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This paper was submitted by the faculty of FAU’s Harbor Branch Oceanographic Institute.
Notice: ©2002 National Shellfisheries Association, Inc. http://shellfish.org/. This manuscript may be cited as: Arnold, W. S., Marelli, D. C., Parker, M., Hoffman, P., Frischer, M., & Scarpa, J. (2002). Enhancing hard clam (Mercenaria spp.) population density in the Indian River Lagoon, Florida: a comparison of strategies to maintain the commercial fishery. Journal of Shellfish Research, 21(2), 659‐ 672. JOllnral ofShellfish Research, Vol. 21, No.2, 659-672. 2002.
ENHANCING HARD CLAM (lYIERCENARIA SPP.) POPULATION DENSITY IN THE INDIAN RIVER LAGOON, FLORIDA: A COMPARISON OF STRATEGIES TO MAINTAIN THE COMIvIERCIAL FISHERY
WILLIAl\tl S. ARNOLD,):\: DAN C. MARELLI,l MELANffi PARKER/ PHILIP HOFFMAN,! MARC FRISCHER,2 AND JOHN SCARPA3 IFlorida Fish and Wildlife COllsenatioll Commission, Florida /l1arine Research Institute, 100 Eighth Avenue SE, St. Petersburg. Florida, 33701-5020; 2Skidaway Institute of Oceanography, 10 Ocean Science Circle. Savannah, Georgia, 31411; 3Harbor Branch Oceanographic Institution, 5600 U.S. Highway 1 North. Ft. Pierce. Florida, 34946
ABSTRACT Hard clams of the genus Mercenaria support an important commercial fishery in the Indian River Lagoon on the east central coast of Florida. The fishery is relatively young but has proven to be quite sporadic, wilh two periods of exceptional landings (mid·1980s and mid-1990s) bounded by periods of almost complete fishery collapse. In response to a request from commercial fishery participants, three strategies for enbancing the abundance of harvestable hard clams in the lagoon were compared. The first strntcgy involved harvesting adult clams from a low·density population and transplanting them at high density in a concentrated area, in an effort to increase fertilization efficiency and thereby maximize reproductive success. 11Ult approach does not appear to be viable in the Indian River Lagoon because mortality of transplanted clams approached lOO% and because Indian River hard clams display a remarkably high incidence (>80%) of gonadal neoplasia. Neoplasia reduces the life span of Indian River hard clams relative to their northern congeners and probably reduces fecundity of those clams that do survive. The second strategy involved seeding juvenile clams at very high densilies (843-7165 m-2 depending upon seed size), again with the intent of maximizing fertilization efficiency but with the additional goal of maximizing residual reproductive value. Various planting treatments were tested in an effort to minimize mortality of seed clams. but losses were still high (generally >70%), and the yield did not appear to justify the cost. TIle final strategy involved spawning large numbers of hard clams in the laboratory, fertilizing the resultant eggs, and immediately releasing the larvae at a predetermined location in the lagoon. Large numbers of larvae did appear to survive the 8-day larval life span, but it remains to be seen whether those larvae will translate into hurvestable hard clams. In any event. enhancement of hard clam populations appears to be feasible only when the density of naturally occurring clams is so low that reproductive viability is compromised. Otherwise, natural reproductive potential will swamp any artificial efforts at population enhancement.
KEY WORDS: Me rcellQria, Indian River Lagoon, population enhancement. spawner transplant, seeding, larval release
INTRODUCTION coast of Florida. For our study, we considered only that area of the Indian River Lagoon (IRL) system that is located north of Sebas Hard clams of the genus support an economically Mercel1aria tian Inlet (Fig. 2) and that is composed of the Indian River (lR), thy. and culturally important fishery in the Indian River Lagoon on the Banana River (BR), and Newfound Harbor. Prehistorically, the east central coast of Florida. This is a relatively new fishery that IR.L was a single continuous basin, and the IR and BR were con developed in the early 1980s (Fig. I) and was originally centered nected to the south at Dragon Point and to the north through near the town of Grant in southern Brevard County (Fig. 2). The Banana Creek. In recent years, development and bridge construc clam population that supported the original fishery collapsed in the tion have resulted in the subdivision of the IRL into a series of late 1980s, probably in response to decreased salinity caused by basins that are defined by the causeways that span the lagoons. the release of St. Johns River floodwaters into the Indian River Water exchange between basins is restricted to the areas under the basin (Barile and Rathjen 1986). Another substantial set of hard bridges that connect the causeway dikes (Evink & Morgan 1982). clams was detected in the northern Indian River Lagoon, between Additionally, the pattern of wuter exchange between the IR and BR Cocoa and Titusville, in the early 19905. That population sup has been altered in the Jast 40 years. Development ofthe Kennedy ported a profitable and productive fishery throughout the mid Space Center essentially severed the Banana Creek connection 19905 (Fig. I), but the population again collapsed and again the between the two lagoons (McCall et al. 1970), and construction of prox.imate cause appeared to be decreased salinity resulting from the Canaveral Barge Canal in the early 19605 (Yusof 1987) created above-average rainfall in the watershed. As of the year 2001, the a new connection south of Banana Creek. fishery for naturally occurring hard clams in the Indian River Reported clam landings from the Indian River Lagoon for the Lagoon has remained depressed, although a small number of fish past 20 years suggest that at least under certain conditions, the ermen continue to pursue lhe few clams !.hat are available. capacity for production of hard clams in the lagoon is substantiaL The Indian River is a shallow, narrow, bar-built lagoonal sys However, no single basin of the lagoon appears to be consistently tem that stretches from Oak Hill to Stuart along the central Atlantic productive. Instead. an individual basin may support a dense clam population for several years, after which the population collapses and remains depressed until another major sel of juvenile clams Present Address of Dan C. Marel1i: Florida Stnte University, 036 Mont occurs eiUler in that basin or in another area of the lagoon. Envi gomery. Tallahassee, Florida, 32306-2310. Present Addrc5s of Philip Hoffman: Pinellas County Department of Envi ronmental conditions can vary substantially among basins, creating ronmental Management.. Environmental Resources Management Division, a potential mismatch between successful spawning events (Hes 300 South Garden Avenue, Clearw:lIer, Florida, 33756. selman et aI. 1989) and the environmental conditions necessary to "'Corresponding Author: E-mail [email protected] support the survival and development of that spawn.
659 660 ARNOLD ET AL.
2000 -,------,
-o g 1500 ~
~>< t/J +'"as ~ 1000 'to-o t/J "C C :::3 500 . o a..
Year Figure 1. Commercial hard clam (Mercenaria spp.) landings from Drevard County. Florida. Data from 1960 through 1985 were provided by the National Marine Fisheries Sen-icc. Data from 1986 onward were provided by the Florida Marine Research Institute's Fisheries Dependent Monitoring Program.
Salinity is one of the key environmental variables affecting the in the lagoon. Here, we compare three approaches that might be success of hard clam populations. Adult dams are not well adapted effectively applied in the IRL. The spawner transplant approach to salinities below 20 practical salinity units Cp.s.u.), and embry involves harvesting mature animals and subsequently replanting onic and juvenile clams tend to be even more sensitive (Castagna them in an area that is either more suitable for survival and repro & Chanley 1973). In the Indian River Lagoon, salinity may range duction or that will allow larvae to disperse to such areas (e.g., from less than 15 p.s.u. (Barile & Rathjen 1986) to more than 40 Carter et al. 1984). Generally, animals are harvested from a rela p.s.u. (Young & Young 1977) with extreme conditions causing tively large area (square kilometers) and replanted into a much even those limits to be exceeded. Furthermore, the salinity condi smaller area (square meters), thus concentrating potenrial spawners tions in one basin of the lagoon may be ideal for clams, whereas in an effort to increase reproductive success. This strategy has been those in an adjacent basin may be inimical to clam survival (e.g., used in efforts to increase the population abundance of a variety of McCall et al. 1970, Barile & Rathjen 1986). Thus, a suitable match organisms, including hard clams (e.g., Carter et a1. 1984), bay between environment and biology may be very localized. which is scallops (Peterson et a1. 1996), and abalone (TegI.1er 1992). The why a commercially successful hard clam spawn is a rare event in rationale for the seeding approach is similar to that for the spawner the Indian River Lagoon. transplant approach: the animals are concentrated in a small area in The diversity of water quality conditions in the lagoon may the hope of increasing reproductive success. Seeding differs from create difficulties for the natural clam populations occupying the spawner transplants in that young, generally prereproductive ani IRL, but it also may create an opportunity for enhancement of mals are planted (e.g.• Marelli & Arnold 1996). Thus, the residual those populations. Although it is difficult to predict when condi reproductive value (Ricklefs 1979) of the planted animals should tions will be suitable for reproduction, recruitment, growth. and be maximized relative to a spawner transplant operation that may survival of hard clams, gross conditions (e.g., salinity. dissolved include a variety of age classes of the target organism. Seeding as oxygen, turbidity; Arnold et al. 2000) can be evaluated from the an enhancement strategy also has a rich history in the population results of frequent water-quality monitoring activities. Such moni enhancement literature, including numerous efforts involving hard toring programs are ongoing in the lagoon, under the auspices of clams (e.g., Castagna & Kraeuter 1977. Walker 1985, Peterson the St. Johns River Water Management District and the Florida ct al. 1995, Marelli & Arnold 1996). The third approach dis Department of Agriculture and Consumer Services. These pro cussed herein involves the release of recently fertilized eggs grams make it possible to identify water-quality conditions suit directly into the lagoon, thereby circumventing the spawning pro able for clams. although it is not possible to ensure that those cess entirely. This strategy has been tested with abalone (Preece conditions will occur at a time and place coincident with a spawn et a1. 1997, Shepherd et al. 2000). but to our knowledge had not ing event. been tested with marine bivalves such as the hard clam prior to our A variety of options are availoble to increase clam abundance study. CLAM POPULATION ENHANCEMENT IN FLORIDA 661
Atlantic Ocean
Cape Canaveral
-.0 -";;"~..---....,
Figure 2. Indian River Lagoon, Florida, showing locations of spawner transplant and seeding studies in the Indian River and Banana Rh'er lagoons. Star indicates site where adult clams were collected [or the spawner transplant study. Closed circles indicate sites of the spawner transplant and seeding studies in the Indian River lagoon and of the spawner transplant stud)' in the Ban:ma River lagoon.
MATERIALS AND /'"IETHODS spray paint to allow for later identification, and then replanted the clams into an area closed to shelltish harvest. The objective of this Spawller Trallspla1lts project was to concentrate reproductively mature clams to maxi mize the number of successfully fertilized eggs (Levitan 1995). In the spawner transplant component of this study, we har During October 1998 (fall relay), we used professional clam har vested adult clams of a variety of sizes, marked the shells with vesters to gather 5,000 clams from an area of the Indian River 662 ARNOLD ET AL.
Lagoon north of Titusville (Fig. 2). The harvested clams were fernan (1994), with the addition of categories for early spawning returned to shore, where the shells were allowed to dry for several and for unreadable samples. hours and then labeled with yellow spray paint for later identifi cation. The labeled clams were split into two groups of 2,500; Lhe Seeding following day, the clams in one group were planted at a site in the Indian River and clams in the other group at a site in the Banana On October 13, 1998, we planted three size-classes of seed River. To transplant the clams, we poured them over the side of clams under four protective conditions at our study site in the our research vessel as we traversed the extent of a IOO-m 2 study Indian River lagoon. Seed clams were obtained from the Division plot demarcated at each corner by n crab trap float. Similar trans of Aquaculture, Harbor Branch Oceanographic Institution, Ft. plants were conducted during January (winter relay; blue paint), Pierce, Florida. Seed size-classes were 2 mm mean shell height April (spring relay; green paint), and August 1999 (summer relay; (SH = maximum distance from umbo to ventral margin), 8 mm red paint) to assess the best season for conducting transplant op mean SH, and 16 mm mean SH. Protective conditions included no erations. cover, oyster shell cover, plastic mesh netting cover, and a com Sampling of the adult relay plots was conducted 2 wk after each bination ofoyster shell and plastic mesh netting cover. Oyster shell relay event and again 3, 6, 9, and ]2 mo after each relay event was purchased from a commercial aggregate company and aver (Tables 1 and 2), thus allowing us to assess reproductive status and aged approximately 5 cm in maximum shell diameter. Fifteen mortality during each season of the year for each transplant date. millimeter mesh plastic netting (Vexar) was purchased from a To sample, we thoroughly hand-raked the contents of 20 randomly commercial aquaculture supply company. Two replicates of each selected 0.25-m2 quadrants within each plot, returned all recovered size-class by protective-cover combination were deployed, for a hard clams to the research vessel for identification and counting, total of 24 treatments, each of which was assigned to an individual and subsampled 15 randomly selected clams from each plot (2-wk I-m2 plot. We planted approximately 7,165 2 mm SH clams (27 g samples excepted) for later reproductive analysis. The location of wet weight), 2,340 8 mm SH clams (259 g wet weight), or 843 16 each sample quadrant was marked with a PVC stake to ensure that mm SH clams (119 g wet weight) in each plot, which equated to plots were not resampled. Note that the Indian River spawner approximately $30 of clam seed per plot. Before planting, all transplant plots could not be effectively sampled after the passage clams in the 2 and 8 mm SH size-classes were marked with tet of Hurricane Irene in October 1999 because the sample plots were racycline (Marelli & Arnold 1996), and the valves of all clams in destroyed by that storm. The Banana River spawner transplant the 16 nun SH size-class were painted so that we could identify plots appeared to be unaffected. them later. Clam samples for reproductive analysis were processed accord On October 27 and 28, 1998, we used a hydraulic suction ing to the following procedures. Live animals were returned to the dredge (e.g.• Peterson et a1. 1983) to sample five O.0278-m2 cores laboratory, where the gonad was excised from each animal and from each replicate plot to determine initial planting mortality. On stored for 24 hours in a solution of5% formaldehyde in seawater. November 11 and 12, 1999, we again collected five 0.0278-rn2 Large gonads were removed fro~ the fixative solution after about' suction dredge samples from each replicate plot to estimate mor four hours, lacerated to ensure penetration of the fixative, and tality and shell growth after one year. The location of each repli returned to the fixative for the remaining 20 hours. Afterwards, the cate was determined by using a string grid to ensure that the gonads were thoroughly rinsed in tap water to remove fixative, replicates within each plot did not overlap. dehydrated through a series of alcohol concentrations, and infil trated with JB-4 mounting plastic. Two 3.5-J.1m sections, separated Lan'al Release from each other by at least 50 J.1m, were then cut from each embedded gonad using a diamond blade microtome and the sec The larval release study was designed to determine the feasi tions mounted on labeled glass slides. Mounted sections were bility of directly introducing fertilized clam eggs into the lagoon stained with hematoxylin and eosin, covered, and stored for later and allowing them to grow and disperse as a natural population. microscopic analysis. This approach to clam population enhancement allows us to cir Gonad sections were examined under a binocular light micro cumvent the expensive and labor-intensive process .of growing scope and each sample assigned a qualitative ranking of gonad clams in the laboratory, while still ensuring that large numbers of developmental stage as described by Arnold et al. (1997) and fertilized eggs will be available in the natural environment. Larval summarized in Table 3. This ranking scheme is a composite of release also allows us to target areas of the lagoon that are suitable gonad development ranking schemes previously used by for the growth and survival of clams and to rapidly respond to Loosanoff (1937), Jaramillo et aL (1993), and Walker and Hef- changing conditions. However, to be able to determine the success
TABLE 1. Planting and sampling dates for the hard clam (l'-lerulJar;a spp.) spawner transplant study in the Indinn Ri\'er lagoon.
Season Plant Date 2-wk Sample 3-mo Sample 6-mo Sample 9-mo Sample 12-mo Sample
Fall 10/28/98 (100) 11/9/98 (127) 2/3/99 (107) 4/28/99 (114) S/lO/99 (57) 10/26/99 (17) Winter 1/27/99 (l00) 2/3/99 (59) 4/28/99 (238) 8/1 0/99 (53) 10/26/99 (IS) N/A eN/A) Spring 4120/99 (100) 4/28/99 (180) 8/10/99 (131) 10/26/99 (38) N/A (N/A) N/A (N/A) Summer 8/3/99 (121 ) 8/10/99 (171) 10/26/99 (6) N/A (N/A) N/A (N/A) N/A (N/A)
Numbers in parentheses indicate sample size of hard clams collected on Ihat dale. Note that on each initial seasonal sampling date, clams were randomly sampled from nil of those harvested, whereas on later dales sample size reOccts the number of clams actually collected from each study plot. 1
CLAM POPULATION ENHANCEMENT IN FLORIDA 663
TABLE 2. Planting and sampling dates for the hard clam (Mercenaria spp.) spawner transplant study in the Banana m"cr lagoon.
Season Plant Date 2-wk Sample 3-mo Sample 6-mo Sample 9-mo Sample l2·mo Sample
Fall 10/28/98 (l00) 11/9/98 (73) 2/3/99 (73) 4/28/99 (96) 8/11/99 (97) 10/25/99 (3) Winter 1/27/99 (100) 213/99 (200) 4/28/99 (144) 8/1 1/99 (91) 10/25/99 (16) 1125/00 (2) Spring 4/20/99 (100) 4/28/99 (161) 81L 1/99 (87) 10/25/99 (31) 1/25/00 (10) 4/25/00 (1) Summer 8/3/99 (121) 8/11199 (229) 10/25/99 (20) 1/25/00 (7) 4/25/00 (9) 8116/00 (2)
Numbers in parenthcses indicate sample size of hard clams collected on that date. Note that on each initial seasonal sampling dale, clams were mndomly sampled from all of those harvested, whcreas on l of this enhancement strategy, it is necessary to be able to track and aquarium bags and transported 10 the study site for immediate sample the animals during the planktonic phase of their life. release. Spawning Larv:d Tracking The larval release strategy requires the production of large Hard clam embryos were transported from the spawning facili numbers of viable hard clam embryos that can be successfully ties at Harbor Branch to our study site in the Banana River lagoon transported to and released at the site targeted for enhancement. and released at 2030 EDT on May 16, 2001, at a site approxi Adult hard clams were collected from various areas of the Indian mately 1.75 m deep. Water temperature at the site was 28°C and River Lagoon on several dates during 1999. The clams were trans salinity was 22 p.s.u., whereas the temperature of the water in ported to a holding area at Harbor Branch Oceanographic Institu which the larvae were transported was 25.8°C and the salinity was tion, where they were conditioned in preparation for spawning. 27.7 p.s.u. Before larval release, five subsurface drifters (Davis Conditioning consisted of holding the clams for several months in 1985, Hitchcock & Arnold. unpublished data) were deployed in a a small lagoon on the Harbor Branch campus. On the day before box-and-one pattern (one drifter at each corner of a 10 m x 10m spawning, the clams were transferred from the holding lagoon to a box, with a single drifter in the center of the box), and their initial refrigerated storage area and held overnight. On the following day, positions were recorded using a differential Global Positioning the clams were removed from the storage area, placed in equally System. Then, at a depth of approximately 0.5 m, the clam larvae spaced rows on each of three spawning tables, and submerged in were gently poured from the bags into the center of the drifter ~pproximately 10-15 em of 28 p.s.u. seawater. During the next six array. Gradual mixing between the transport water and the la hours, the clams were exposed to cycles of cool and wann water goonal water was allowed in an attempt to minimize osmotic and induced to spawn. As each individual clam initiated spawning, shock. the sex of the animal was identified and the clam was removed During daylight hours on May 16, we collected thirteen 200 L from the table and isolated in containers with other clams in small water samples from the targeted release area to determine the groups of males or females. The clams continued to spawn within prerelease concentration of hard clam larvae in the study basin. On the containers, and the resultant eggs were pooled and concen May 17 and 23, 2000, post-release water samples were collected to trated on a 35-f-Lm-mesh sieve and exposed 10 a sperm concentra determine the distribution and density of the larval mass. For the tion adequate to ensure fertilization of all eggs. Total egg produc May 17 sampling, when the larval mass was predicted to be rela tion and fertilization success were determined microscopically, tively concentrated, the subsurface drifters were visually located after which the developing embryos were transferred to 20 L and the position of each drifrer recorded. Sample collection loca tions for hard clam larvae were then selected based upon the lo cation and distribution of the subsurface drifters. On May 23. after TABLE 3. diffusive processes were anticipated to have spreap the larvae Gonad stnging scheme for female hard dams (Mercenaria spp.) throughout the study basin, samples were collected at each of 23 collected from tbe Indian River Lagoon, including female clams grid nodes equally distributed throughout the basin. transplanted to study sites in the Indian River and Banana River On each sampling date, samples of hard clam larvae were ob lagoons and sampled during various times of the year, and from tained by using a Jabsco Model 34600-0000 diaphragm pump to their undisturbed conspecilics. collect volumes of water that ranged in size from 100-400 L, depending upon the projected density of larvae. Water was Reproductive Numerical pumped through a 150-f.Lm-mesh sieve to remove large objects, Status Description of Gonadal Tissue Stage and then captured in a 63-f-Lm-mesh plankton net. Each resultant No Data Tissue unreadable 0 sample was removed from the cod end of the plankton net and Inactive Gonad tissue undifferentiated 1 carefully distilled to a volume of approximately 30 rnL, then trans Developing Tissue differentiatcd, eggs present 2 ferred to a 50-mL screw-cap centrifuge tube, labeled, and placed Ripe Tissue full of eggs 3 on ice until arrival at the laboratory, where it was frozen at ap Early spawning Eggs being shed, but follicles still 3.5 proximately -5°C. Within one month of the completion of the full in appearance study. all of the frozen water samples were sent to the Skidaway Spawning Many eggs shed, follicles appear 4 Institute of Oceanography for determination of the presence and partially empty abundance ofclam larvae. Samples were analyzed for the presence Spent Follicles nearly empty 5 of hard clam larvae using a previously developed genetic probe 664 ARNOLD ET AL. that is both quantitative and Merccflaria-specific (e.g., Frischer et tially in all Banana River plots between the August 11 and October a1. 2000). 25, 1999, sample dates (Fig. 3). Hurricane Irene swept through our RESULTS study area on October 16, 1999, and salinity near our Banana River Spawner Transplants study site decreased to a study-period minimum of less than 15 p.s.u. at the end of October 1999 (Fig. 4). At both the Indian River and Banana River study sites, the The high levels of mortality that we observed in our transplant mortality ofrelayed hard clams was severe, particularly during the plots may have been influenced by the inability of clams (espe summer and early fall of 1999. Also at both sites, considerable loss cially large clams) to reburrow following initial harvest TIle In of clams from the plots was associated with the initial transplant. dian River study site was characterized by a soft sand/mud sub At each site, on all dates, we transplanted an average of 25 clams strate that appeared to provide little resistance to burrowing clams. 2 m- , and within 2 wk the densities for all plantings had decreased Upon re-sampling that site two weeks after the faU transplant, 26% by' more than 50% (Fig. 3). After the initial transplant event and of all clams collected remained on the surface, and three monlhs the loss of clams associated with that event, clam densities stabi after the fall transplant 20% of all clams collected still remained on lized throughout the winter, spring, and early summer at both sites the surface. In contrast, the Banana River study site was charac (Fig. 3). During late summer and early fall of 1999, the clams terized by a hard sand bottom that appeared to provide consider experienced substantial mortality, possibly as a result of decreased able resistance to clams attempting to burrow. Two weeks afler the salinity associated with Hurricane Irene (Fig. 4). As noted previ Banana River fall transplant, 47% of all clams collected remained ously, that stann destroyed our study plots in the Indian River. n on the surface, and three months after the fall transplant 34% ofall also appears to have had a severe detrimental effect on the clams clams collected remained on the surface. At both sites, failure to planted in the Banana River, as clam density decreased substan- burrow was related to clam size. A comparison of the mean SH of buried versus unburied clams at each site two weeks after trans 25...... ------, plant during both fall and winter, indicated that the clams that 20 A) failed to burrow were significantly larger than those that success 15 fully reburrowed (Hest, see Fig. 5A, B, E, and F for respective P 10 values). We detected no significant difference in SH between bur ied and unburied clams at either site three months afler transplant (Fig. 5C and D), although only clams transplanted during the fall :30 were compared. During the spring and subsequent sampling epi 25 sodes, we discovered few clams at either site that were both alive B) 20 and unburied. 15 After the initial episode of transplant mortality, overall mortal ity of relayed clams did not 3ppcar to be size-related. At the Indian N 10 E River study site, the size distribution of hard clams did not differ ...... significantly among sampling dates (Simultaneous Test Procedure; ClJ 25 Sakal & Rohlf 1995) except during the summer transplant study E (Fig. 6). During the final sampling episode (October 26, 1999) of ro 211 C) the summer transplant study, the size distribution of planted clams 0 15 differed significantly from the size distribution recorded during the 10 previous two sampling dates and appears to have shifted towards a preponderance of small clams (Fig. 6). For all four of the trans 0 plant episodes at the Banana River study site, a significant shift in 25 clam size distribution was detected for the October 25, 1999, 2Q D) sample date, and for all sample dates subsequent to October 25, 15 relative to all sample dates preceding October 25 (~ig. 7). The only 10 exception to this pattern was from the spring transplant study, for which the size shift was not detected until the January 25 sampling episode (Fig. 7). .I. T ...,.. 0 I During each seasonal harvesting event, a subsample of 15 # ~~ ~ 45> ~~ ...... #' ~ III ....# ....r;,# / ~ ~" clams from each plot was returned to the laboratory for analysis of reproductive condition and for a comparison with control samples collected from the natural population on the same date. However, Date the results from only the fall planting date at both study sites are Figure 3. Mean density of hard clams (Mercellaria spp.) on various included in the present analysis because that is the only planting sampling dates nfter transpl::mtation during (A) fall 1998; (B) winter date for which adequate sample numbers were available for all 1999; (C) spring 1999; and (D) summer 1999 :It the Indian Rh'er sample dates from both sites. The female clams in the control lagoon (filled bars) and Banana River lagoon (open bars) study sites. samples had a pattern of reproductive development typical of In Sec Figure 2 for the location of each study site. Note that at the Indian River study sitc, all fOllr study plots ,,'cre destroyed by Hurricane dian River hard clams (Hesselman et a1. 1989). During fall and Irene during October of 1999 and no further samllling was conducted winter, when Hesselman et aI. (1989) reported that spawning oc nftcr tbat date. Error bars represent one standard de\'iation. On the curred in Indian River hard clams, most clams that we sampled November 9, 1998, sampling date, samples were pooled ~lDd no slan~ (control and transplant) were either spawning or were spent (Fig. dard dc\'ialion was calculated. 8). During spring, the season of peak spawning in indian River CLAM POPULATION ENHANCEMENT IN FLORIDA 665 35 ...,------, 30 -"""":' 25 :::s. tn...... C. ~ 15 .-t: -CO en 10 5 Sampling Date Figure 4. Salinity recorded at the Indian River lagoon (closed triangles) and Banana River lagoon (closed circles) study sites during September 1998 through October 2000. The dotted line provides a 20 p.s.u. reference. Data courtesy of the S1. Johns River Water Management District Surface Water Quality Monitoring Program. hard clams (Hesselman et a1. 1989), most of the clams from the towards female clams (0.50 males: 1.00 females, X2 = 42.89, df control sample were ripe, and lesser proportions were either = 1, P < 0.0001). spawning or spenL In contrast. most animals collected from the Seeding [ndian River and Banana River transplant plots during spring were in some stage of spawning. Finally, during summer the V.ISt ma Mortality of 2 mm hard clams was substantial within 2 wk of jority of control and transplant clams that we sampled were in tJle planting under all treatment conditions (Table 4). Only under mesh spent condition, which agrees well with the observation of Hes protection did the 2 mm size-class suffer less than 90% mortality, selman et al. (1989) that most Indian River hard clams are repro but even WitJl mesh protection the small clams experienced a mean ductively spent during summer. mortality of 85.6%. Survival of clams in the 8 mm size-class was Gonadal neoplasia is extensive in Indian River hard clams not much better; those clams also experienced >90% mortality in (Hesselman et al. 1988, Bert et a1. 1993), and this condition ap the open plots and at least 50% mortality within 2 wk after being pears to be related to hybridization between the two species of planted in the remaining plots. In contrast, clams in .(he 16 mm Mercenaria (M. mercenaria and M. campechiensis) that occupy size-class experienced <10% mean mortality in the mesh plots and the lagoon (Bert et al. 1993). We recorded neoplasia in 85% of the a mean mortality of 11.3% in the open plots. However, those clams male clams and almost 92% of the female clams that we collected suffered 30.4% mean mortality under the combined protective from the natural clam population of the lagoon. Male clams suf cover and>70% mean mortality in the shell plots. fered 93% and 86% neoplasia when harvested after transplant to Hurricane Irene also severely impacted our seed clam plots. the Indian River and Banana River, respectively. Female clams Nevertheless, on November II, 1999, we attempted to reconstruct suffered 96% and 93% neoplasia when harvested after transplant the experimental plots and we did conduct suction dredge sam to the Indian River and Banana River, respectively. pling of the reconstructed plots. We found no live clams in the seed When uIl samples of clams collected for reproductive analysis pIaL I y after planting. However, we cannot detennine whether that during the course of the spawner transplant study were pooled, lack of clams was due to the effects of Hurricane Irene or due to there were significantly more female than male clams (0.53 males: factors independent of the hurricane. 2 1.00 females, X = 42. I3, df = 1, P < 0.0001). However, sex ratio Lan'al Release was dependent upon clam size-class. For all clams that were .s; 60 mOl SH, we detected no significant difference in sex ratio (0.75 Spawning males: 1.00 females, XL = 1.43, df = 1, P = 0.232). In contrast, On May 16, 2000, approximately 550 million hard clam eggs for clams> 60 mm SHy the sex ratio was significantly skewed were spawned and collected. The eggs were then exposed to an 666 ARNOLD ET AL. 24 h, the drifters that tracked the water mass within which the A) larvae were released were transported towards the west until they approached the western shore of the lagoon (Fig. 9). As the drifters approached the shoreline, they gradually swung around to the ~I n , I I ,n north, but four of the five drifters contacted the bottom, hung up, and were relrieved. The fifth drifter passed through a small bridge B) at the western end of the SR 528 causeway and was retrieved to prevent its 105s. Analysis of water samples collected on May 16. before the .n ~l ,0 n ~ , release of cultured larvae, detected no evidence of naturally oc >. 0 curring hard clam larvae in the study basin. On May 17, after our c C) larval release during the evening of the 16th, we detected larvae at 0.> p P=09~11 :J ulli only one sampling station (Fig. 9), probably because the larval 0- mass remained tightly constrained in the area around that station. m ~l n t- r I By May 23, the larval mass had spread throughout the basin (with LL the exception of the southeast corner), although several peaks of +-' c D) abundance were detected. Two of these peaks were in the north 0.> () west corner of the basin, where larval density exceeded 7 veligers t- o.> ~I L-1 (Fig. 10). 0... P P DISCUSSION 0043 E) P = 0. 1 We tested three strategies for enhancing the abundance of har vestable hard clams in the Indian River Lagoon, Florida. The first n Jilll~,n~, ~I Ip p I , r strategy involved harvesting adult clams from low-density popu lations and concentrating them in high-density patches in an effort to increase fertilization success and the production of viable larvae. This strategy does not appear to be cost-effective because most of the clams that we transplanted did not survive the 12-mo moni toring period. Furthermore, the vast majority of the clams that we transplanted were infected with gonadal neoplasia, a disease that progressively reduces fecundity and probably proceeds to a fatal Size-Class (mm) outcome (Yevich & Barry 1969, Hesselman et a1. 1988, Eversole Figure 5. Percent frequency of occurrence of hard clams (llferce,wria & Heffernan 1995). The second strategy involved planting small spp.) at each of the Indian River lagoon and Banana Rh'er lagoon seed clams at relatively high densities, again with the intention of study sites during fall and winter sampling events. Filled bars repre creating concentrated patches of reproductively active clams. This sent the pcrcentage of clams that werc recovered on the surface of the approach has one anticipated advantage (n longer life span for the plots, and open bars represent the percentage of clams tbat were bur spawners) and one unanticipated advantage (avoidance of gonadal ied upon reco\'ery. (A) Indian River Call planting, 2-wk sampling; (B) neoplasia) when compared with the spawner transplant strategy. Banana River Call planting, 2-wk sampling; (e) Indinn River fnll plnnt However, the firsl advantage may be offset by the high rates of ing, 3-mo sampling; (0) Banana Rh'er fall planting, 3-mo sampling; mortality experienced by most size-classes of the seed clams, and (E) JndiclO River winter planting, 2-wk sampling; and (F) Banana the second advantage may be temporary because the seed clams River winter planting, 2-wk sampling. The P values included in each become increasingly susceptible to gonadal neoplasia, as they plot represent the probability (i-test) that the mean size of clams re covered on the surface was not significantly different than the melln grow older (Bert et al. 1993). Although survival t:ates were rela size of clams that were buried upon recovery. A P'::;: 0.05 indicates a tively high for the largest size-class of clams (16 mm mean SH), statistically significant difference in the menn size of surficinl vs. bur the considerable cost of those clams ($0.036 each) reduces the ied cl3ms. cost,effectiveness of this strategy. The third strategy involved cir cumventing the entire process of natural fertilization by releasing already fertilized eggs directly into the lagoon. Our results suggest amount of sperm sufficient to ensure fertilization of all eggs as that this strategy may be effective, but more information is needed. determined by microscopic examination. Spawning occurred in Ir appears that large numbers of clam larvae survived to an age at three "batches" beginning at 1200 EDT and ending at 1600 EDT. which settlement can be reasonably expected (8 days to set in Transport from Harbor Branch (1800 EDT departure) to the Ba culture conditions during May using Indian River water, B. Leem nana River release site (2030 EDT release) required approximately ing, personal communication). However, the ultimate fate of those 2.5 h, so the clams ranged in age from 4.5 to 8.5 h at the time of larvae has not been determined, and successful settlement needs to release. be demonstrated it! villo for this approach to have any validity. Larval Release The common currency with which to gauge the success of each of these strategies will be an increase in the abundance of hard Larvae were released at a site in the Banana River lagoon clams available for harvest by the fishermen. Hard clams in the between SR 520 and SR 528 (28°23.320' N latitude, 80°37.951'W Indian River Lagoon require approximately 18 months to achieve longitude) at 2030 EST on May 16.2000 (Fig. 2). During the next the legal harvest size of2.54 cm in shell width (Arnold et al. 1996). CLAM POPULATION ENHANCEMENT IN FLORIDA 667 50 ~------, 40 A) 30 20 10 lu~ .. 1 50 40 l/) C/} B) 3D E E m ~ > ::J .,1 ,I ::J 0 ..... ill1J I ..... o 0 50 III 100 Q) C'l 1 Cl 1 40 J9 511 C) ...m C) c Q) 40 t: 3D o CD L.. 30 (J Q) L.. 20 20 Q) 0. 10 a. 10 0 50 -r------. 100 1 1 40 50 D) D) 40 30 3D 20 10 0 Hard Clam Size·Class (mm) Hard Clam Size-Class (mm) Figure 7. Percentage of live hard clams (Mercenaria spp.) recorded Figure 6. Percentage of live flllrd clams (Mercenaria spp.) recorded within various size-classes during each seasonal sampling event fol within various size-classes during each sea<;onal sampling event fol lowing the (A) fall; (B) winter; (C) spring; and (0) summer transplants lowing the (A) Call; (B) winter; (C) spring; and (D) summer transplants in the Banana River lagoon. For each size.:class category within each in the Indian Riv~ lagoon. For each size-class category within each transplant season, sampling dates are plotted from left to right. Thus, tmnsplant season, sampling dates arc plotted from left to righL Thus, the leftmost bar within each size-class represents the sample collected the leftmost bar within each size-class represents the sample collected on the original date of harvest, and the rightmost bar represents the on the original date of harvest, and the rightmost bar represents the sample collected on the final sampling date. Sec Table 2 for sampling sample collected on the final sampling date. See Table 1 for sampling dates and sample size for each date. Missing bars indicate that no dates and sample size for each date. Note that samples were collected clams were collected from that size-c1llSS on that date. on only five dates for the winter transplant, four dates for the spring transplant, and three dates for the summer transplant because of the ducted in May 2000, surviving animals would have been expected impacts of Hurricane Irene, which destroyed the sample ploL~ on Oc to achieve harvest size during fall 2001. Information from the tober 16, 1999. Othenvise, missing bars indicate that no clams were fishery and preliminary results from our own sampling efforts collected from that sizc·class on that date. provide no evidence of a substantial yield of harvestable hard clams, as might be expected from the above timetables. Of course, For the adult transplants, it would have been possible to detect our study was conducted on a much smaller scale than would be adult offspring as early as spring 2000, assuming tllat clams trans· necessary to realize a significant contribution to future year-classes planted in fall 1998 spawned very soon after transplant. That is a (McHugh 1981). Our primary objective was to experimentally reasonable assumption considering that a fall spawn has been de compare three possible approaches to hard clam population en scribed for Indian River hard clams (Hesselman et a1. 1989) and hancement in the lagoon. From those results, we hoped to be able was similarly indicated by our fall 1999 reproductive data. For the to choose a single approach that has the greatest likelihood of seeding study initiated in fall 1998, we would have expected that success and then to apply that approach on a scale appropriate for maternally derived offspring would be available for harvest no success. earlier than fall 2000. Hard clams as small as 27 mm SH arc For the Indian River hard clam fishery, success is a quantifiable reproductively active in the Indian River Lagoon (Hesselman el al. parameter. The primary fishing grounds fall within the boundaries 1989), bUI clams are generally male during the first year of life of Brevard County, and commercial clam harvest in that county is (Loosanoff 1937). Thus, we would not have expected egg produc strictly regulated. Each clammer must be licensed, and only 500 tion from the 16 mm size~cIass of seed clams for at least 6 months licenses are allOCaled for the fishery. The goal of our enhancement post-planting (i.e., spring 1999). For the larval release study con- work is to provide the clam harvesters with a resource base that 668 ARNOLD ET AL. c::::::J Undifferentialed ~ Developing ~ Ripe ~ Earty Spawning E:::3 Spawning lITIIID Spent 8 8 7 6 B 9 11 13 13 B 10 2 100 -.------.-ITTT..------",=::r-- 80 >. (,) c Q) ::J 0- 60 ~ ll...... C Q) 40 (,) L. Q.) a.. 20 o Control IR 8R Control IR BR Control IR BR Control IR 8R Control IR BR Fall 199B Winter 1999 Spring 1999 Summer 1999 Fall 1999 Season Figure 8. Reproductive stages of female hard clams (Mcrcclloria spp.) collected on various seasonal sampling dates after the fall trunsplant Stages are as described in Table 3. Numbers across the top of the plot arc sample sizes. Control, clams that were harvested fmm the natural population during each season; DR. clams that were transplanted to the Banana River lagoon study site during fall and sampled during ench season; JR. clams that were transplanted to the Indian River lagoon study site during fall and sampled during each season. would allow them to survive a nadir in Lhe abundance of naturally the larvae survive to produce clams of legal harvest size. then 5 occurring clams. If il is assumed that $20.000 is a minimum ac billion larvae must be released. We have been able to consistently ceptable annual income for each clam fisherman and that the sale produce approximately 50 million eggs per spawning table per day price for each clam is $0.20, then it can be estimated that 50 during subsequent tests of the larval release strategy. and we have million harvestable clams must be produced each year to satisfy used four spawning tables per day during those trials. At thal rate, that harvest goal. Using the larval release strategy as an example, it would require 25 spawning days to produce 5 billion larvae. it is apparent that 50 million larvae must be released and must all Carriker (1961) estimated the mean survival rale of hard clam survive to harvest size to meet that goaL Similarly, if only 1% of larvae to be 2.6% when rates of flushing were low, but the mini mum survival rates that he reported were 0.1 % or less. At those minimum larval survival rates it would be necessary to increase TABLE 4. daily larval production by a factor of five and to increase the time Percent mortality of hard clam (MerceIJorio spp.) seed 2 wk after span of larval release to approximately 50 days to achieve our being planted under various protective conditions at the Indian stated goal. 11mt may be possible. but it would require a broad River stud)' site. scale effort that includes participation by the clam fishennen. Ad ditionally. our best-case larval production estimates require the Clam Open Mesh Shell Mesh/Shell provision of 800 broodstock per day. because we place approxi Size·Class Plots Plol., Plots Plote; mately 200 clams on each spawning table (equivalent to about 135 female clams with a 2: 1 ratio of females to males). Considering the 2 mm (rep 1) 96.6 86.9 91.0 92.5 present status of the hard clam fishery in the Indian River Lagoon, 2 mm (rep 2) 98.0 84.2 92.5 98.3 2 mm (menn) 97.3 85.6 91.8 95.4 the limited availability of broodstock to support egg production 8 mm (rep I) 91.7 53.8 71.7 84.9 may prove to be a serious problem that limits the effectiveness of 8 mm (rep 2) 97.5 88.9 62.5 90.2 the larval release strategy. Moreover, broodstock availability may 8 nun (mean) 94.6 71.4 67.1 87.6 also be adversely affected by the high incidence of gonadal neo 16 mm (rep 1) 20.9 5.2 65.2 48.7 plasia in Indian River hard clams. Large clams, which are eco 16 mm (rep 2) 1.7 7.6 80.9 12.2 nomically less valuable than small clams and are therefore more 16 mm (mean) 11.3 6.5 73.0 30.4 available for use in the spawning program. arc relatively rare in Protective treatments arc listed across the lOp of the table and the results Indian River waters and produce fewer eggs than would be pre from replicate treatments (and the mean of the paired replicates) are pre dicted based upon allometric considerations alone (e.g., Peterson sented within the body of the table. 1983, 1986). The clams that \ve have successfully used in previous The boldface rows represent the mean of the two observations. spawning efforts generally fall within the "topneck" commercial CLAM POPULATION ENHANCEMENT IN FLORIDA 669 ~ Land '* Release Point o Banana River Drillers 5116100 5117100 LaNai Distribution • 5118100 LillV;ll Concl!nlralmn IViay 17, 2000 n.2 (Vcli(lersll) 0.4 D.li 0.8 1 1.2 ~11 0 00 0.0 0.0 DO 0.0 00 00 00 At/antic 0.0 Ocean Banana River N A 0.5 0 0.5 1 1 Figure 9. C'ontour plot of the distribution of hard clam (Mercenaria spp.) larvae as estimated by sampling conducted on May 17, 2000, in the Banana River lagoon, Florida. Data are presented as number of clam larvae L-1, and htrva) conccntrations are depicted at the location of each samplc station. Also plotted are the locations of the subsurface drifters that were deployed during the evening of May 16, 2000. classification (average SH = 60 mm). There is a strong market site (Arnold et a1. 1997) and were similarly rare in the vicinity of demand for these clams, and seafood processors are reluctant to the Indian River study site. As a result, the artificial densities that sell them to us even at a premium price for fear of upsetting \Ve initially established in each planting study (spawner transplants previously established buyers. During times when native Indian and seeding) substantially exceeded the background density at River broodsrock are readily available from seafood processors each study site. The 550 million eggs that we contributed during because of high levels of harvest in the natural fishery, there is our larval release study equate to an average contribution from the little need for an enhancement program, and the contribution that spawn of more than 75 female "cherrystone" size hard clams could be realized from enhancement would be swamped by natural (Bricelj 1992), assuming 100% fertilization of naturally spawned production (Kassner & Malouf 1982). eggs. Two factors increase the value of the fertilized eggs that we Our estimate of the yield of harvestable clams from larval released. First, it is unlikely that 100% fertilization efficiency is release does not take into account post-setrlement losses, which realized in the natural environment (Levitan 1995). Second, be typically exceed 80% and may approach 100% under some con~ cause of the prevalence of gonadal neoplasia, egg production in ditions (Gosselin & Qian 1997). However, hard clam populations Indian River hard clams may be considerably less than that pre continue to thrive in Florida and throughout the eastern seaboard sented by Bricelj (1992) for northern US waters. If we estimate ofthe United States, so conditions suitable for survival must occur. mean production to be 1 million eggs per female, and we estimate 2 Perhaps it is more appropriate to consider our enhancement efforts a fertilization efficiency of 1% at the low clam densities «< 1 m- ) within the context of natural clam densities in the lagoon. During currently found in the lagoon, then the number of larvae that we the early 19805 in the area near Grant and during the mid-l 990s in released is equivalent to the number of larvae produced by ap the area north of Cocoa, clams were extremely abundant and peak proximately 55000 female clams or a bed of >80000 clams as densities exceeded 10 clams m-2 (Arnold, unpublished data). suming a 2: 1 female:male sex ratio. However, during the time frame of the present study. clams were We did not anticipate the rdte of loss of transplanted clams that practically nonexistent in the vicinity of the Banana River study was actually realized during this study. Similar transplants have 670 ARNOLD ET AL. . j~,-,·-_~I Land Banana River Release Point * o Larval Concenlratlon Larval Distribution o(Veligersll) 1 o May 23,2000 2 3 .1\ 5 6 7 Atlantic Ocean < Banana River 0.5:, 0.5 0 0.5 1 Kilometers ~=- Figure 10. Contour plot of the distribution of hard clam (Mercellaria spp.) larvae as estimated by sampling conducted on May 23, 2000, in the 1 Banana River lagoon, Florida. Data are presented as number of clam lan'ae 1- , and larval concentrations are depicted at the location of each sample smtion. been conducted in other areas throughout the range of Mercenaria, ference in the frequency of neoplasia between the transplants and either for eventual hnrvest of the relayed clams (e.g., Rice et al. their undisturbed conspecifics was greater in the Indian River than 2000) or to increase or expand larval production (e.g., Kassner & in the Banana River. This difference was minor and may reflect Malouf 1982, Ganz 1991). However, the high frequency of go sampling bias related to differences in the frequency of neoplasia nadal neoplasia in the clams that we collected for transplant will that have been reported for various size·classes and genotypes of mitigate against the success of this approach. Gonadal neoplasia is hard clams (Bert et al. 1993). Nevertheless, gonad.al neoplasia was common in Indian River hard clams (Hesselman et a1. 1988), par considerably more prevalent in clams collected during our study ticularly in the northern lagoon where we caHecled clams for trans than in clam collections reported by either Hesselman et aL (1988) plant (Bert et al. 1993). Although gonadal neoplasia has been or Bert et al. (1993). Neoplasia is more common in hybrid hard reported in hard clams collected from northeastern US coastal clams (Bert et aL 1993) and hybrid clams are more common in the waters (e.g., Barry & Yevich 1972), the frequency of occurrence northern Indian River Lagoon where we collected our transplant was less than 5% versus greater than 80% in our study. That animals (Bert & Arnold 1995). Our spawner transplant study disease appears to substantially reduce the reproductive potential would perhaps have been more successful if we had collected of hard clams (Hesselman et al. 1988) and probably contributes to clams from more southerly Indian River waters, but we were lim the relatively short life span of hard clams in Indian River waters ited in our choice of hnrvest sites by the availability of clams. (Jones et a1. 1990). Considering that the reproductive potential of Seeding as a means of increasing the abundance of hard clams Mercenaria increases with age (Bricelj & Malouf 1980, Peterson has been attempted in various areas throughout the range of Mer 1983, 1986), the high frequency of gonadal neoplasia in Indian cenaria, including both coasts of Florida (Menzel & -Sims 1962, River hard clams appears to render spawner transplants an inef Menzel et al. 1976, Marelli & Arnold 1996), Georgia (Walker feclive strategy for enhancing the abundance of harvestable clam 1985), North Carolina (peterson et a1. 1995), Virginia (Castagna & populations in the lagoon. Kraeuter 1977), New York (Flagg & Malouf 1983), and Rhode In all ca.«>es, gonadalllcoplasia was more prevalent in the trans Island (Rice et a1. 2000). With the exception of the work in North planted clams than in their undisturbed conspecifics, and the dif- Carolina (Peterson et al. 1995), these efforts have met with limited '" CLAM POPULATION ENHANCEl'vrENT IN FLORIDA 671 success due to the high rate of loss of seeded clams even when published record of those efforts indicates that success has been protective measures are used. In North Carolina, relatively large rare if not nonexistent. Clearly, efforts to enhance even moderately seed clams (14-22 mm shellienglh) were planted at relatively low dense populations are superfluous, as the reproductive potential of 2 density (l m- ) in shell hash habitat in late fall, resulting in 35% the natural population is sufficient to swamp any directed enhance survival after 14 months (Peterson et al. 1995). However, clam ment efforts. Only when population density is very low, such as it density below 5 m-2 is considered to be inadequate for commercial would be in an essentially collapsed population, might such efforts harvest in Florida waters (Arnold et at. 2000), and increasing the yield success. At that point, we shift from an enhancement effort density of planted seed clams might result in a loss of economic to a restoration effort, and the goals of the project shift from viability attributed to this approach. Moreover, achieving even the increasing the abundance of an ecologically functional population limited enhancement goals that ·we have set for our project (50 to restoring reproductive viability in an ecologically dysfunctional million harvestable clams) would require seeding almost 15,000 population (Arnold 2001). Nevertheless, user groups and manage hectares of submerged land with almost 1.5 billion clams. Even if ment agencies continue to request that population enhancement the available clam hatcheries could produce that many clams, the efforts be undertaken, and we will continue our efforts to deter cost of the clams alone would be exorbitant. At least for the 500 mine if or under what conditions we can meet those requests. hard clam fishermen currently licensed to work Brevard County waters, seeding for direct harvest does not appear to be a cost ACICNOWLEDGIVIENTS effective means of ensuring a minimum annual income, although that approach may be feasible for projects of a smaller scale. Kate Hagner, Tracy Idocks, Micah Humphrey, and Melissa We are familiar with only one application of larval release as a Harrison of the Florida Marine Research Institute provided invalu means of stock enhancement. Shepherd and colleagues (Preece et able field and laboratory assistance. Commercial clam harvesters a1. 1997, Shepherd et a1. 2000) released various densities of aba Perry McMahon, Bill Leeming, Peter Barile, Doug TeIgen, Mason lone larvae at several sites in South Australia and monitored their Bowen, Bill Bowler, John Condos, David Panizzi, Jay Anderson, survival. They found that because larval and post-larval survival George Vincent, Allen Ellingharn, Richie Luck, George Rotsch, was density~dependent, the releases of relatively low densities of James Horst, Pele Roy, and Riley Bergman assisted with clam larvae'w,ere more successful than were the high-density releases. collections for the spawner transplant study. Frederico Prahl, The overall conclusion of those authors (Shepherd et at. 2000) was Sandy Zeiner, Richard Baptiste, and David Vaughan of the Divi that larval release was not a viable strategy for abalone stock sion of Aquaculture, Harbor Branch Oceanographic Institution enhancement because of the density-dependent nature of larval were essential to successful spawning and fertilization of hard mortality. Those results, and results from our own analyses of clam eggs. Gary Hitchcock of the University of Miami assisted diffusive processes acting on artificially introduced hard dam lar with larval tracking and was instrumental in the design and de vae (Hitchcock & Arnold, unpublished data), suggest that a point ployment of the shallow-water drifters. Winnie White (FMRI) pre release of the larvae is not the best strategy. Instead, higher sur pared the maps and provided GIS support. Dick Moravec (FMRl) vival rates may be obtained by spreading the lan'ne throughout the provided assistance with vessels and vehicles, and Gerry Bruger target basin, thereby enhancing diffusive processes that will take (F1vlRl) ensured that our electronic database management was place anyway (Hitchcock & Arnold, unpublished data). We will seamless and uneventful. This smdy was made possible by the test that approach in future experiments. generous financial support of the commercial clam harvesters of Efforts to enhance the population abundance of commercially Brevard County, Florida. This is Harbor Branch Oceanographic important marine molluscs have been ongoing for decades, and the Institution contribution number 1483. LITERATURE CITED Arnold, W. S. 2001. Bivalve enhancement and restoration strategies in Unpublished report, Marine Resources Council of East Central Florida Florida, U.S.A. Hydrobiologia 465:7-19. and Florida Institute of Technology. 171 pp. Arnold, W. S., T. M. Bert. C. Crawford, 1. A. Guenthner, K. G. Hagner, M. Barry, M. M. & P. P. Yevich. 1972. Incidence of gonadal cancer in qua M. Harrison, P. L. Hoffman, R. Hudson, D. C. MarcHi, M. L. Parker & haug Mercenarja mercl!f1aria. Ollcology 26:87-96. 1. R. Styer. 1997. Hard clam (Merccnoria spp.) population of the Ba Bert, T. M. & W. S. Arnold. 1995. An empirical test of predictions of two nana River lagoon, Florida: present status and future prospects. Final competing models for the mainten;mce and fate of hybrid zones: both Report to the Florida Department of Environmental Protection, Bureau models arc supported in a hard-clam hybrid zooe. Evolution 49:276 of Marine Resource Regulation and Development, 46 pp. 289. Arnold, W. S., T. M. Bert, D. C. Marelli, H. Cruz-Lopez & P. A. Gill. Bert, T. M., D. M. Hesselman, W. S. Arnold, W. S. Moore, H. Cruz-Lopez 19~6. Genotype-specific growth of hard clams (genus lIfercellaria) in & D. C. Marelli. 1993. High frequency of gonadal neoplasia in a hard a hybrid zone: variation among habitats. Mar. BioI. 125:129-139. clnm (Merccnoria spp.) hybrid lonc. Mar. Bioi. 117:97-104. Bricclj, V. M. 1992. Aspects of the biology of the northern quahog, Mer Arnold, W. S.• D. C. Marelli. K. Hagner, M. Harrison & P. Hoffman. 1997. ccnoria mercellaria, with emphasis on growth and survival during early Annual report of the bay scallop project, 1996. Report to the Florida life history. In: M.A. Ricc & D. Grossman-Garber, editors. Proceedings Marine Fisheries Commission. 50 pp. of the Second Rhode Island Shellfish Industry Conference. RllOde Is Arnold, W. S., M. W. White, H. A. Norris & M. E. Berrigan. 2000. Hard land Sea Grant, Namlganse£t, Rhode Ishmd. pp. 29--48- clam (ft.fercenaria spp.) aquaculture in Florida, USA: geographic in Bricelj. V. M. & R. E. Malouf. 1980. Aspects ofreproduction of hard clams fonnation system applications to lease site selection. Aquacull. Eng. (Mercenaria mcrcenaria) in Greal South Bay, New York. Prnc. Natl. 23:203-231. Shellfish Assoc. 70:216-229. Barile, D. D. & W. Rathjen. 1986. Report on rainfall event of September Caniker. M. R. 1961. Interrelation of funclional morphology, behavior, and October 1985 and the impact ofstorm discharge 011 salinity and the and autecology in early stages of the bivalve MerCCllQria mercetraria. clam population (lI1ercenoria mcrce/wria) of the Indian River Lagoon. .T. Elisha Mitchell Sci. Soc. 77: 168-2-11. 672 ARNOLD ET AL. Carter, H. H., K. C. Wong & R. E. Malouf. 1984. Maximizing hard clam IvIcHugh. J. L. 1981. Recent advances in hard clam mariculture. J. Shellfish sets at specific locations in Great South Bay by meanS of a larval Res. 1:51-55. dispersion model. Marine Science Research Center Special Report #54, Menzel. R. W. & H. W. Sims. 1962. Experimental farming of hard clams, Ref. 84-1. 65 pp. MerCClloria mercellaria, in Florida. Proe. Natl. Shellfish Assoc. 53: Castngna, M. & P. 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