Aquaculture America 2023

February 23 - 26, 2023

New Orleans, Louisiana USA


 Meghan Keough, Jade Salis, David Bradshaw, Richard Baptiste, and Susan Laramore*

 Harbor Branch Oceanographic Institute at Florida Atlantic University

 Aquaculture and Stock Enhancement Program

 5600 US 1 North

 Fort Pierce, FL 34946 USA


The land based IMTA system located at Harbor Branch (HBOI-LB-IMTA) utilizes various fed, assimilative and extractive components. Good extractive candidates for these systems include bivalves. As filter feeders, they cycle nutrients by removing particulates from the water and storing nitrogen and carbon. The hard clam, Mercenaria mercenaria, is the largest food aquaculture industry in Florida, and therefore the natural choice for HBOI-LB-IMTA system. Maintaining a stable environment is important for consistent production, yet this system is subject to large pH swings (e.g., 7.4 to 8.2 over 24 hours). These fluctuations may lead to stress resulting in decreased production, and low pH has been shown to have a detrimental effect on larval bivalves. To better understand the impact of pH on post-set clam production and health, a short-term eight-week study examined the effects of stable (low, high) and fluctuating pH levels in the LB-IMTA system.

Three-week-old clam post-set (1,200+75, 537+85 µm) were randomly assigned to one of three treatments, 8.0, 7.4 or fluctuating (system) pH (N=5 replicates/treatment).  The pH in the 7.4 and 8.0 treatments was maintained with Pinpoint pH controllers 120 Vac, a CO2 tank and MP810 dosing pumps to deliver a soda ash solution. Clams were fed a 1:1 ratio of Tisochrysis lutea and Chaetoceros neogracile twice daily (50,000 cells/mL initial rate). Dissolved oxygen, temperature, and salinity were monitored daily and pH continuously, via a PASCO wireless sensor. Ammonia, nitrite, and alkalinity were measured weekly. Growth was evaluated bi-weekly, and survival at experimental termination. Condition index, bacterial analysis (Marine agar, TCBS), and shell composition were evaluated at experimental termination.

Significant differences (P<0.05) were seen in growth at weeks 6 and 8, with lower growth at pH 7.4 compared to pH 8 and the fluctuating pH treatments. Survival was similar between treatments (P=0.126). No difference was seen in the condition index. Total bacterial (P<0.05) and Vibrio (P<0.012) counts were higher at pH 7.4 and the percentage of colony types varied between treatments. Clams from all three treatments showed high levels of calcium, strontium, and sodium, with higher calcium and strontium levels in clams subjected to fluctuating pH levels.

These results indicate that post-set clams can be successfully grown under the fluctuating pH levels inherent in the LB-IMTA system, with no short-term negative effects compared to clams maintained at a stable pH of 8.0. These results likewise have implications for wild or cultured clams in estuarine ecosystems that may be subjected to short-term fluctuations in pH due to natural processes or human impacts and confirm that clams subjected to low pH experience negative impacts as have been reported previously.