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Life History of Bay Scallops Argopecten irradians in Florida Waters

BACKGROUND
In Florida, bay scallop populations that comprise the Argopecten irradians metapopulation (a population composed of smaller, isolated local populations) have historically occupied bays and nearshore waters from Palm Beach to the Chandeleur Islands in Louisiana. In recent decades that range, and the abundance of individuals comprising the populations occupying that range, has contracted considerably. Scallops are now rare or non-existent in southeast Florida and in areas west of St. Joseph Bay in the Florida panhandle. Moreover, areas such as Pine Island Sound, Sarasota Bay, Tampa Bay, and Pensacola, which once harbored very dense scallop populations, now support few if any individuals. Essentially, until 1999 only Steinhatchee and St. Joseph Bay harbored healthy scallop populations.

LIFE HISTORY
Bay scallops occur in discrete “local” populations that are isolated from neighboring populations by areas of inhospitable habitat (e.g., salinity < 20 psu, no seagrass). Most members of each population only live for 12–18 months; although, a small proportion of each year class may survive for 24 months or more. Thus, each population must replace itself, or receive offspring from neighboring populations sufficient to replace itself, each year. Because individual scallops only live for about one year, population fluctuations are extreme and the collapse of local populations is a natural feature of bay scallops in Florida. As a result, the stability of scallop populations in Florida is realized at the metapopulation level, not the local level. That stability derives from the existence during any year of multiple local populations throughout the state; although, the location of those individual populations may change from year to year. It is reasonable to assume that increasing the number of healthy local populations extant during any year increases the stability of the metapopulation.

In Florida waters, bay scallops appear to spawn only once, generally during fall; although, recruitment monitoring by FMRI staff indicates settlement of larvae beginning in August of each year and extending through May of the following year. Larval scallops are pelagic (living in the water column rather than on the bottom) for 10–14 days. During that time they may be dispersed a considerable distance from the source population. The pelagic dispersal phase connects local populations and is the critical link in maintaining the metapopulation. Any local population that becomes disconnected from this linkage will eventually become extinct unless that linkage is reestablished.

POPULATION MONITORING
To assess the status of bay scallops in Florida waters, we conduct adult population surveys during June of each year at various sites along the Florida west coast. Survey sites include Pine Island Sound in Charlotte Harbor (PIS), Anclote Anchorage near Tarpon Springs (ANC), the coastal waters of Pasco and Hernando Counties (HER), coastal waters in the vicinity of the Homosassa and Crystal Rivers (HOM), the Steinhatchee area (STN), St. Joseph Bay (SJB), and the Crooked Island Sound/St. Andrew Bay estuarine complex (SAB). At each site, we conduct transect surveys at 20 randomly located stations. Each transect survey consists of paired scuba divers swimming either side of a 300-m transect line and counting all scallops within 1 m on each side of the line. Thus, we count scallops within a 600 m² area at each station, and we sample a 12,000 m² area at each site. Surveys have been conducted since 1994 at most sites; although, we did not begin sampling the Hernando area until 1997 (Figure 1).

Using the results of our transect surveys, we apply the following criteria to determine the health of a local scallop population:

1) Abundance: In a healthy scallop population, mean population density generally exceeds 25 scallops per 600 m² transect. Mean population density below 5 scallops per transect generally suggests a collapsed population. Mean density between 5 and 25 scallops per transect suggests a transitional population.

2) Distribution: Scallops should be widely distributed throughout the sample area. We sample 20 stations at each study site, and we expect to recover scallops from a minimum of 10 of those stations in a healthy population.

3) Resilience: Even healthy scallop populations fluctuate in abundance from year to year, but a healthy population should recover from a low point within one or two years. Good examples of resilient populations include Anclote and Steinhatchee, but the resilience of the St. Joseph Bay population is currently unknown. (Figure 1)

MANAGEMENT
Prior to 1985 there were no statewide regulations regarding the harvest of bay scallops in Florida waters; although, local rules applied in Bay, Gulf, and Pinellas Counties. In 1985, the Florida Marine Fisheries Commission instituted statewide regulations (Rule 46-18), governing the harvest of scallops and simultaneously repealed all local rules. Those statewide regulations included a closure of the fishery between April 1 and June 30 of each year and a recreational bag limit of five gallons of whole scallops, or ½ gallon of meat per person per day. Limits were also placed on the size and number of drags that could be used by vessels harvesting scallops for commercial purposes, and restrictions were placed on commercial harvest in the south end of St. Joseph Bay.

In 1994, there was an emergency closure of the bay scallop fishery in response to the perceived collapse of bay scallop populations in many areas of Florida. All state waters were closed to commercial harvest, and the recreational fishery was closed south of the Suwannee River. Additionally, the length of the recreational harvest season was reduced to three months (July–September). A formal modification of bay scallop harvest regulations, which continued both the commercial and recreational closure south of the Suwannee River, was instituted in 1995. The 1995 regulations also included a more restrictive individual bag limit (two gallons of whole scallops or one pint of meat per person per day), a boat limit of 10 gallons of whole scallops or ½ gallon of meat per boat per day, and a further reduction of the season to July and August of each year. In 1997, the season was lengthened by 10 days (July 1–September 10); otherwise, the 1995 regulations remain in effect.

RESTORATION
The goal of the 1994 bay scallop management modifications is restoration of local populations in an effort to further stabilize the metapopulation. To achieve this objective, between 1997 and June of 2002, FMRI scientists conducted a federally funded program to restore bay scallop populations in Florida waters. FMRI’s initial efforts targeted the most recently collapsed scallop populations, those between Anclote and Crystal River, because those populations were in close proximity to the relatively healthy Steinhatchee population.

A resurgence of scallop populations has been observed in the area from the Weeki Wachee River north to Crystal River. From 1993 through 1996, the Homosassa scallop population averaged less than eight scallops per 600 m² transect during June of each year and density decreased during each successive year (Figure 1). A slight, 15 scallops per transect, resurgence in scallop abundance was recorded in 1997. Numbers again fell during 1998, probably as a symptom of degraded coastal water quality during that year. Scallop abundance increased substantially during 1999 (29/transect), 2000 (243/transect), and 2001 (299/transect). Genetic evidence and the initial increase in abundance observed during 1997, prior to any restoration activities, suggest that management rather than restoration may have fundamentally influenced this resurgence. Regardless, there are substantially more scallops in the area between Weeki Wachee and Crystal River than were observed during the 1990s.

Prior to July 1, 2004, the Fish and Wildlife Research Institute was known as the Florida Marine Research Institute.