Bay Scallop Monitoring
To assess the status of bay scallops in Florida waters, Fish and Wildlife Research Institute (FWRI) scientists conduct adult population surveys each June along the state's west coast. Researchers monitor 10 sites from Pine Island Sound to St. Andrew Bay. At nine of the 10 sites, they survey 20 stations (10 stations at the other site) located in seagrass beds in depths up to 10 feet. At each station, researchers deploy a 300-meter (984.3 feet), weighted transect line. Two divers – one on either side of the line – each count all scallops within a meter-wide (3.3 feet) area along the line, for a total survey area of 600 square meters (1,968.5 square feet).
Divers measure the first 30 scallops at each station to determine the average size of the population. Researchers compare estimates between years and sites to determine if bay scallop populations are maturing at different rates. Using the results of transect surveys, researchers can determine the health of a local scallop population based on the following criteria.
- Abundance: Healthy scallop populations must have a certain scallop density (number per area) in order to increase the chance of successful reproduction. For each local population researchers calculate the average number of scallops per square meter and classify abundances as: Collapsed, 0 - 0.01 scallop per m2; Vulnerable, 0.01 - 0.1 scallop per m2; Stable, 0.1 – 1 scallop per m2
- Distribution: Healthy populations have scallops widely distributed throughout seagrass beds. For each local population, researchers calculate the percentage of stations that have 60 or more scallops (equal to an abundance of 0.1 scallop per m2) and classify distributions as: Sparse, less than 25 percent; Patchy, 25-50 percent; or Dense, greater than 50 percent. Sites with dense distributions generally have higher numbers of scallops the following year.
- Resilience: Scallop populations fluctuate in abundance from year to year, but a healthy population should recover from a low point within one or two years.
Results of the adult population monitoring data can be found in the most recent 2015 Florida Bay Scallop Annual Report.
In addition to monitoring the local adult populations, scientists study juvenile bay scallops as they recruit to, or settle into, the population. Most juveniles come from the local population where they were spawned, but some come from distant populations, relocated by the ocean currents. To study the recruitment of bay scallops to local populations, scientists use a simple but effective method.
They anchor citrus bags stuffed with black mesh to a block (pictured left) to collect juveniles. The collectors are deployed every month in the nearshore, grassy habitats found along Florida’s Gulf coast. They simulate grass blades, and juvenile bay scallops, called spat, settle out of the water column and attach to collectors using small fibers called byssal threads.
Researchers leave the collectors underwater for eight weeks. The location of each is marked by a surface float. They then retrieve the collectors and take them to the lab for processing. Researchers count any scallops found on the collectors. They standardize data by dividing that count by the number of days the collector was in the water.
Researchers then average the counts from each collector to determine the recruitment rate for each deployment period. Scientists use average recruitment rates to compare local populations, determine timing of spawning events and evaluate the health of a population over time.