest and the easement (control).
Investigator: Susan Laramore, Florida Atlantic University-Harbor Branch
Objectives: The objective is to determine how environmental changes seen during summer impact clam health by employing bacterial and histological analysis. Temperature has a known impact on the proliferation of bacterial and parasitic diseases in clams, while salinity is a factor in the proliferation of parasitic diseases. High temperature and salinity and low dissolved oxygen, typically seen in the summer increase stress and decrease immune response. This project will take an integrated approach by documenting the presence of bacterial and other disease-causing organisms in clams in response to environmental changes that occur during the summer months at Gulf Jackson and Dog Island lease areas. In addition, physiological condition, and responses, such as changes in the digestive tract tubules and presence of food in the gut will be evaluated. Harvest-size clams will be collected monthly at three different lease parcels within each lease area.
