Protracted recruitment in the bay scallop Argopecten irradians in a West Florida estuary.
Scallops (Environmental aspects)
Estuaries (Environmental aspects)
Spawning (Environmental aspects)
Geiger, Stephen P.
Stephenson, Sarah P.
Arnold, William S.
|Publication:||Name: Journal of Shellfish Research Publisher: National Shellfisheries Association, Inc. Audience: Academic Format: Magazine/Journal Subject: Biological sciences; Zoology and wildlife conservation Copyright: COPYRIGHT 2010 National Shellfisheries Association, Inc. ISSN: 0730-8000|
|Issue:||Date: Dec, 2010 Source Volume: 29 Source Issue: 4|
|Topic:||Event Code: 310 Science & research|
|Product:||Product Code: 0913070 Scallops NAICS Code: 114112 Shellfish Fishing SIC Code: 0913 Shellfish|
|Geographic:||Geographic Scope: United States Geographic Code: 1USA United States|
Many studies have indicated that annual spawning of bay scallops
(Argopecten irradians) peaks during discrete and limited periods each
year. Spawning in most Florida bay scallop subpopulations has been shown
to occur in fall, whereas more northerly U.S. populations typically
spawn in late spring or summer. In this article we describe our efforts
to expand our understanding of the seasonality of bay scallop
recruitment dynamics in Florida. Visual surveys were conducted by divers
each spring from 1994 to 2009 to estimate adult scallop abundance. Adult
abundance was low (6.1 scallops/600 [m.sup.2]) during the first 3 y of
the study (1994 to 1996), prompting a 7-y restoration effort intended to
enhance the number of spawning adults and thereby enhance the local
production of larval scallops. Adult abundances increased to an average
of 21.9 scallops/600 [m.sup.2] in 1997 to 2006, and then rose
dramatically to an average density of 154.8 scallops/600 [m.sup.2] in
the most recent years (2007 to 2009). Artificial recruit collectors (n =
12) were deployed monthly near the Anclote River estuary beginning in
1997 and were allowed to soak for 2 mo at a time. Each collector's
deployment period overlapped with the preceding and following
trap's deployment period by 1 mo. The project is ongoing, but only
data collected through December 2009 is included here (bay scallops
recruited to the collectors during 163 of the 185 deployment periods).
For the entire study period, the average recruitment rate was 0.3
scallops/collector/day, the maximum average for a single deployment
period was 5.5 scallops/collector/day during November 2001 to January
2002, and the highest rate for a single collector was 19.6
scallops/collector/day during November 2001 to January 2002. In most
years, the collectors retrieved in late fall and early winter had the
highest settlement rate; a secondary recruitment peak was observed in
the spring. A period of protracted recruitment (December 2005 to
December 2009) occurred, during which scallops recruited to at least 1
of the 12 deployed collectors deployed during 53 consecutive deployment
periods. The average recruitment rate for this protracted period was 0.4
scallops/collector/day; the maximum recruitment rate for a single
deployment period (3.5 scallops/ collector/day) and individual collector
(17.7 scallops/day) occurred during December 2008 to February 2009.
Early in our study, scallops were detected in a majority of our recruit
collectors, and a protracted period of recruitment (October 2001 to
February 2004) coincided with the multiyear restoration effort. However,
the recent high adult densities and protracted period of recruitment
occurred in the absence of any active restoration in this subpopulation,
suggesting that, at least within the Anclote River estuary, the
population has stabilized for the short term.
KEY WORDS: bay scallop, Argopecten irradians, recruitment, Florida
The bay scallop is distributed as at least 3 subspecies along the U.S. east coast from the waters of the Atlantic Ocean near Cape Cod, MA, through at least New Jersey (Argopecten irradians irradians Lamarck, 1819), south from North Carolina throughout Florida waters (A. irradians concentricus Say, 1822), and west to the Texas and Mexico waters of the western Gulf of Mexico (A. irradians amplicostatus Dall, 1898). A fourth putative subspecies, A. irradians taylorae, originally described from the Florida Keys (Petuch 1987), was shown to be similar morphologically and genetically to other Florida subpopulations and is probably invalid (Marelli et al. 1997). Florida subpopulations are currently limited to bays and coastal seagrass beds, from Florida Bay in the south to Pensacola Bay in the northwest. The bay scallop has not been observed recently along Florida's Atlantic coast, and the westernmost Florida population in Pensacola Bay may be present only sporadically, possibly relying on an exogenous supply of larvae from more stable populations.
In Florida waters, bay scallops typically live 12-18 mo. They are thought to spawn once, as water temperatures decline in the fall, and generally the larval scallops are pelagic for 10-14 days (Loosanoff & Davis 1963, Sastry 1965, Castagna & Duggan 1971). During that time they may be dispersed a considerable distance from the source population. The larvae develop into pediveligers (larvae that have developed a foot) and then settle to the bottom, where they attach to seagrass blades. After a few days the pediveligers grow into juvenile scallops, or spat, and gradually move up the seagrass blades out of the reach of bottom-dwelling predators. When the juveniles are large enough, about 20 mm in shell height (SH; defined as the linear distance from the ventral margin of the shell to the center of the hinge), they detach from the grass blades and fall to the bottom, where they spend the rest of their lives. They can move small distances using their large adductor muscle to clap their shells together rapidly, expelling water and scooting either backward or forward through the water.
Reproduction of bay scallops typically occurs in midsummer in northeastern U.S. populations, toward the late spring or early summer in mid-Atlantic waters, and is thought to peak in fall or early winter in Florida (Barber & Blake 2006), although bimodal peaks in reproduction have been reported in New York (Tettelbach et al. 1999). One method that has been used to assess the reproductive activity of bay scallops is through the examination of the gonad. Florida bay scallops undergo somatic growth during the spring and summer, after which most individuals are believed to have a decline in somatic growth and increased rate of gonadal development in late summer (Barber & Blake 1981, Barber & Blake 1985, Geiger et al. 2006). Concurrent with increases in size of the gonad, oogenesis begins, and spawning occurs in the fall, resulting in declines in individual biomass and the mortality of broodstock (Barber & Blake 1981, Barber & Blake 1983). Spawning in Florida populations is thought to occur in a strong fall peak, when water temperatures begin to decline, which can begin as early as August (Sastry 1963) and continue through fall (Barber & Blake 1981, Barber & Blake 1983) and into winter (Bologna 1998). This spawning event is in general followed by widespread mortality, which occurs after energetic reserves have been transferred from the muscle and digestive gland to reproductive output (Barber & Blake 1981, Barber & Blake 1985, Geiger et al. 2006). However, evidence suggests that spawning may also occur at other times of year. Bologna (1998) conducted histological examination of captured scallops and reported that there was a protracted period (August to May) during which scallop gonads were reproductively ripe and a similarly long period (November to August) during which the scallop gonads were in postspawn condition.
A second method of assessing reproductive activity in bay scallops is through recruitment monitoring. Studies of recruitment in Florida have focused on the estuarine and coastal systems from the Anclote River northward through the Big Bend region, and west to St. Joseph Bay. Arnold et al. (1998) reported that at least some level of recruitment was observed on larval eollectors deployed in the Anclote River estuary as early as September (early fall) and those retrieved as late as mid February (late winter). Bologna (1998) detected scallop recruits (<20 mm) in St. Joseph Bay, FL, during winter, spring, and summer, and recently settled spat (<4 mm) in July to September, clearly indicating summer spawning. Geiger et al. (2006) found that large fall peaks in recruitment occurred in Anclote, Homosassa, and St. Joseph Bay, but also, observed that a low level of recruitment occurred at each site during most of the nonpeak months of the 2-y study. During a long-term study, we collected evidence suggesting that recruitment commonly occurs outside the presumed temporal window of spawning activity (Arnold et al. 2009). In this study we present further evidence that, within a localized area of west central Florida--the Anclote River estuary recruitment can be continuous and has been observed over 2 protracted periods. Traditional peaks in recruitment during the fall are evident, as are smaller peaks during the spring.
Scientists from the Fish and Wildlife Research Institute (FWRI) monitor bay scallop populations in several estuaries along Florida's gulf coast. Seagrass beds available to the Anclote subpopulation (Fig. 1) cover approximately 405 k[m.sup.2] and extend roughly 40 km north to south and 5-10 km east to west. The area is fed by 2 rivers (the Anclote River and the Pithlachascotee River) and is partially enclosed by a series of disconnected barrier islands. The area described in this study is closed to harvest of bay scallops but adjoins the Homosassa and Crystal River area, with subpopulations that are subject to recreational harvest, as well as the Tampa Bay and Sarasota Bay systems, also closed to any harvest (Leverone et al. 2010). FWRI staff monitors water quality in this area as part of our long-term, bay scallop monitoring program. Throughout a 10-y period (1999 to 2009) salinity ranged from 22.2-36.9 ppt and temperature ranged from 9.0-33.4[degrees]C (Fig. 2). Data were recorded hourly using a Sea-Bird Electronics, Inc. data logger (www.seabird.com; Bellevue, WA). Fluctuations of 5 10[degrees]C/wk were common in the spring, when cold fronts passed through every 3-10 days. Temperatures of 28-31[degrees]C were achieved by June and were maintained through August in most years. As early as September, temperature decreases of [greater than or equal to] 5[degrees]C/wk occurred.
[FIGURE 1 OMITTED]
Visual surveys were conducted by divers in the Anclote estuary during late spring or early summer each year (1994 to 2009) to estimate scallop abundance (Fig. 1). At the onset of the survey program, 20 station locations were randomly selected between the 1-m and 3-m, mean low-water-depth contours in appropriate seagrass habitat (primarily Thalassia testudinum, Syringodium filiforme, and Halodule wrightii). These stations have been repeatedly sampled throughout the study. At each station, a 300-m weighted transect line was deployed and a pair of scuba divers (or snorkelers when depths were shallow and the water was clear) counted all scallops within 1 m of the transect line (Marelli et al. 1999).
We also conducted year-round recruitment monitoring at 12 stations near the Anclote River estuary (Fig. 1). Recruitment stations were selected by a matrix of multiple criteria, including depth (1-2 m), habitat (adequate seagrass cover), salinity (average, >20 ppt), and location (away from major boating traffic and commercial fishing areas). On September 17, 1997, a recruit collector was placed at each of the 12 stations within seagrass beds at a depth of 1-2 m. Three weeks later (October 8, 1997), a second set of collectors was deployed at those same 12 stations. The collectors were allowed to soak for 6 wk (1997 to 2006). Each collector's deployment period overlapped with the preceding and following trap's deployment period by 3 wk. The overlapping deployment schedule allowed detection of recruits that could have been missed if a collector had been removed when the spat were still too small to be detected visually (<1.0 mm) by ensuring that settling recruits on 1 of the 2 sets would always have 3-4 wk to grow prior to retrieval. Beginning in 2007 and continuing through the remainder of the study, traps were deployed monthly and allowed to soak for 8 wk. Each collector's deployment period overlapped with the preceding and following trap's deployment period by 1 mo. For the entire study, recruit collectors were soaked for an average of 48 days with a minimum deployment of 34 days and a maximum deployment of 84 days.
Recruit collectors were constructed from a 35-L citrus bag containing 1 47 x 31-cm flat sheet of Vexar with 3.2-mm mesh. The Vexar was not folded or bunched in any way. Each collector was attached to a half cinder block (20 x 20 x 20 cm) and a 15-cm crab-trap buoy with a 5-m length of 7.9-mm polypropylene line (Arnold et al. 1998, Brand et al. 1980) as modified from a design described in Motada (1977). The collector was cable-tied to a loop in the line, directly above the block, and a small secondary float was attached to the free end of the collector bag so that it floated roughly 30 cm off the bottom.
Upon retrieval, each collector was removed from the block-and-line apparatus, placed in a plastic garbage bag to capture loosely attached spat, and returned to the laboratory for processing as follows. The collector was removed from the trash bag and placed on a large, shallow tray. The inside of the trash bag was inspected for loose scallops. The outside of the collector was inspected by eye and then cut open. The inside surfaces of the citrus bag were then inspected and then the bag was discarded. Both surfaces of the Vexar were examined visually. Finally, the tray was inspected for any additional loose scallops or shells. The quantity of scallop spat were estimated by counting the number of paired valves. When single valves or numerous pieces of valves were encountered, we attempted to match valves and estimated the number of pairs present. The number of spat observed on each collector was divided by the number of days that it had been deployed to standardize the data to number of spat per collector per day.
The average adult bay scallop abundance from 1994 to 2009 was 43.4 scallops/600 [m.sup.2] (Fig. 3). At the onset of our surveys (1994 to 1996), adult abundance averaged 6.1 scallops/600 [m.sup.2], and scallops were found on 50% of the survey transects (an average of 10 of the 20 transects; 30 of 60 total for 3 y). During the following 10-y period (1997 to 2006), the average adult abundance climbed to 21.9 scallops/600 [m.sup.2], and scallops were found on 72% of the survey transects (144 out of 200 total). During the final 3-y period of our study (2007 to 2009), the average adult abundance surged to 154.8 scallops/600 [m.sup.2], and scallops were found at 97% of the stations surveyed (58 out of 60 total).
Scallops recruited to some collectors during 163 of our 185 deployment periods (Fig. 4). There were 2 prolonged time spans during which scallops recruited to collectors for 24 or more consecutive deployment periods. The longer of these 2 protracted periods occurred from December 2005 to December 2009, when scallops recruited to collectors during the final 53 consecutive deployment periods of the study. Of the 20 deployment periods in which no spat were found, 7 occurred in periods with August retrievals and 4 in periods with September retrievals. Minima in recruitment occurred in general from June through November, although they varied in duration or timing from year to year. Throughout the 11-y recruitment study, recruits were always observed in some of the collectors retrieved in March, April, May, June, and December. Recruitment varied from month to month, but in general had a bimodal recruitment pattern (Fig. 5), as it was on average low from May through October, increased in November with a substantial peak in December through January, declined briefly in February and March, and then increased for a secondary, weaker peak in April.
[FIGURE 3 OMITTED]
The average recruitment rate was 0.30 spat/collector/day. The greatest average recruitment rate for all collectors deployed over a single deployment period was 5.5 spat/collector/day (November 2001 to January 2002; Fig. 4). The maximum recruitment rate to a single collector was 19.6 spat/day (November 2001 to January 2002). Average monthly recruitment rate was greatest for December retrievals (October to November deployment), with 1.10 spat/collector/day, and was least for June retrievals (April to May deployment), with 0.02 spat/ collector/day. The annual average for years in which sampling was conducted throughout the year (1997 was excluded) ranged from 0.03 spat/collector/day in 2004 to 0.60 spat/collector/day in 2009.
The abundance of adult bay scallops fluctuated greatly during the first 13 y of this 16-y study, dropping to levels we considered to be transitional during 5 y, and collapsed during 3 y (defined later). There were 2 y out of the first 5 y when most scallops were found at only a few of the survey stations. In a study of Florida's bay scallop populations, Arnold et al. (1998) observed that when average bay scallop abundance was more than 25 scallops/600 [m.sup.2], spat on scallop recruit collectors placed in those areas was 1.5-2 orders of magnitude higher than in those areas where adult bay scallop abundance was less than 5 scallops/600 [m.sup.2]. FWRI scientists began to classify the former populations as "healthy." When scallop abundance dropped to less than 5 scallops/600 [m.sup.2], few spat were observed on recruit collectors, and FWRI scientists began to classify those populations as "collapsed." Populations with adult densities between 5 scallops/600 [m.sup.2] and 25 scallops/600 [m.sup.2] were defined as "transitional."
A secondary criterion supporting these observations was the distribution of scallops. Healthy populations had scallops at more than half the stations, whereas transitional and collapsed populations had scallops at half the stations or less. As restoration efforts were implemented near the Anclote estuary (Arnold et al. 2005a), the distribution became more widespread, and during 7 of the last 8 y of the current study (2002 to 2009), scallops have been detected at almost every station surveyed, even after being impacted by red tide in 2005 and 2006, when scallop abundance was low. In cases when the 2 criteria are inconsistent (such as low abundance but wide distribution), a third criterion, resiliency, is also considered. Some populations tend to recover within 1 or 2 y after a year of low abundance, whereas other populations take up to a decade or longer to recover naturally, and yet others show no ability to recover without restoration activities.
Throughout the 13 y of our juvenile monitoring study, we observed fall peaks in recruitment, minor spring peaks, and frequent, prolonged periods of recruitment involving small numbers of recruits. The typical pattern of recruitment included a fall increase that peaked in collectors retrieved during December, which was expected. Collectors retrieved in December generally had been deployed since October, thus they were soaking during the presumed peak spawning period for bay scallops in Florida. Periods during which no recruits were observed were increasingly rare as the study progressed, but were most common in collectors retrieved during August or September. Collectors retrieved during August and September had been deployed since June or July, a period when most studies have indicated scallops in Florida's waters are devoting energetic resources to somatic growth, rather than reproduction (Barber & Blake 1985, Geiger et al. 2006).
No species of scallop with a recruitment period as extended as that observed in this study was reported in a review of scallop reproduction (Barber & Blake 2006). We therefore have several explanations to consider. The Anclote scallop population may have a markedly asynchronous spawning period, related to a protracted recruitment period and the resultant asynchrony in maturation. Or, Anclote scallops may have synchronous spawning, yet larval settlement may be asynchronous, as a result of various phenomena such as patchy larval food supply, phenotypic diversity, heterogeneous environmental factors, and protracted larval durations (described later). Regardless, recruits could have 2 origins: entirely local spawning or spawning by both local and more distant stocks. In this latter case, the most likely explanation would be that peak recruitments result from local spawning, whereas much of the off-peak recruitment results from spawning in more distant places, such as Crystal River to the north, or Tampa Bay to the south.
[FIGURE 4 OMITTED]
Although bay scallop spawning probably has a distinct peak in October and November, evidence suggests that Florida bay scallops have the capacity for a prolonged breeding season. Sastry (1963) indicated that, during a 7-mo period, at least some scallops from Alligator Harbor, in the Florida Panhandle, were ripe. Barber and Blake (1983) indicated that although peak spawning occurred in September and October, mean oocyte diameter varied widely between individuals, suggesting that some individuals may be capable of spring spawning. Our observations of year-round recruits support this hypothesis. Spawning of scallops in northern Florida had been thought to be limited to late summer and fall, and strictly tied to temperatures of 23-28[degrees]C, but earlier observations (Sastry 1966) suggested that scallops that had adapted to cooler temperatures spawned at lower temperatures than those adapted to warmer temperatures. It appears that the spawning peak is increasingly more distinct the farther north the population; more distinct for North Carolina populations than for Florida ones, and more distinct still for those in New England (Sastry 1970), although secondary peaks and off-peak recruits have been observed in other studies (Peterson & Summerson 1992, Tettelbach et al. 1999). Arnold et al. (2005a) indicated continual spawning in caged scallops during an 11-mo span at Anclote, and Way (2001) and Arnold et al. (2006) observed similarly protracted spawning periods in wild Anclote scallops. Lastly, Bologna (1998) detected recruits at times other than the fall peak.
Florida scallops may be behaving as a tropical rather than a temperate species. Although less common than a distinct spawning season, extremely protracted spawning seasons have been observed in other scallop species such as the variant scallop (Chlamys varia (Margus 1991)) and Catarina or speckled scallop (Argopecten circularis (Ruiz-Verdugo & Caceres-Martinez 1991)). Tropical bivalves in general have a more protracted spawning season, presumably because seasonal variation in primary productivity and climate in their range is relatively minor (Parsons et al. 1977, Raymont 1980). Studies using histological assessment of populations of the pearl oyster, Pteria penguin, in Thailand found spawning and mature individuals year-round, but temperatures ranged only from 26.8-30[degrees]C (Arjarasirikoon et al. 2004). Other tropical species displaying some level of year-round spawning and recruitment include the flame scallop (Ctenoides scaber (Dukeman et al. 2005)) and the tropical oyster Crassostrea corteziensis (Rodriguez-Jaramillo et al. 2008). Some individuals of the Antarctic soft-shelled clam Laternula elliptica, which also experiences limited seasonal fluctuation in temperature, also maintain ripe oocytes throughout the year, which presumably allows them to spawn whenever environmental conditions become favorable for larval survival and development (Bigatti et al. 2001). In Tampa Bay, a native mussel (Brachidontes exustus) and the introduced green mussel (Perna viridis), both exhibit at least a spring and fall peak in spawning, with some level of gametogenesis occurring year-round (Barber et al. 2005). In the tropics, where P. viridis is native, it spawns year-round (Walter 1982) and recruits over protracted periods (Rajagopal et al. 1997). Species that can continually reproduce despite a more extreme range in seasonal temperature include Limnoperna fortunei (Darrigan et al. 1999), the blood ark (Anadara ovalis (Power et al. 2004)), the ponderous ark (Noetia ponderosa (Power et al. 2005)), and the venerid clam (Tawera gayi (Morriconi et al. 2007)).
[FIGURE 5 OMITTED]
Wilson et al. (2005) showed that oysters (Crassostrea virginica) can recruit almost year-round, but that recruitment rate can vary over short distances in an estuary, probably related to water quality at broodstock and settlement sites. Similarly, the New Zealand scallop (Pecten novaezelandiae) was found to spawn mainly in summer, but had other minor spawns almost year-round (Williams & Babcock 2004). Populations separated by only a few degrees of latitude exhibited differences in the timing of minor recruitment peaks, which presumably occurred when more food became available (Williams & Babcock 2004). Winter spawning related to increased food abundance has been demonstrated in scallops from South Africa (Arendse et al. 2008).
Another possible explanation for the continuous settlement observed at Anclote is that larvae may spend extended periods in the plankton. The bay scallop has a larval phase of 10-14 days at temperatures more than 20[degrees]C (Castagna & Duggan 1971, Loosanoff & Davis 1963), but the potential for an extended larval phase during cooler temperatures exists. Argopecten purpuratus reared at 14[degrees]C had a larval phase of 35 days (Piquimil et al. 1991), and Chlamys islandica reared at 7[degrees]C had a larval phase of 70 days (Gruffydd 1976). Beaumont and Budd (1982) showed experimentally that veligers of mussels (Mytilus edulis) and scallops (Pectin maximus) could survive prolonged exposure to 5[degrees]C water, and would metamorphose and grow as spat if returned to 17[degrees]C water. In a review of the processes that affect planktonic larvae, Bradbury and Snelgrove (2001) postulated that for a typical invertebrate, a decrease in temperature from 20[degrees]C to 10[degrees]C could prolong the larval phase from ~20 to ~54 days, whereas a 10[degrees]C increase in temperature might shorten the larval phase by as much as a week. We observed new recruits of less than 5 mm SH at Anclote when temperatures were as low as 10.9[degrees]C on the day of retrieval. If we assume that the duration of their larval phase had been prolonged by low temperatures, increasing the chances that they had been transported from distant sources before settlement, it is quite possible that some of those larvae had been developing for 6-8 wk. In addition, Moran and Manahan (2004) have shown that when food is limiting, the larval stage of bivalves may last as much as 4 times longer than when food is not limiting, with the larvae possibly surviving and developing through the use of detritus or dissolved organic matter. We also observed new recruits at temperatures as high as 27.4[degrees]C on the day of collection. We have collected larger recruits (>20 mm SH), when water temperatures during the entire collection period probably exceeded 30[degrees]C. This suggests not only that scallops are capable of extremely fast growth when conditions are ideal, but also that recruitment occurs over the entire temperature range observed for the study area. The most likely explanation for the presence of summer recruits, with a larval phase duration that is likely to be short, is that they were derived from relatively local broodstock.
Another possible explanation for the continual settlement at Anclote is that winds and currents can work together to export larvae from natal seagrass areas and advect them long distances to their final settlement site. Hjort (1914, as cited in Heath (1992)) first postulated that the larval phase of marine organisms was characterized by extreme threats of mortality. One component of his theory was that advection could result either in mortality if currents transported larvae into unsuitable settlement habitat, or in success if larvae reached suitable settlement habitat; this has been borne out for bivalves (Chicaro 2001). Relative abundance of planktonic larvae may also change through diffusion (Arnold et al. 2005b) or concentration by frontal systems (Le Fevre 1986). Although a majority of the larvae settling in Anclote probably resulted from local spawns, the estuary is located at the intersection of currents that may supply larvae that originated in Tampa Bay, Homosassa, and, rarely, Panhandle populations. Populations to the south, although less likely to be the source of larvae settling in Anclote because their populations have been reduced for decades, have exhibited marked fluctuations in population recently (Leverone et al. 2006) and would have been more likely to contribute to the Anclote population during periods of higher abundance. The wide and shallow Florida shelf is readily affected by shifting wind patterns (Yang & Weisberg 1999), which could result in accumulation of larvae near shore or transport them offshore, away from their preferred habitat. Krause et al. (1994) showed that Florida and North Carolina populations of a sympatric congener, the calico scallop (Argopecten gibbus), maintained genetic similarity through relatively frequent larval migration over large geographic distances, showing scallops of the genus are capable of larval transport at greater distances than those among Florida's bay scallop subpopulations. Genetic evidence suggests that the A. irradians population is either well mixed (T. M. Bert in prep.) or has a slight regional structure among subpopulations (Marelli & Arnold 2001). We conclude that most peaks of settlement result from synchronized spawning within the Anclote subpopulation. Additional prolonged periods of low but constant recruitment result from asynchronous "dribble" spawning from Anclote broodstock and from a supply of immigrants from the peaks of other subpopulations around Florida's gulf coast.
The Anclote subpopulation is located in a region that could receive larvae from a number of other possible sources. The northeast corner of the Gulf of Mexico has a broad, shallow continental shelf. The currents along the edge of the shelf in general flow parallel to the isobaths (Carlson & Clarke 2009), but on the shelf form a gyre known as the Steinhatchee Gyre (Yang & Weisberg 1999). The gyre's typical flow is clockwise from October through March, potentially bringing entrained larvae southward to Anclote from populations in the Homosassa, Steinhatchee, and St. Mark's rivers, and counterclockwise from April through September. In some summers, currents along the shelf could .bring larvae from southern Florida estuaries Tampa Bay, Sarasota Bay, and Pine Island Sound. Each of these areas has had strong positive fluctuations in adult abundance during the past few years (Arnold et al. 2009) and may be at least episodically exporting larvae. Larval import may also result from flow from Florida's Panhandle bays--St. Joseph Bay and St. Andrew Bay--if the larvae were entrained in bottom waters (Yang et al. 1999), but this has been deemed unlikely (Yang & Weisberg 1999). Regardless of which of these currents were entraining larvae, a discontinuity between water bodies is likely to exist near the Anclote estuary, and the resulting accumulation of larvae could result in an increased supply of potential recruits.
Scallops in Florida waters may be much less fecund than those in more temperate regions. Using the assumption that each population has a single spawning peak and reproductive event, populations in New York were estimated to have 7 times the size-specific fecundity as populations in Florida (Bricelj & Malouf 1987). A prolonged spawning period may allow for a greater reproductive output over a lifetime. Understanding the reproductive potential of Florida populations would require better understanding of individual life history and population-level dynamics. In particular, if a majority of the recruits in a population with a prolonged recruitment period are derived from itself, and not from other populations, the individuals may be semelparous and the population may spawn asynchronously, or, the individuals may be iteroparous and spawn multiple times. A prolonged reproductive season may be of benefit during events that disrupt a population (Bishop et al. 2005, Tettelbach et al. 2001). In Florida, two especially pervasive disruptions are harmful algal blooms (Steidinger et al. 1998, Landsberg et al. 2009) and extreme variations in salinity in estuarine reaches of major rivers (c.f. Baker et al. 2005). The stress of harmful algal blooms can be exacerbated by accompanying hypoxia in severe events (Hu et al. 2006, Yentsch et al. 2008, Gannon et al. 2009). One of the main benefits of the exportation of larvae is that it allows repopulation of regions that have experienced declines or extinctions. In New Jersey's Barnegat Bay, after an almost complete loss of bay scallops, recruits in a resurgent population were shown to be genetically similar to the neighboring Long Island population. The bay was most likely to have been repopulated by larval transport mechanisms (Campanella et al. 2007).
What contribution do off-peak scallops make? Toba et al. (2007) showed that the population of the Manila clam (Ruditapes philippinarum) in Tokyo Bay originated mostly from locally spawned recruits, but that some recruit classes disappeared during extreme summer conditions. The small cohorts of recruits presumed to be exogenously supplied may then be important in maintaining local populations, because they arrive in the fall after the worst environmental stresses have passed. The observed prolonged settlement periods of bay scallops in the Anclote estuary may also serve to perpetuate the population. A range of settlement dates may result in a range of maturation dates and thus times of peak spawning that, for this region, seem to have produced at least short-term stability in the adult population.
Numerous FWRI coworkers assisted with collection and analysis of scallop spat collectors and visual transect surveys described in this article: D. Marelli, C. Bray, M. Harrison, K. Hagner, P. Hoffman, M. Parker, M. Humphrey, M. Julian, T. Idocks, A. Feldberg, K. Ferenc, L. Gentile, J. Bickford, C. Meyer, C. Matterson, J. Cobb, A. Granholm, C. Beals, B. Pittinger, M. Gambordella, J. Stone, M. Poplaski, B. Brown, A. Vasilis, M. Drexler, S. Bergeron, A. Dowling, and numerous participants in the FWRI intern program. Reviews were provided by D. Chagaris, J. Herrera, Jim Colvocoresses, Judy Colvocoresses, Bland Crowder, and 2 anonymous reviewers. Primary funding was provided by funds generated by the Florida Salt Water Fishing License.
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STEPHEN P. GEIGER, (1), * SARAH P. STEPHENSON (1) AND WILLIAM S. ARNOLD (2)
(1) Florida Fish and Wildlife Conservation Commission, Fish and Wildlife Research Institute, 100 8th Avenue SE, St. Petersburg, FL 33701; (2) NOAA Southeast Regional Office, 263 13th Avenue South, St. Petersburg, FL 33701
* Corresponding author. E-mail: firstname.lastname@example.org
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