Settlement of the Bay Scallop (Argopecten irradians) along the Gulf Coast of Florida

Before collapsing, bay scallops (Argopecten irradians) supported commercial fisheries in Florida but, following decades of restoration efforts and management actions, still support a recreational fishery. Settlement was monitored along the west coast of Florida from 1992 through 2018, using collector traps. The primary environmental variables retained in the analyses included temperature, salinity, and red tide concentration (Karenia brevis). The settlement rate generally increased for the first days of trap deployment, then leveled off or declined. Settlement peaked when the average water temperatures were between 21°C and 23°C at four sites: St. Andrew Bay, St. Joseph Bay, West Coast, and Pine Island Sound, indicative of a fall peak in spawning. At the Tampa Bay and Sarasota Bay sites, settlement peaked when temperatures were around 16°C, indicative of winter spawning. At most sites, the highest peak in settlement occurred in association with declines in temperature of 10°C–15°C during the fall, with a smaller, secondary peak occurring with similar increases in temperature during the spring. Warming winter temperatures may already be negatively affecting the ability of the subpopulations to synchronously spawn, with unknown impacts to population dynamics. At all sites, settlement declined rapidly when salinity fell below 30 with little settlement occurring below 27, and approached zero when salinity fell below approximately 20. The occurrence of K. brevis exceeding 10,000 cells/L resulted in reduced settlement. In the core populations of West Coast and Steinhatchee, K. brevis was uncommon, and settlement resumed rapidly when blooms abated. For sites outside the core population, settlement was reduced for 2–4 y after red tides. Recovery of populations in these noncore sites probably relies on exogenous supply of larvae from the core. If blooms of K. brevis that penetrate the core region, as was observed in 2021, become more severe, more frequent, or have longer durations, the entire Florida scallop population could be endangered.