Enhancing the potential for population recovery: restoration options for bay scallop populations, Argopecten irradians concentricus, in North Carolina.
KEY WORDS: bay scallop, Argopecten irradians, scallop spat collector, recruitment, restoration
Justifications for investing resources to restore depleted shellfish populations have relied on the need to revitalize moribund fisheries (MacKenzie 1983, Stotz 2000, Booth & Cox 2003) or to recreate lost ecosystem services (Coen & Luckenbach 2000, French McCay et al. 2003) that are dependent on the target species. Multiple methods have been used in shellfish restoration including: (1) transplanting and protecting broodstock (e.g., Peterson et al. 1996), (2) seeding with hatchery-reared juveniles (e.g., Karney 1991, Summerson et al. 1995, Wilbur et al. 2005), and (3) habitat modification designed to enhance survival of early life-history stages (e.g., Thayer 1992). The method used has depended on the status of the target species population and socio-economic factors (Booth & Cox 2003).
Bay scallop populations have decreased along the entire eastern coast of the United States in the past 50 y as a result of overexploitation, harmful algal blooms, and habitat loss (Cosper et al. 1987, Peterson & Summerson 1992, Tettelbach & Wenczel 1993, Marelli et al. 1999, Bologna 2008, Murphy & Walton 2008). Within North Carolina, bay scallops (Argopecten irradians concentricus) have experienced dramatic declines in population size over the past two decades. From October 1987 to February 1988, a red tide (Karenia brevis) outbreak in North Carolina sounds induced mortalities so severe in bay scallop populations that scallop recruitment was suppressed for several subsequent years (Peterson & Summerson 1992). An attempt to restore bay scallop populations by transplanting adult bay scallops into a depleted water basin revealed that this species is recruitment-limited and that populations in hydrographically separate basins are largely reproductively isolated from each other (Peterson et al. 1996). This transplantation effort with subsequent natural recruitment from the spawning adults was successful such that during the latter half of the 1990s bay scallop populations and fisheries recovered in the sounds where red-tide suppression had occurred (NCDMF 2007). After 2000, North Carolina bay scallops have suffered a progressively severe and persistent decrease in population size that ultimately led to a collapse of the fishery, with no harvest season opening from 2005 through 2008 (NCDMF 2007). Schools of fall-migrating cownose rays (Rhinoptera bonasus) that have undergone dramatic population increases in response to release from their own predators, the great sharks (Myers et al. 2007), migrate southward through North Carolina sounds in late summer and now apply sufficient predation pressure to consume virtually all adult bay scallops before they complete spawning and before the opening of the fishery (Peterson et al. 2001).
Bay scallop biology in North Carolina has been described in detail in several reports (e.g., Peterson & Summerson 1992, Bishop et al. 2005, NCDMF 2007). Bay scallop populations exhibit discrete, patchy distributions over their range (Gutsell 1930), within a region (Peterson et al. 1996, Arnold et al. 1998) and on a local scale (Thayer & Stewart 1974, Peterson et al. 2001), where higher abundances are generally restricted to patchily-distributed seagrass beds. The patchiness of their populations requires that restoration efforts be directed at appropriate spatial scales to be effective. For example, water exchange among some North Carolina sounds is limited by hydrographic barriers, which can create semi-isolated basins (Peterson et al. 1996). Restoration efforts must recognize this low connectivity and direct separate efforts towards each basin. In addition, the important processes of scallop spawning and cownose ray predation are discrete temporally. Most bay scallop spawning occurs in mid August to late September in North Carolina: a smaller spring spawning event occurs but generally contributes few recruits to the fishery (Bishop et al. 2005). Cownose ray predation occurs from mid August to mid September (Peterson et al. 2001, Myers et al. 2007). Consequently, management practices enhancing scallop spawning and reducing cownose ray predation need occur only during this late summer, early fall window of time.
Tettelbach and Smith (2009) demonstrated that restoration of bay scallop populations and fishery landings can be aided using annual hatchery production of scallop seed. However, Milke et al. (2006) state that scallop restoration efforts that depend on hatchery-reared seed are limited generally by costs, insufficient numbers of juvenile scallops, and the slow growth of hatchery-produced scallops prior to seeding. Developing inexpensive means of producing large numbers of larger juvenile scallops from natural sets would help in bay scallop restoration efforts throughout its range. In addition, relying on natural set, as opposed to hatchery-raised larvae, avoids costs associated with maintaining and operating a hatchery and problems associated with inbreeding that have burdened some scallop breeding programs (Zheng et al. 2008), although selection for scallops that accept and survive on spat collectors may also reduce genetic variance. In our study we compare several, relatively inexpensive, practical methods designed to: (1) enhance adult bay scallop survival up to the time of spawning and (2) increase bay scallop spat set, survival, and growth. For the first goal, we use temporary stockades made of plastic pipe to exclude rays from dense concentrations of adult scallops prior to and during their late summer spawning period. For the second goal, we deploy spat collector bags to increase the available surface area for scallop settlement and then subsequently to serve as field-based nurseries for those scallops. Spat collectors have been used to collect and grow natural spat for enhancing Placopecten magellanicus stocks (Parsons & Robinson 2006), but collectors have been underutilized as a means of enhancing bay scallops. We also examine alternative spat collector designs to test whether spat collector bags are the best design for attracting and growing scallops. Finally, we explore the use of shore-side salt ponds for scallop production. Bishop and Wear (2005) provide evidence that increased fish predation on juvenile crabs in the fall enhances juvenile bay scallop survival through a trophic cascade that suppresses crab predatory pressure. Isolated ponds can be managed to reduce the abundance of scallop predators and, by suppressing predation on juvenile scallops, allow methods for increasing production of scallop spat and juveniles that are not possible in the field.
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MATERIALS AND METHODS
In North Carolina, commercially viable bay scallop populations have historically been highest in 3 contiguous sounds: Core, Back, and Bogue Sounds (Fig. 1). In almost every year in which harvest of bay scallops occurred in NC, at least 75%, and usually more, of the harvest came from these three water bodies (NCDMF 2007). Our study sites encompassed this entire region in 2007 and 2008. In the first year (2007) we established spawner sanctuaries inside stockades in Back Sound (at Middle Marsh) and western Bogue Sound (near Lovett Island) and in the second year (2008) at Middle Marsh, Lovett Island, and in northern Core Sound (at the Swash). In both years, we also introduced spat collectors at Middle Marsh, Lovett Island, Yellow Shoal (southern Core Sound), and in a shore-side, mesocosm pond at the UNC Institute of Marine Sciences (eastern Bogue Sound). In the second year we also placed spat collectors at the Swash (Fig. 1).
Detailed descriptions of these sites can be found in other studies (Peterson et al. 1989, Peterson et al. 2001, Bishop et al. 2005, NCDMF 2007). In general, all the sites (except the mesocosm pond) were shallow (generally <1 m water depth at low tide), subtidal locations within or near seagrass beds dominated by Zostera marina. All of the sites have supported abundant bay scallops historically, experienced cownose ray predation during their September migrations in the past decade, and maintained persistent, but greatly reduced, scallop production since 2007.
The 18 m x 29 m shore-side mesocosm pond at IMS was 1.2-m deep at the center and remained ~1-m deep to within 1-2 m of the pond edge, where the basin graded upwards to dry land. The basin substrate consisted of fine sand. Unfiltered seawater, pumped from nearby Bogue Sound, entered the pond in 2007 at 6 L [s.sup.-1] from a pipe on the southern edge and flowed out of the pond through a centrally located standpipe. In April 2008 before introducing spat collector bags, we repositioned the pipe that supplies water from the southern edge to the southwest corner of the pond to increase the distance between the inflow and drain. We chose to use a shore-side salt pond as one of our sites for several reasons. The pond: (1) provided greater environmental control so spat collector bags would not be damaged or lost by wind or waves; (2) enabled us to reduce predation risk dramatically on juvenile scallops by using baited, commercial crab pots, lined with 2-mm plastic mesh, as well as by draining the pond periodically to kill predators not captured by the crab pots; and (3) allowed efficient examination, manipulation, and retrieval of spat collectors.
Spawner Sanctuaries Inside Stockades
During the latter half of August or very early September in 2007 and 2008, prior to the expected season of southerly migration of cownose rays through Core, Back, and Bogue Sounds, we created spawner sanctuaries for adult scallops and protected them by erecting stockades modified from those used by Peterson et al. (2001). Each stockade consisted of 2.5-m long poles of 2.54-cm diameter PVC pipe, inserted 30-40 cm vertically into the sediments at 0.25-m intervals, extending 11.5 m above the water at high tide. Except for the stockade at Middle Marsh in 2007, all of the stockades were circular in shape with a 13-14 m radius. The Middle Marsh stockade in 2007 was rectangular (10 m wide x 27 m long). After we erected each stockade, we collected, by hand, all adult bay scallops readily located in the immediate vicinity (5 m) of the stockade, and added them to those already inside. When a minimum of 1,000 scallops had been transferred into the stockade, we estimated initial scallop densities at low tide inside and within a 5-m wide perimeter immediately outside the stockade by haphazardly placing at least 30 replicate 0.25 [m.sup.2] quadrats throughout each area and counting the adult bay scallops in each quadrat.
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In October and November of each year, we resampled the stockades several weeks after we had evidence that: (1) bay scallops had spawned locally (based on visual inspections of bay scallop gonads) and (2) cownose rays had completed their migration (based on reports from local gill-net, pound-net, and trawl fishermen in addition to our own visual searches). We assessed the density of scallops inside and outside of the stockades (using the same sampling procedure as before) and then dismantled the stockade to retrieve the poles. The poles were brought back to land, allowed to sit outside and dry for two months, and then scraped clean of all fouling organisms in preparation for construction of stockades in a subsequent year.
Deployment of Spat Collector Bags
Our spat collector bag design was modified from Ambrose et al. (1992) and consisted of 4 [m.sup.2] (4.33 x 0.93 m) of transparent, 8.33-mm mesh netting stuffed into a 5-mm mesh "onion" bag (0.82 x 0.48 m) in which two, 30 x 10 x 1.5 cm pieces of Styrofoam had already been placed as far from the opening as possible. The drawstring on the open end of each "onion" bag was cinched and knotted to secure the Styrofoam and netting. Based on the circumference of the plastic strands of the mesh, the linear distance of strands per unit area of mesh, and the total area of mesh used, the internal netting provided a maximum of 850 [cm.sup.2] of potential surface area for setting (the actual area would be somewhat less and variable depending on how each piece of netting folded when placed within the bag). The flat surfaces of the interior of the "onion" bag and of the Styrofoam could also serve as setting substrate, bringing the total, possible available surface area on the interior of each spat collector to 1.48 [m.sup.2].
In 2007, as we deployed the spat collector bags, we attached 10 (or occasionally five) bags at 1.0-m intervals to a continuous, 12-m length of 0.95-cm diameter polypropylene rope, by braiding the onion bag draw strings into the tope strands. Cinder blocks were tied to each end of the tope and at the middle to anchor the cinched ends of all bags within 20-cm of the bottom substrate (Fig. 2). We placed each spat collector array in water depths sufficiently deep (~1.5-m at MLW) to ensure that the bags would remain fully submerged at low water. When we returned to the sites on subsequent occasions, we found that chaffing of the anchor lines against the cinder blocks often led to torn anchor lines and loss of collectors before their recovery. In response, we modified the method of securing collector bags in 2008. We replaced each cinder block in the collector array with a 3.05-m long piece of 5.08-cm diameter PVC pipe. One end of each pipe was beveled and a hole was drilled through the pipe 0.75 m from the beveled end. In the field, we threaded the anchor rope through the hole in the pipe, tying it firmly, and inserted the pipe vertically (beveled end down) into the sediments until the hole and attached anchor line were at the substrate surface. Pipes were placed at both ends and at the middle of the anchor line. We attached spat collector bags, as before, at 1-m intervals to complete the array.
In August and September 2007, we installed 35 spat collectors (three 10-bag arrays and one 5-bag array) in each of five locations (Fig. 1): Yellow Shoal (southern Core Sound), Middle Marsh (western Back Sound), Lovett Island (western Bogue Sound), Hoop Pole Creek (eastern Bogue Sound) and the UNC Institute of Marine Sciences (IMS) mesocosm pond (also eastern Bogue Sound). In October, November, and December, we revisited each site and removed five, haphazardly selected, spat collector bags, which were immediately enclosed within large plastic bags for transport to the laboratory. There, each bag was immediately examined or held within a cold-room at 40[degrees]C until it could be processed. Examination of each collector involved: (1) opening the onion bag exterior and removing the interior mesh; (2) collecting any scallops attached to the interior of the onion bag or the Styrofoam pieces; (3) collecting all scallops attached to the interior mesh; and (4) washing all surfaces onto a 1-mm mesh screen that was then examined for the presence of any scallops. All recovered scallops were placed in labeled jars containing formalin. We counted all scallops from each spat collector and estimated their size-frequency distribution by measuring, to the nearest 0.1 mm, shell height (umbo to ventral margin distance) of each scallop (up to a maximum of 40 individuals per collector bag). Because haphazard sampling can underestimate small individuals, we identified and counted all scallops <3mm in shell height in each sample. In March 2008, we retrieved all remaining spat collector bags and processed them as before, except that we counted and measured (up to a maximum of 50 individuals per bag) the scallops while alive.
In August and September 2008, we placed a total of 20 spat collector bags, in strings of 10 as before but anchored by PVC poles, in each of five locations: Yellow Shoal, Middle Marsh, Lovett Island, the Swash, and the IMS mesocosm pond. The bags were retrieved in December and processed like the March set: all counts and measurements were made on live individuals and a maximum of 50, haphazardly-selected individuals was measured from each collector bag.
Spat Collectors Reclining on the Bottom
We tested the ability of alternative spat collector designs to collect scallops by comparing their returns against the standard spat collector bag in a protected environment, the IMS mesocosm pond, in spring and Yall 2008. The natural spring recruitment of bay scallops in North Carolina is generally trivial in large part because of intense predation on juveniles by crabs and fishes entering the estuarine nursery (Bishop et al. 2005). We hypothesized that our ability to control predators through the use of traps and periodic drainings of the pond could allow us to collect and protect substantial numbers of recruiting bay scallops even when recruitment success in nature was minimal. Three new spat collector designs (Fig. 3), with contrasting substrate flexibility, orientation, and probable surface flow characteristics, were created to test spat collecting success across a range of alternative designs and to take advantage of the expected higher growth rates experienced by juvenile scallops when they are attached close to the substrate (Ambrose & Irlandi 1992) as opposed to suspended in bags higher above the bottom. One design was intended to mimic the physical attributes of seagrass as much as possible. This "artificial seagrass unit" (ASU) collector consisted of a 1 m x 1m frame made of 1.27-cm diameter PVC pipe with a grid of monofilament lines criss-crossing the frame at 5-cm intervals along each margin. At every intersection of lines, four 12-cm long, 0.5-cm wide pieces of green plastic ribbon were tied: the ribbon has the buoyancy and dimensions of an eelgrass blade (Micheli et al. 2008). A second design provided stable vertical surfaces that allowed flow through the collector. An X-shaped (X) collector was made by interweaving the middle of two, 90cm long x 31-cm high strips of 0.5-cm plastic mesh strips to produce a structure that, when standing vertically, was x-shaped as viewed from above. At the intersection of the mesh strips and at each end, a 40-cm long, 3/8" iron rebar stake was attached, with the top of the rebar flush with the top of the mesh and extending about 10 cm below the bottom of the mesh. The final design combined fixed horizontal and vertical surfaces but restricted flow to just across those surfaces. A half-pipe (HP) collector was produced by cutting a 16-cm diameter, 94-cm long, corrugated, plastic drainage pipe in half longitudinally. A brick was attached to the lower surface of the cut pipe so that the collector would rest on the bottom with the concave surface upward. Based on overall dimensions and direct measurements of surface dimensions, we estimated the potential surface area (on all exposed surfaces in each case) of each collector type available for setting by juvenile scallops as: ASU = 2.08 [m.sup.2] ; X 0.42 [m.sup.2]; and HP = 0.93 [m.sup.2] (vs. the 1.48 [m.sup.2] available on the inside of each spat collector bag).
On April 29, 2008, a few weeks before we expected spring recruitment to occur, three replicates of each collector type were arranged randomly in a Latin Square design within each of four blocks, spatially defined by each of the four quadrants of the IMS mesocosm pond. The pond, which had been drained and dried for three weeks, was filled with unfiltered seawater two days before the experiment began. Two linear arrays (using the same design that we used in the field, as described earlier), each with 10 spat collector bags, were placed along the midline of the pond. Two, baited, commercial crab pots that had been completely covered with 0.5-cm plastic mesh were placed in the pond and fished every 2-3 days throughout the period of the experiment. On June 2, 2008 the pond was drained. Before the spat collectors were completely exposed, all were carefully lifted from the substrate, a 0.1-cm mesh net was placed under each, and all scallops and any crabs that were present were collected from the collectors or mesh net by hand and counted. The spat bags were processed as described earlier.
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On August 19, 2008 two to three weeks prior to when we expected fall recruitment, three replicates of the X-shaped collector and three replicates of the half-pipe collector were randomly placed into each of four blocks located within the IMS mesocosm pond. As in the experiment conducted in the spring, the pond had been drained and allowed to dry for three weeks prior to starting the experiment. Two linear arrays of spat bag collectors (10 bags each) were installed along the midline of the pond. The pond was fished in the same manner as in the spring, using two meshed crab pots. On December 19, 2008 sixteen of the spat collector bags were removed from the pond and processed as in the spring (four collector bags were left in the pond for use in another experiment). While retrieving the spat collector bags, we observed that some of the scallops on the bottom collectors had reached the size at which they detach from substrate. Because some of the scallops that had been collected by these devices may have already fallen to the substrate surface, we modified our procedure from what we did in the spring trial, when we sampled the bottom collectors. Specifically, on December 22, 2008 the pond was drained completely and all scallops attached to each bottom collector, or below each bottom collector, were gathered, counted, and measured (shell height to the nearest 0.1 mm). The presence of any crab was noted.
Spawner Sanctuaries Inside Stockades
In both years and at all locations, stockades maintained almost complete structural integrity during the entire field deployment. The stockades were visited at irregular intervals of four to six weeks. Very few (0-5) gaps (created by poles falling into the water) occurred along the perimeter of any stockade and never was any gap greater than 0.5 m. Any gaps noticed during a visit were repaired by restoring the pole to its vertical position.
At Lovett Island (in western Bogue Sound) in 2007 and 2008, and the Swash (in northern Core Sound) in 2008, adult scallop densities inside the stockades remained elevated above those outside from the time when we first established the sanctuaries through the season when cownose rays typically migrate through the region and prey on scallops (Table 1). The changes in scallop density through late summer-fall differed between years at Lovett Island and between sites in 2008. As revealed by separate, two-way ANOVAs, at no site in either year was there a significant interaction between time and location (inside or outside of a sanctuary) (Table 2). In 2007, the density of scallops inside the Lovett Island sanctuary decreased by 21% from August to October, whereas the scallop density outside decreased by 79%. At the same location in 2008, adult scallop density stayed roughly the same inside the sanctuary (increased by 0.3%), whereas the outside density increased by 42% from August to November. In the Swash, scallop density decreased to a similar degree inside (-55%) and outside (-57%) from August to November. The only significant decrease in scallop densities over the fall time interval occurred in the Swash in 2008 (Table 2).
In 2007 and 2008, either no, of almost no, adult scallops were found, either inside or outside of the stockade at Middle Marsh (in Back Sound) when we returned to sample at the end of the experiment. In 2007, we received an eye-witness report that the scallops in the Middle Marsh stockade had been poached. In 2008, we moved the location of the sanctuary to a more publicly visible site, hoping to reduce the chances of scallop piracy. The similarity of the 2008 Middle Marsh sanctuary results to the 2007 results and the difference between the performance of Middle Marsh's sanctuary and those at other sites in both years suggest a lack of success in avoiding human interference.
Spat Collector Bags--Year One: August 2007--March 2008
The late of spat collectors themselves differed among sites. In October 2007, all spat collectors remained where they had been installed. In November 2007, three spat collectors were missing from Yellow Shoal and two from Lovett Island. Inspection of collector moorings at Middle Marsh, Yellow Shoal, and Lovett Island revealed that some anchor lines had abraded near where the cinder blocks held the array to the bottom. We repaired the spat collector arrays at that time. In December 2007, only one collector remained at Yellow Shoal, three at Lovett Island, whereas only four collectors were missing at Middle Marsh. By March 2008, collectors were present only at Middle Marsh. We never lost collector bags from the IMS mesocosm pond throughout the entire experiment. Unlike the other field sites, Hoop Pole Creek suffered burial of its collector bags by sediments. In November, some of the Hoop Pole collector bags were discovered during laboratory processing to be partially filled (>15% of collector volume) with sand. During the December through March period, all of the remaining collector bags at Hoop Pole Creek became completely buried by sand.
Sites differed in the numbers of scallops collected per bag (Fig. 4A). A two-way (site by month of collection) ANOVA conducted on the homoscedastistic (based on a nonsignificant O'Brien test), untransformed data from all sites for October through December produced significant main effects and a significant interaction (Sites F = 70.97, P < 0.0001; Month F = 3.43, P = 0.040, Sites x Month F = 2.19, P = 0.043). An unplanned Tukey HSD test revealed overlapping sets of nonsignificant interaction means attributable primarily to temporal differences at two sites, Yellow Shoal and Hoop Pole Creek (Fig. 4). Only one spat collector remained at Yellow Shoal in December so the low value observed here must be interpreted cautiously: it may not be representative. The sand in the bags retrieved in November from Hoop Pole Creek would have decreased the surface area available for scallops to settle and induced mortality on any scallops buried within the bag. If we omit the unreplicated datum and the November bag counts from Hoop Pole Creek, two patterns emerge: (1) there are site-specific differences in the numbers of scallops per bag (Middle Marsh > Lovett Island and Yellow Shoal > Hoop Pole [much greater than] IMS pond) and (2) there is no evidence of decreasing scallop numbers per bag throughout the period. The last observation is further supported by the results of a two-way ANOVA conducted on counts from collector bags located at just Middle Marsh and the IMS pond for all months (October 2007 through March 2008), in which only the site main effect was significant (Sites F = 394.00, P<0.0001; Month F = 1.38, P = 0.26, Sites x Month F = 1.53, P = 0.22). Replicate spat bags collected similar numbers of scallops at Middle Marsh, Lovett Island, and Yellow Shoal, where coefficients of variation among replicates within a sampling time were low (<40%) and consistent throughout the experiment (Fig. 4B). Hoop Pole Creek and IMS pond spat collector bags showed much greater variability among replicates in collecting scallops.
Shell height of scallop spat differed among times and sites. Scallops <3 mm in shell height were found only in spat collector bags retrieved in October 2007 (Fig. 5). A two-way ANOVA conducted on the mean shell height of juvenile scallops in each bag (based on the subsamples of 40 individuals) from each site in October, November, and December produced a nonsignificant interaction of site x month (F = 1.44, P = 0.20), whereas both main effects were highly significant (Site F = 8.23, P < 0.0001 and Month F = 104.68, P < 0.0001). Mean shell height, pooled across all sites, increased by 84.3% from October to November and by 21.2% from November to December (Fig. 6A). Two sites, Middle Marsh and Hoop Pole Creek, had significantly larger scallops than Yellow Shoal and the IMS pond, whereas Lovett Island scallops were intermediate in size (as determined by a Tukey HSD test, Fig. 6B). The December 2007 difference in mean size between Middle Marsh and IMS persisted into the subsequent spring (Fig. 7). A one-way ANOVA on scallop shell height (power-transformed to achieve homoscedasticity) contrasting the only two sites to yield scallops in March 2008 (Middle Marsh and IMS) was significant (F = 370.71, P < 0.0001), with shell height 34% larger at Middle Marsh than IMS.
When we drained the pond in March of 2008 to retrieve the last spat collector bags, we hand-collected 2,060 scallop recruits not associated with any collectors, mostly from areas of the pond several meters away from where the collectors had been located. The mean (+1 SE) shell height of these scallops was 37.02 (0.74) mm, exceeding the mean size recorded from any spat collector bag at any location on any date. We hypothesize that these scallops had set on the abundant stubble of terrestrial grasses that persisted in the pond when we filled it with water in the fall (the pond had gone unused for several years prior to our experiment and terrestrial grasses had grown in much of the empty basin). By measuring the dimensions of 100 of these haphazardly-chosen grass blades and using replicate tosses of a 0.25-[m.sup.2] quadrat to estimate blade (stubble) density, we calculated that the total approximate surface area available for scallop spat attachment, not including the inside of spat bags, in the entire pond was about 40 m2 (surface area of the plant remnants as well as the outsides of the spat collector bags held in the pond).
Spat Collector Bags--Year Two: August 2008--December 2008
As in 2007, the mean number of scallop recruits per spat collector bag differed among sites (Fig. 8A). A one-way ANOVA (on power-transformed data), followed by a Tukey HSD comparison of the means, demonstrated that the spat collectors in the IMS pond collected fewer scallops than any of the field sites, which did not differ from each other (F = 6.46, P = 0.004). Unlike 2007, there was no relationship among sites between mean numbers of scallops per bag and the coefficient of variation among bags.
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Mean shell height of scallop recruits found in the collector bags also differed significantly among sites (one-way ANOVA on reciprocal-transformed data: F = 75.94, P < 0.0001). Tukey comparison of means revealed that the largest scallops were found at Middle Marsh and Lovett Island, with the smallest at the Swash (Fig. 8B). Scallops from Yellow Shoal and the IMS pond were intermediate in mean shell height.
A two-way, factorial ANOVA on the mean shell height in December of scallops collected from those sites that held collectors in 2007 and 2008 (Middle Marsh, Lovett Island, and IMS pond) produced a significant site effect but no significant year effect or interaction of site x year (site F = 40.81, P < 0.0001, year F = 3.36, P = 0.075, and site x year F = 1.64, P = 0.20). In both years of the study, scallops were significantly larger in collector bags held at Middle Marsh and Lovett Island than at the IMS pond in December.
Alternative Spat Collector Designs
In the April-May 2008 experiment, scallop recruits were virtually absent from all three substrate-based collectors. Only 4 recruits were found (3 on the artificial seagrass unit (ASU) design and 1 on the X-shaped design). In contrast, all three of these collectors harbored predatory crabs (primarily xanthids with some portunids; Fig. 9). A mixed model, two-way ANOVA (collector type vs. block) on square-root transformed crab abundances (# individual crabs / collector) indicated that each main effect, but not the interaction, was significant (collector type F = 22.98, P = 0.0015, block F = 7.90, P = 0.017, and collector type x block F = 1.12, P = 0.38). A Tukey HSD test detected significantly more crabs associated with the ASU design than the other two. Collector bags in those blocks closer to the pond's water inlet had higher crab abundances than those further from the inlet. Only three, large (>5 cm carapace width) blue crabs (Callinectes sapidus) were captured in the meshed crab pots (and removed) during this experiment.
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In the IMS pond, the mean number of scallops per spat collector bag in the spring did not differ from that observed in fall2007 (mean [+ or -] 1 SE for spring, 20.4 [+ or -] 2.7 vs. 25.7 [+ or -] 5.7 for the preceding fall; t-test P = 0.51). Crab abundances in the bags averaged 12.0 [+ or -] 2.6 individuals per bag, which is intermediate between the mean abundances seen among the different substrate-based spat collectors.
The significantly high numbers of crabs in the ASU collectors led us to exclude this design from our comparison of substrate-based collectors in the September to December 2008 experiment. In contrast to the preceding spring, the X-shaped collector and the half-pipe collector attracted scallop recruits (Fig. 10). Inspection of the half-pipe collectors showed that fine sediments had accumulated on the upper surfaces of the half-pipes in a continuous layer ranging from 1-2 cm in depth. The X-shaped collectors supported some algal growth on their surfaces but very little sediment was present on any of them. To compare the capabilities of the two types of substrate-based collectors and the bag collectors to achieve bay scallop recruitment, we standardized the number of scallops found on each type of collector to the respective surface areas available for scallop spat settlement. Separate, one-way ANOVAs conducted first on the untransformed, standardized number of scallops [m.sup.-2] of collector area and second on the mean scallop shell height were both significant. Tukey HSD tests revealed that the half-pipe collector had significantly fewer scallops per unit surface area than either of the other two collectors (x-shaped or bag), whereas the scallops retrieved from the collector bags were smaller on average than those from either bottom collector. Xanthid crabs were observed in association with all three types of collectors. Over the three months of this rail experiment, we captured a total of 8 blue crabs in the meshed crab pots.
[FIGURE 6 OMITTED]
Site- and Year-dependency of Benefits of Installing Stockades to Protect Spawners
We were able at different locations and in separate years to establish and maintain sufficiently high adult scallop densities to promote effective spawning (>2 scallops [m.sup.-2], Peterson & Summerson 1992) throughout the season when migrating cow-nose ray predation had decimated North Carolina scallops in previous years (Peterson et al. 2001, Myers et al. 2007). Increasing the density of scallops inside a stockade has a potential benefit for improving scallop spawning success because externally fertilizing, free-spawning, marine invertebrates show low fertilization rates at low population densities (Levitan & Petersen 1995). In western Bogue Sound in 2007, the relative decrease in adult scallop abundances outside the stockade was almost x4 greater than inside the stockade, implying that the stockade successfully inhibited predation by some source. The results in 2008 were different in both Bogue and Core Sounds. In Bogue Sound unprotected adult scallops outside the stockade did not experience apparent mortality, even by mid November. In Core Sound, adult bay scallop abundances decreased inside and outside of the stockade with roughly the same percentage decline ([congruent to] 55%). Scallops outside the Core Sound stockade could have experienced predation relatively early in this experiment, such that the imbalance between immigration and emigration over a period of weeks resulted in a homogeneous decline inside and outside the stockade (Powers & Peterson 2000, Peterson et al. 2001). However, changes in scallop density at this site could also have resulted from a mortality source unaffected by the presence of the stockade. The results in 2008 from both Lovett Island and the Swash are consistent with the near absence of cownose ray predation. Our observations and reports from fishermen suggest that migrating cownose rays were much less abundant in Core, Back, and especially Bogue Sounds during late summer-fall 2008 than any other fall migration season in the previous decade. Our suggestion that cownose ray predation on bay scallops in these sounds during the fall migration season was trivial in 2008 gains support from the North Carolina Division of Marine Fisheries scallop dredge surveys showing no decline in adult scallop density in Core or Bogue Sound from July to October 2008 (T. Moore, NCDMV, pers. com).
[FIGURE 7 OMITTED]
Peterson et al. (2001) and Myers et al. (2007) demonstrated that cownose ray predation on bay scallops can be site-specific. Spatial and temporal variability in predation intensity would provide unpredictable, but potentially important, refuges for spawning scallops that could serve to repopulate bay scallops in many areas within the same hydrographic basin. Such predation variability could influence how management protects spawning scallops. If the loss of adult scallops prior to spawning can be confidently predicted for a particular location, then even an expensive protection intervention may have merit. If, however, ray predation is less predictable, either from location to location or year to year, then an expensive protection program may not be cost-effective. Nevertheless, ah inexpensive protection could still be justified in this situation. Our stockades can be erected or removed in a single tide by three to four individuals. They can be placed where scallops are already abundant, reducing or eliminating the need to collect, transport, and concentrate adult scallops. The cost of the pipe for a stockade of about 15-m diameter is $600-$700 (depending on prevailing PVC costs) and the poles show no structural decay or wear even after two years of use and are thus reusable.
[FIGURE 8 OMITTED]
Human depredation of bay scallops inside spawner sanctuaries is not a trivial concern. The stockades exclude cownose and other rays but not humans. The constituent poles are designed to have high visibility above the waterline, allowing boaters to easily avoid them. Unfortunately, stockade visibility can serve asa beacon to those who would illegally exploit the scallops concentrated in the sanctuary. Human behavior and inadequate fisheries law enforcement may limit the locations where stockade-style sanctuaries may be effectively used.
[FIGURE 9 OMITTED]
[FIGURE 10 OMITTED]
Efficacy of Bags as Spat Collectors and Fall-Winter Nurseries
In 2007 and 2008 spat collector bags deployed in the field during late summer to fall allowed us to accumulate thousands of bay scallop recruits. Pooling across all dates of retrieval, a total of 12,041 juvenile scallops was collected from 106 spat collectors deployed in 2007 and 11,334 juvenile scallops from 44 spat collectors deployed in 2008 (mean ([+ or -] l SE) number of scallops per collector: 122.9 (10.2) in 2007 and 257.6 (30.3) in 2008). Both of these annual totals include scallops gathered from spat collector bags held in the IMS pond, which collected significantly fewer juvenile scallops per collector on average compared with most field sites in either year. Excluding the IMS pond data shows that bag spat collectors located in the sounds produced a mean ([+ or -] 1 SE) of 176.8 (10.6) scallops in 2007 and 328.3 (40.6) scallops in 2008.
Based on previous studies (Ambrose et al. 1992, Peterson et al. 1996), we had expected that the spat collector bags would successfully yield substantial numbers of scallop spat. These earlier studies used spat collectors to test questions about short-term scallop survival associated with height in the water column or as a means to assess the abundance of competent scallop larvae in specific water bodies. We tested whether scallops could be maintained in spat bags for several months, using these bags not only as a means to collect scallop recruits but to serve as nurseries for grow-out of scallops to sizes large enough to reintroduce into the natural habitat. Although we found crabs in the spat collectors at all locations in both years, predation by these crabs did not prevent us from collecting 100's of scallops per bag, even after allowing the bags to sit undisturbed for 4-7 mo. From October 2007 to March 2008, we detected no significant decline in the numbers of scallops per bag at either the Middle Marsh or IMS pond sites, which are where we could make this assessment.
In situ scallop size within bags was similar in both years. In the winter months of 2008, the mean size of scallops held in spat collectors at the IMS pond and Middle Marsh (the only two locations where bags remained until March) increased beyond the December shell heights by 56% and 33%, respectively. The majority of scallops that we collected in March 2008 from the collectors held at both sites had achieved large enough size that they would suffer relatively low predation in the spring (Bishop et al. 2005) and exceeded the size of byssal detachment from substrate (Peterson & Summerson 1992). Enhancing native stocks could be accomplished by visiting the field site where the collectors have been held until spring of the year, retrieving the spat collector bags, opening them, discarding crabs, and immediately introducing the scallops into a nearby seagrass bed.
The design of our spat collector bag experiment prevented marking and following the late of individual scallops through time. Nevertheless, we infer that the observed stasis in scallop abundance per bag from October onwards at each site corresponds to high juvenile scallop survivorship and that the increases in mean scallop size reflect scallop growth. The presence of very small scallops (<3 mm) only in October implies that subsequent settlement of new cohorts was absent. Inspection of the size-frequency distributions further supports this conclusion. No detectable mode of such small scallops appeared after October at any field site: at IMS, where juvenile scallops grew so slowly in some locations, small scallops persisted until December, although not as small as the 3-mm size class. A temporal increase for the dominant (or sole) mode is apparent at all sites, attributable to the growth of scallop cohorts already present in October (Figs. 5 and 7).
Previous studies demonstrated that current speeds affect scallop growth (Kirby-Smith 1972, Eckman et al. 1989). Although we do not have direct measurements of current flow in or around our spat collector bags, we suspect that decreased flow into spat collector bags held in the IMS mesocosm pond accounts for the lower numbers of recruiting scallops, their generally smaller sizes, and their higher size variation at any given date. The >2,000 scallop recruits collected in the IMS pond outside of spat bags in March 2008 were much larger than those inside the bags at that time, implying that the slow flows of currents inside the protected pond limited the capacity of the water to penetrate into the bags and thereby caused lower fluxes of scallop larvae and greater food limitation inside bags. At all field sites the numbers of scallops per bag were greater than the those in the IMS pond and at all field sites except Swash in 2008 (a location often subjected to persistently lower salinities), bay scallop recruits in bags were larger than those in the IMS spat collectors, consistent with higher flow and food delivery into the bags under the higher-energy conditions of tidal currents and wind-driven exchanges in nature. Redesigning flow in the IMS pond to increase water exchange into spat collector bags held there could substantially improve their scallop production in the protected environment of the pond.
Attrition of spat collectors at the field sites decreased the yield of scallops and provides guidance to where spat collector bags should be installed in future deployments. Our modifications to the anchoring system in the second year reduced, but did not eliminate, the losses of bags at Yellow Shoal (74% lost by December 2007 vs. 50% lost by December 2008) and Lovett Island (63% lost by December 2007 vs. 30% lost by December 2008). We observed no change in the loss of spat collector bags at Middle Marsh from 2007 2008 (26% vs. 30%). Persistence of bags was generally greater at sites that were protected from long wind fetch by surrounding marshes, namely Hoop Pole Creek and Middle Marsh. However, Hoop Pole Creek suffered from unexpectedly high sediment deposition, resulting in loss of all bags by March 2008. Thus, to some degree, appropriate site locations will have to be determined by trial and error.
Performance of Alternative Spat Collector Designs
Observed high settlement of bay scallop spat on grass stubble on the bottom of the IMS mesocosm pond in fall 2007 at 51.5 scallops [m.sup.-2] at levels much higher than on collector bags in the pond (5.9 scallops [m.sup.-2]) and equivalent to the highest recruitment efficiency observed at any field site in that year (51.2 scallops [m.sup.-2] at Middle Marsh) motivated us to design and test new substrate-based spat collectors. The spring 2008 trial of the three substrate-based designs produced little but crabs. Recruitment of scallops onto spat collector bags in spring (May) 2008 exhibited a density equivalent to the rail (October) 2007 recruitment onto identical bags in this pond, so lack of larval settlement potential cannot explain why substrate-based collectors failed to produce spat in spring 2008. In fall 2008, the results for the X-type collector were dramatically different. The X-shaped collector had a high recruitment (38.5 scallops [m.sup.-2]) of scallops that grew well, as compared with those in the spat collector bags in the same pond at the same time. In fact, the scallop recruitment on X-shaped collectors was four times greater per unit surface area than on the traditional spat collector bags deployed in the pond and sampled at the same time (December 2008). The half-pipe collectors again produced few recruits, with extensive sedimentation on the upper surfaces a likely inhibitor of success. Consequently, X-shaped collectors may hold real promise for producing bay scallop recruits. However, understanding the radically different results between spring and fall is necessary before reaching any firm conclusions. In addition, the mesh bag around the internal collecting mesh of traditional spat collectors has been assumed to provide some protection against predators such that bags would represent better long-term nurseries. Further studies of temporal change in scallop numbers on the X-shaped and bag-type collectors are needed to test the hypothesis that bags do limit predation on early life stages.
Funding was provided by the NC Sea Grant Fishery Resource Grant Program (FRG 06-EP-04) to C.H. Peterson and D. Gaskill. Field and laboratory support was provided by C. Bland, L. Brown, R. Dunn, B. Fegley, E. Hansen, K. Hart, C. Logan, C. Martin, J. Meiners, J. Purifory, L. Rath, T. Revelle, I. Rodil, G. Safrit, E. Satterthwaite-Phillip, G. Shaw, C. Tommerdahl, B. VanDusan, and M. Wong.
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STEPHEN R. FEGLEY,* CHARLES H. PETERSON, NATHAN R. GERALDI AND DAVID W. GASKILL
UNC Institute of Marine Sciences, University of North Carolina at Chapel Hill Morehead City, North Carolina 28557
* Corresponding author. E-mail: email@example.com
TABLE 1. Mean ([+ or -] 1 SE) densities (individuals [m.sup.-2]) of adult bay scallops at each location within each year immediately after each spawner sanctuary inside a stockade was created (initial) and several months later (final). The Middle Marsh stockade may have been poached in both years of the study (see text for details). Dates when the respective stockades were created and sampled are presented in the top row of each year. x--not tested at this location in this year. Location Lovett Island (Bogue Sound) Year Initial Final 2007 29 Aug 22 Oct Inside 2.8 (0.7) 2.2 (0.6) Outside 1.4 (0.4) 0.3 (0.2) 2008 18 Aug 10 Nov Inside 6.3 (0.8) 6.5 (1.2) 1.9 (0.4) 2.7 (0.6) Location Middle Marsh (Back Sound) Year Initial Final 2007 10 Sep 23 Oct Inside 2.8 (0.7) 0.2 (0.1) Outside 0 0 2008 14 Aug 7 Nov Inside 18.4 (2.6) 0 Outside 2.5 (0.6) 0.3 (0.2) Location Swash (Core Sound) Year Initial Final 2007 x x Inside x x Outside x x 2008 11 Aug 31 Oct Inside 25.3 (3.1) 11.4 (1.1) Outside 2.8 (0.5) 1.2 (0.4) TABLE 2. Results of separate, model I, two-way ANOVA (time = initial vs. final, location = inside versus outside of the stockade) conducted on the densities of scallops in- and outside the stockade spawner sanctuaries. In each case transformation (shown after the site name) of the original data was needed to achieve homoscedasticity (as determined by a modified Levene test) and residual plots with no obvious patterns. Means associated with significant factors were compared using Tukey HSD test (results of these tests are described in the text). Bogue Sound (Lovett L) in 2007: ((x + 1).sup.-1] -1)/(-0.176) Time 1 13.19 2.38 0.1249 Location 1 53.68 9.69 0.0022 Time x location 1 9.09 1.64 0.2021 error 158 5.54 Bogue Sound (Lovett L) in 2008: In(x +1) * 3.2 Time 1 4.68 0.47 0.4926 Location 1 233.49 23.62 -0.0001 Time X Location 1 3.20 0.32 0.5702 Error 156 9.89 Core Sound (the Swash) in 2008: In(x +1) * 5.596 Time 1 351.00 14.5 0.0002 Location 1 4626.70 191.1 -0.0001 Time x Location 1 11.99 0.5 0.4826 Error 158 24.24
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|Author:||Fegley, Stephen R.; Peterson, Charles H.; Geraldi, Nathan R.; Gaskill, David W.|
|Publication:||Journal of Shellfish Research|
|Date:||Aug 1, 2009|
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