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Short-term effects of rapid salinity reduction on seed clams (Mercenaria mercenaria).

ABSTRACT Sudden salinity drops in Gulf Coast clam growing areas in Florida have been suggested as a cause of seed (juvenile) clam mortality. Laboratory experiments were used to assess short-term impacts of rapid salinity drops on hatchery-produced juvenile northern quahog (= hard clam), Mercenaria mercenaria, in two separate trials. Mortality and condition index (CI) were measured as response parameters. In Trial I, clams were exposed to a salinity drop of 5 ppt, 15 ppt, or 24 ppt over a span of 24 h, and the duration of this exposure was either 3 or 6 days. In Trial II, clams were either immediately immersed or dry-stored for 24 h prior to immersion, representing common treatment patterns by clam growers. In this trial, clams were exposed to acute salinity drops of either 10 ppt or 20 ppt for up to 7 days. Both trials were conducted at ambient seasonal temperatures.

Juvenile hard clams were surprisingly robust and resilient to changes in salinity, experiencing less than 5% mortality after relatively abrupt reductions in salinity of 10 to 15 ppt. Nonetheless, salinity declines of the magnitude occasionally observed at clam culture sites, up to 24 ppt, resulted in significant mortality; 17% (Trial I) and 100% (Trial II). Condition index (CI) was an insensitive response parameter. Dry storage of clams did not appear to have an effect on their ability to withstand changes in salinity; storage decreased final survival by <2.5%. Our results suggest that the salinity declines typically experienced at the Gulf Coast aquaculture sites are not of a magnitude or speed to account for the particular seed clam mortality events that spurred this research. However, long-term effects of salinity changes remain to be tested. In addition, reduced salinity may be indicative of a variety of other stressors, such as increased temperature and turbidity, or decreased phytoplankton concentration, which compound the effects of salinity on clam health and survival.

KEY WORDS: Mercenaria mercenaria, clam, salinity, mortality, condition index, Florida

INTRODUCTION

Florida ranks third in the United States in aquaculture production values, and the culture of the northern quahog (=hard clam), Mercenaria mercenaria (Linnaeus, 1758), represents the fastest growing segment of the state's aquaculture industry; between 1989 and 1999 revenue from farm-raised hard clams increased 15-fold (USDA 1990, USDA 2000a). Florida farm-raised clams are now a recognized commodity on the national market, with Florida's crop accounting for more than 40% of the nation's total aquacultured production (USDA 2000b). Today, approximately 400 active shellfish growers farm over 1800 acres of sovereign submerged state lands in coastal waters, producing a crop worth $18.2 million with an economic impact of about $55 million (USDA 2002, Philippakos et al. 2001). The Florida Department of Agriculture and Consumer Services (FDACS), Division of Aquaculture, has designated tracts of submerged lands suitable for aquaculture, called high-density lease areas (HDLAs), which are located in coastal counties including Franklin, Dixie, Levy, Charlotte, Lee, Indian River, and Brevard. Over half of the statewide production of clams is attributed to the highly productive HDLAs in Dixie and Levy counties on the west coast of Florida. The phenomenal production of clams in Florida is attributed to the high natural productivity of subtropical waters that allow for almost year-round growth.

In 2000, clam growers became the first aquaculturists in the United States eligible to purchase crop insurance. The US Department of Agriculture (USDA) Risk Management Agency developed a program under which hard clam growers are able to buy subsidized Cultivated Clam Crop Insurance. Eligible HDLAs are located in Florida's Dixie, Levy, Brevard, and Indian River Counties, as well as counties in South Carolina, Massachusetts, and Virginia. The clam crop insurance program covers losses due to "unavoidable damage" such as storms, low oxygen, and changes in salinity (USDA 2000c). To date, approximately 95% of the clam growers in the four eligible counties in Florida have purchased crop insurance.

On implementation of the insurance program, there were inadequate provisions for correlating water quality or weather events with crop loss. That same year, however, we had the opportunity to begin deploying multiparameter monitoring systems (YSI 600XLM Sonde or YSI 6600 Sonde, YSI, Yellow Springs, Ohio). In cooperation with the FDACS, monitoring sites were established on the Horseshoe HDLA in Dixie County and the Gulf Jackson HDLA in Levy County. The monitoring data revealed important details of temporal variability, previously unresolved from monthly water samples obtained by the FDACS Shellfish Environmental Assessment Section for management of shellfish harvesting areas. For example, in 2001, mean daily salinity fluctuation at Gulf Jackson HDLA was 5.3 ppt, with a maximum 24-h change of 24.5 ppt. Salinity values for the year ranged from a low of 5.4 ppt to a high of 37.2 ppt (Fig. 1).

[FIGURE 1 OMITTED]

In spring 2000, several growers reported high mortalities of seed (juvenile) clams recently planted in Levy County. Upon examination of the concurrent water quality data, we discovered that the seed had been planted just hours prior to rapid declines in salinity. In these particular cases, the salinity dropped 11 to 14 ppt in 28 h; for example, from 30 ppt to 19 ppt. Adjacent adult clams and previously planted seed were unaffected, making crop insurance claims difficult to substantiate because the mortalities could have been the result of poor seed quality or mistreatment of seed prior to planting. However, the reporting growers were known to be reliable, and the previously planted seed, which did not experience the same mortality, came from the same source. This suggested that the newly planted seed clams might be susceptible to rapid changes in salinity. Spring mortalities of seed clams have continued; high losses, especially in 2001 and 2003, have corresponded with large salinity drops (Fig. 1) associated with flood stage at the mouth of the Suwannee River.

A limited number of studies have explored the effects of salinity on seed clams (Chanley 1957, Roegner & Mann 1991). We are not aware of any studies on the effects of salinity declines on Florida seed clams of the magnitude, or short temporal scale, that appear to occur at the Gulf Coast aquaculture sites. In this study, we initially examined the impacts of salinity declines on the mortality and CI of juvenile seed clams (Trial I). Subsequently, we examined the effects of acute changes in salinity and the influence of two common seed handling and transportation methods (Trial II).

MATERIALS AND METHODS

Trial I. Salinity Reduction Integrated Over 24 h

Experiments were conducted in static 10-gallon (38 L) aquaria, each with a 3-cm layer of cleaned river sand in the bottom. The aquaria were equipped with power-head water circulators and placed on racks. Temperatures were kept constant at 23[degrees]C by use of aquarium heaters with thermostats. Seed clams (juvenile hard clams, M. mercenaria) were obtained from a commercial hatchery and held in flowing seawater at ~29 ppt at a private Cedar Key Florida clam nursery facility until use at the Department of Fisheries and Aquatic Sciences (FAS), University of Florida, in Gainesville. Mean seed clamshell length (SL) was 5.98 mm (sd = 0.46 mm). Florida growers purchase this seed size to plant directly on the HDLAs in field nursery bags. Scaled-down versions (25 x 25 cm) of commercial clam culture bags were sewn from woven polyester mesh. Clams were divided into lots of 150 each and placed inside the bags; this density approximates commercial planting densities. Two bags were randomly assigned to each of 12 aquaria (8 experimental and 4 controls).

From an initial salinity of 29 ppt, seed clams were exposed to 1 of 4 salinity treatments: reductions in salinity of 0 ppt (controls), 5 pt, 15 ppt, or 24 ppt. Logistic constraints prevented the salinity treatments from being conducted simultaneously; salinity treatments were conducted in three sequential weeks, with the smallest salinity reduction first (5 ppt) and the largest reduction (24 ppt) last. Controls (0 ppt change) were conducted each week. Salinity was reduced either on Day 1 (four aquaria) or Day 3 (four aquaria) of the salinity treatments, resulting in 2 duration treatments of 6 or 3 days, respectively. Salinity reduction was accomplished by incremental addition of reverse osmosis-purified water to the aquaria over a period of 24 h. Identical volumes of seawater were removed and replaced in the four control aquaria, which remained at 29 ppt. The clams were fed a commercially obtained algal paste, at a ration of ~2% of their total wet weight per day, or about 300 [micro]L of paste (0.058 g dry weight) per aquarium (300 clams each). As an indication of normal activity, we observed whether the clams produced feces and pseudofeces. One bag was removed from each aquarium on Day 3 (without replacement) and the final bags were removed on Day 6.

Two response parameters were measured: mortality and CI. Mortality was quantified as counts dead per replicate bag. To reduce experimental error due to small specimen size, CI was estimated for groups of 20 juvenile clams from a bag. These 20 clams comprised a single replicate and resulted in a single datum. Shell cavity volume was estimated from the difference between the volume of water displaced by whole live animals and the volume displaced by clean, separated shells. Tissues were dried (100[degrees]C) to constant weight. CI was calculated according to Rainer and Mann (1992), where CI = (dry meat weight x 100) / shell cavity volume.

Mortality counts were arcsine transformed. Two-factor analyses of variance were performed to test the null hypotheses that there were no effects of salinity treatment, duration, or interaction, on the two response parameters. If a null hypothesis was rejected, multiple comparison tests were used to identify differences between specific salinity treatments and exposure durations. Statistical analyses were performed using JMP version 3.2.6 software (SAS Institute Inc. 1995).

Trial II. Immediate Salinity Reduction and Two Handling Regimes

Aquaria were similar to those used in Trial I, with the following changes. Aquaria were equipped with individual mechanical and biologic filters and partially submerged in three separate but identical water baths, eight aquaria each. Aquaria were randomly assigned to 1 of 3 treatments; salinities either 0 ppt (control), 10 ppt, or 20 ppt lower than that at which the seed clams were obtained (24 ppt). Therefore, salinity was either 24 ppt (control), 14 ppt, or 4 ppt. Temperatures were kept constant at 26[degrees]C.

Seed clams were obtained from a commercial hatchery outlet in Cedar Key (24 ppt) and transported directly to FAS. Mean seed clamshell length was 6.34 mm (sd = 0.72). Bags of approximately 150 seed clams were prepared as for Trial I. Half of the bags were randomly assigned, one to each aquarium, and immediately immersed (8/7/01). The remaining bags were dry-stored in a cooler (16[degrees]C), using commercial hatchery-recommended storage and handling protocols for shipping clam seed from the hatchery to the field (Hadley et al. 1997). Following 24 h of dry storage, these bags were also randomly assigned, one to each aquarium (8/8/01). Therefore, each of the 24 aquaria had 2 bags; one immediately immersed and one immersed following 24 h dry storage. This resulted in six total treatments: 3 salinity treatments x 2 handling treatments.

Beginning 24 h after immersion of the stored bags (8/9/01, Day 1), 2 replicate bags from each handling treatment (immediate immersion vs. storage) from each salinity treatment (24, 14, or 4 ppt salinity) were randomly selected for analysis. Bag collection was repeated on Days 2, 3, 4, and 6 (8/14/01). Because of unresolved problems with CI data for very small bivalves, the only response parameter measured was mortality, which was quantified as counts dead per replicate bag.

Mortality counts were arcsine transformed. A 3-factor analysis of variance was performed to test the null hypotheses that there were no effects of salinity treatment, handling treatment, duration, or interaction, on mortality. If a null hypothesis was rejected, multiple comparison tests were used to identify differences between specific treatments.

RESULTS

Trial I. Salinity Reduction Integrated Over 24 h

Clams appeared to feed and produced feces and pseudofeces in all treatments. Both the magnitude of salinity change and length of exposure (3 or 6 days) had significant effects (P < 0.0001) on mortality of seed clams (Fig. 2). There was also a significant interaction effect (P < 0.0001) that was accounted for by the high response of a single duration x salinity treatment. Mortality was slight (<2%) and not significantly different from controls in all treatments, with one exception; seed clams subjected to a 24 ppt magnitude salinity reduction (to 5 ppt) for 6 days experienced mortalities of 17%. CI was highly variable, with no significant effect of salinity reduction (P = 0.1089).

[FIGURE 2 OMITTED]

Trial II. Immediate Salinity Reduction and Two Handling Regimes

Clams appeared to feed and produce feces and pseudofeees in all treatment groups, although less so at the 4 ppt salinity level (salinity reduction of 20 ppt). Mortality was low (<5%) in the 24 ppt (control) and 14 ppt treatments throughout the trial (Fig. 3). However, magnitude of salinity change (P < 0.0001), length of exposure (P < 0.0001), and their interaction (P < 0.0001), had significant effects on mortality of seed clams; beginning on the second day of the trial, mortality was significantly greater in the 4 ppt salinity treatments and reached 100% by Day 6. Handling regimen also had an apparently significant impact on seed clam mortality (P = 0.0053). However, within salinity treatments, the difference between the two handling regimes was usually <2.5%, with a difference as high as 7% in the 4 ppt treatment on Day 3.

[FIGURE 3 OMITTED]

DISCUSSION

Juvenile hard clams, M. mercenaria, are surprisingly robust and resilient to changes in salinity, experiencing less than 5% mortality after relatively abrupt reductions in salinity of 10-15 ppt. Nonetheless, salinity declines of the magnitude occasionally observed at HDLAs, 20-24 ppt, resulted in significant mortality; 17% (Trial I) and 100% (Trial II) following 6 days of exposure.

Significant mortality was observed after only 3 days in Trial II, compared with 6 days in Trial I (Fig. 2, Fig. 3). This delay in the onset of clam mortality may have been due to the rapidity of salinity change; in Trial I the salinity was dropped step-wise over 24 h, whereas in Trial II clams were immediately immersed in the test salinity. A period of acclimation can have a significant impact on the ability of molluscs to perform under altered salinity conditions (Berger & Kharazova 1997).

Temperature differences between the two trials may also partially account for the differences in mortality. Ambient water temperature was approximately 3[degrees]C warmer in Trial II, potentially increasing metabolic rate of the clams and speeding the depletion of energy reserves. Hibbert (1977) found that, between 20[degrees]C and 25[degrees]C, the increase in oxygen consumption by M. mercenaria (50-mm SL) from the northeastern US exceeded the increase in clearance rate, resulting in a net energy loss at these temperatures. However, both temperatures used in these experiments, 23[degrees]C and 26[degrees]C, were well within normal growth and reproduction ranges cited for hard clams (Wells 1957, Roegner & Mann 1991). Further information on the effects of temperature on the energy budgets of juvenile hard clams is required to determine whether the 3[degrees]C difference in the two Trials was physiologically significant. In addition, the two trials used different seed sources.

Our study suggests that rapid salinity drops of 10-15 ppt have little acute effect on seed (6-mm SL) clams. However, long-term effects of salinity changes remain to be tested. Acute reductions in salinity may have a chronic impact on clam survival. Reductions in salinity may also have sublethal effects on metabolic or feeding rates, resulting in reduced growth rate. For example, oyster spat subjected to low salinities for longer than 2 wk did not immediately attain normal feeding levels on return to full salinities, and mortality continued to be high (Rodstrom & Jonsson 2000). In addition, changes in salinity may reduce resistance to other environmental stressors or pathogens (Kraeuter et al. 1998). Any long-term reduction in survival or growth rate would reduce profitability to the grower.

Our results suggest that the salinity declines experienced at the Gulf Coast aquaculture sites in the spring of 2000 (11 ppt over 28 h) were not of a magnitude or speed to account for the particular seedclam mortality events that spurred this research. However, additional stressors are likely to accompany the Suwannee River flood waters that cause the rapid salinity drops in the Dixie-Levy County clam growing areas. These potentially confounding stressors include sudden temperature shifts (Hamwi 1969, Diaz 1973, Savage 1976), hypoxic events (Walsh 1974, Baker & Mann 1992), changes in phytoplankton species composition (Rice & Smith 1958, Beals 2004), and decrease in phytoplankton concentration (Foster-Smith 1975, Bricelj & Malouf 1984, Navarro & Iglesias 1993). For example, poor nutrition during a low salinity event caused digestive tubule atrophy in oysters from Apalachicola Bay (Winstead 1995). Thus, reduced salinity may be indicative of various other stressors that, in combination, result in mortality or impact clam health.

Seed clams are often shipped from suppliers to the growers, who will then plant the seed at nursery sites immediately upon receipt (http://shellfish.ifas.ufl.edu/seed_tips.htm); care is taken to limit transit time to less than 24 h. Salinities at the hatchery and nursery sites may differ. We attempted to replicate this handling and shipping procedure by dry storing seed clams for 24 h in coolers at 16[degrees]C, prior to immersion in the test salinity. While statistics suggested that handling regimen had a significant impact on seed clam mortality (P = 0.0053), overnight storage decreased final survival by <2.5%. These results suggest that the shipping procedures typically used by clam farmers do not have an economically significant impact on clam mortality.

This study allowed us to examine the statistical behavior of 2 response parameters: CI and mortality. CI has the potential to be more sensitive than mortality as an indicator of seed clam stress; CI is a continuous variable for each clam, whereas mortality is dichotomous (dead or alive). In these short trials, however, CI proved to be an insensitive indicator of clam health. The apparent lack of sensitivity of CI is likely due to the high variability in CI measurements, compounded by the difficulty in measuring CI in such small organisms. Small errors in measuring volume displacement, due to adherence of an unnoticed air bubble to a shell, for example, are amplified into large errors in CI (Crosby & Gale 1990). Despite this problem, we believe that, with methodological refinement, CI may yet prove to be a sensitive response indicator in seed clams.

Mortality data present their own statistical difficulties. Most values in this data set were near zero, with a few in the 15% to 25% range in Trial I and a few in the 100% range in Trial II, resulting in a non-normal data distribution. Data transformations only partially corrected non-normality problems. Fortunately, a balanced ANOVA design is robust, and the statistical tests bore out what is clearly apparent from a graphical presentation of the data (Fig. 2, Fig. 3). Nonparametric tests are an imperfect solution; while robust, they are also less powerful (Zar 1996). If future studies on this problem present mortality data with a similarly non-normal distribution, but without the nonoverlapping variances seen between the single high-mortality treatment and all other treatments, the researcher must make the unpleasant choice between parametric tests with possibly spurious null hypothesis rejection and nonparametric tests with a loss of statistical power.

ACKNOWLEDGMENTS

The authors thank Ayana McCoy and Heather Herb for their assistance in the lab. Dr. Derk Bergquist completed QAQC procedures on the environmental monitoring data. This research was sponsored by NOAA, Office of Sea Grant, Department of Commerce, under Grant Number NA76RG-0120 and was also supported by Department of Agriculture, Cooperative State Research, Education, and Extension Service, Initiative for Future Agriculture and Food Systems, under Award No. 00-52103-9641. Some of the multiparameter monitoring systems (YSI Sondes) were on loan from the University of Florida's Tropical Aquaculture Laboratory and all were maintained by The Florida Department of Agriculture and Consumer Services, Division of Aquaculture, Shellfish Environmental Assessment Section. This research was supported by the Florida Agricultural Experiment Station and approved for publication as Journal Series No. 19-10528

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SHIRLEY M. BAKER, (1) * PATRICK BAKER, (1) DAVID HEUBERGER (1) AND LESLIE STURMER (2)

(1) Department of Fisheries and Aquatic Sciences IFAS, University of Florida, 7922 NW 71st Street, Gainesville, Florida 32653; (2) Shellfish Aquaculture Extension Program, IFAS Extension, University of Florida, Cedar Key Field Lab, P.O. Box 89, Cedar Key, Florida 32625

* Corresponding author. E-mail smbaker@ifas.ufl.edu
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Author:Sturmer, Leslie
Publication:Journal of Shellfish Research
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Date:Jan 1, 2005
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