Synchronous spawning of the core population of bay scallop s (Argopecten irradians ) in Florida was described by early studies of this species, and may be critical to population dynamics, but later research has shown that in some of the Florida subpopulations, settlement of spat is nearly continuous. S ettlement rates of bay scallops were monitored seasonally from 1992 through 1999 then almost continuously until 2017 in the open gulf and most of the enclosed bays along the west coast of Florida, after which the focus of the program switched to collection of wild harvest of juveniles for restoration. The principal methodology was deployment of artificial spat collectors (&fraq12;-bushel citrus bags ) for 6-8 weeks. Analysis of the long-term data set is still ongoing.
Spat settlement rate was analyzed with generalized linear mixed models using a negative binom ial distribution (spat data is zero-inflated) in R software. Primary environmental drivers for settlement models included temperature (T), salinity (S), chlorophyll (when available), turbidity and red tide (Karenia brevis ) concentration . Statistical models also included variations of these terms including non-linear factors (T2 and S2), the change in temperature during deployment ( ?T and ?T2) , and the change in temperature or salinity during the month preceding deployment (as an estimate of spawning cues; ?ST , ?ST2 , ?SS, ?SS2). The dependent factor in models was the number of spat settling to the collector. Deployment duration was included in all models.
T emperature, salinity, and temperature changes during presumed spawning period were all important contributors to the model. Temperature drops of -10 °C or more during the fall are predicted to produce large settlement events , presumably by serving as a strong spawning cue to the broodstock, either directly or as the waters warm after the passage of cold fronts. Temperature increases in the spring precede smaller settlement peaks. Climate change could result in t he loss of the coldest winter temperatures which could reduce synchrony in bay scallop spawning during the fall but increased intensity of weather systems could offset those changes. Frequency and intensity of red tide can both repress settlement, but red tides are infrequent in the regions where the core population persists. Intrusion of red tide to the core region , either increased frequency or intensity, could be devastating to the long-term viability of bay scallops in Florida.