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Growth of seed clams, Mercenaria mercenaria, at various densities in a commercial scale nursery system
Aquaculture, 36 (1984) 369-378 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands
369
GROWTHOFSEEDCLAMS, MERCENARIAMERCENARIA, AT VARIOUS DENSITIES IN A COMMERCIAL SCALE NURSERY SYSTEM
NANCY H. HADLEY and JOHN J. MANZI* Grice Marine Biological (U.S.A.) *Marine Resources
Laboratory,
216 Fort Johnson Road, Charleston,
Research Institute,
P.O. Box 12559,
Charleston,
Contribution No. 60 from Grice Marine Biological Laboratory Carolina Marine Resources Center. (Accepted
SC 29412
SC 29412
(U.S.A.)
and NO. 175 from South
7 May 1983)
ABSTRACT Hadley, N.H. and Manzi, J.J., 1984. Growth of seed clams, Mercenaria mercenaria, various densities in a commercial scale nursery system. Aquaculture, 36: 369-378.
at
Hatchery-raised Mercenaria mercenaria (mean size = 3.9 mm) were placed in commercial nursery raceways at densities approximating 740, 2220, 6660 and 19 980 seed/m’. Each density wss replicated eight times in the nursery and the highest and lowest densities were replicated four times in adjacent subtidal field controls. All replicates were monitored monthly from February to August 1981 to determine growth and survival. Temperature and salinity were measured daily and inflow and outflow water were sarnpled monthly to determine chlorophyll a concentrations. Results indicated that growth was significantly affected by planting density in both raceways and field controls. Although total mean growth for the raceway and the field was similar, a number of observations indicated that different factors influence growth in the two locations. Growth in the raceways was inversely proportional to both distance from inflows and planting density. Greatest growth was observed in the lowest density nearest the inflow and slowest growth was observed in the highest density nearest the outflow. Growth rates were analyzed in relationship to effective water flow rate (volume water/volume clams/min), effective density (number clams/unit water) and chlorophyll a stripping rates.
INTRODUCTION
The hard clam, hfeFCenUFia mercenaria, is an important commercial shellfish species on the east coast of the United States. Decreasing natural harvests and increasing value have led to interest in mariculture of this species. Because the space and food requirements increase exponentially as clams grow, it has proven most economical to grow them in a natural environment at controlled densities (Castagna and Kraeuter, 1977). However, mortality of seed clams less than 10 mm long (greatest anterior-posterior dimension) is high in field plantings (Eldridge et al., 1976; Manzi et al.,
0044-8486/84/$03.00
o 1984
Elsevier Science Publishers B.V.
370
1980). Commercial hatcheries generally supply seed clams at sizes appreciably below this minimum (Manzi and Whetstone, 1981), thus necessitating an intermediate system for growing small seed to a size suitable for planting in the field. The South Carolina Marine Resources Research Institute, in cooperation with private industry (Trident Seafarms Company, Charleston, SC), has established a pilot nursery to provide intermediate growth of seed clams in raceways. This report summarizes a study performed between February and August 1981 to examine the growth of small seed clams at various densities in raceways in South Carolina, using field plantings for comparison. Relationships between seed density, water flow rate, chlorophyll a removal rates, and seed growth were examined. MATERIALS
AND METHODS
This study was conducted in the clam nursery facilities of Trident Seafarms Company, located on Folly Island, SC (Fig. 1). Two experimental raceways constructed of epoxy resin coated wood were subdivided into 16 compartments (0.07 m2), arranged in rows of four compartments each. Water was pumped from the Folly River, a barrier island river, to a header tank and gravity-fed through a PVC manifold to the raceways at a flow rate of approximately 8 1 per min per row. Field units providing the same bottom area as the raceway compartments were constructed of vinyl-coated wire (5-cm mesh) lined with- fiberglass window screening. In February 1981, seed clams averaging 3.9 mm were planted in the raceways at densities corresponding to 740, 2220, 6660, and 19980 clams/m2. Each density was replicated eight times in the raceways. The highest and lowest densities were replicated four times at a shallow station in Folly River, adjacent to the nursery. All compartments were monitored monthly from February to August 1981 to determine growth and survival. Direct counts were made of all replicates and replacements added as necessary to maintain the original densities. Length was measured by ocular micrometer or Vernier calipers across the longest anterior-posterior axis, and recorded to the nearest 0.1 mm. Packed volumes were determined using appropriate graduated cylinders. Temperature and salinity were measured daily. Water samples were taken at monthly intervals from the inlet and outlet of each raceway row for chlorophyll a determinations. Chlorophyll a was determined by standard fluorometric methods (Strickland and Parsons, 1968), using a Turner fluorometer Model 111. RESULTS
Growth rate, as reflected by changes in size and volume, was significantly affected by planting density in both raceway and field control (Table I).
371
:v- f
CHARLESTON
I
‘.
STOUO
HARBOR
FOLLY ISLAND INLET
Fig. 1. Map showing location of experimental facilities.
Raceway populations exhibited mean increases in length over the study period of 13.03, 12.92, 10.60 and 8.79 mm at densities of 740, 2220, 6660 and 19 980 clams/m’, respectively. Analysis of variance followed by Student -Neuman-Keuls- test showed that the highest density (19 980) grew significantly slower than the two lowest densities (740 and 2220). Growth at 6660 clams/m2 did not differ significantly from growth at the lower densi-
372 TABLE I Mean total increase in length (mm) and volume (~1) of seed clams (initial size i=3.9 mm, 24 ~1) in raceway (R) and control (C) replicatesover the 6 month study period Density (seed/ml) 740 C
R Length
Z Volume
Z
2220
6660
19 980
R
R
R
C
increase (mm) 16.70 13.28 9.30 8.13 16.17 14.07 13.47 13.09 i3.03
11.44 11.03 11.56
14.42 14.11 10.77 9.45 15.65 13.63 12.89 12.37
13.26 9.48 9.48 9.66 13.19 11.00 9.96 8.83
10.33 9.50 9.50 8.47 10.26 8.56 7.47 6.22
10.41 10.21
11.43
12.92
10.60
8.79
10.35
increase (fil)
2935 1803 856 581 3101 2060 1916 1676 1866
1405 1226 1539 -1421
2203 2003 1165 730 2268’ 1910 1586 1315
1610 797 674 707 1559 1253 939 679
749 735 567 875 619 444 311
1005 1022
1648
1027
652
1005
923
ties or from growth at the highest density. Clams in the field controls grew 11.43 mm (low density) and 10.35 mm (high density) over the study period. This difference was significant at the 95% confidence level. Increase in biomass was similarly affected by planting density. Growth in the raceway was inversely proportional to distance from the water inflow (Table II). Highest growth was recorded in the lowest density nearest the inflow; lowest growth was observed in the highest density nearest the outflow. Because of the linear flow arrangement, the compartments further from the inflow experienced higher effective densities (number of clams per unit water). Regression of mean total length increase on effective density (log-transformed) was highly significant (P < 0.001). As a result of the significant relationship between growth rate and position within the raceway, or effective density, there was high with-in group variance for the raceway data, making parametric statistical comparison with
313 TABLE II Effect of distance from the inflow (cm) on mean total increase in length (mm) of seed clams (initiaI size = 3.9 mm) grown in raceways at four densities from February to August 1981 (190 days) Approximate distance from inflow (cm)
Density (clams/m*)
x
740
2220
6660
19 980
O-26
16.70 16.17
14.52 15.65
13.26 13.19
10.33 10.26
13.76
29-55
13.28 14.07
14.11 13.63
9.48 11.00
9.50 8.56
11.70
58-84
9.30 13.47
10.77 12.89
9.48 9.96
9.50 7.47
10.36
87-113
8.13 13.09
9.45 12.37
9.56 8.83
3.47 6.22
9.52
x
13.03
12.92
10.60
8.79
field controls (which had low within-group variance) difficult. The KruskalWallis non-parametric test demonstrated no significant difference between growth in the raceway and in the field controls. On the assumption that the conditions in the sample compartments nearest the inflow most closely resembled the natural environment, those were compared with the field controls of the same density, using one-way analysis of variance (Table III). At the low density, growth in the raceway was significantly greater than growth in the field controls (P < O.OOl), while at the high density there were no significant differences between growth in the two locations. TABLE III Statistical comparison of mean total increase in length (mm) of seed clams (initial size = 3.9 mm) grown at two densities in raceways (samples nearest inflow) and field control units Density (seed/m’) 19 980
740
x
Raceway
Control
Raceway
Control
16.70 16.17
11.44 11.03 11.68 11.56
10.33 10.26
10.41 10.21 10.42
16.44
11.43
10.30
10.35
F = 351.69***
F = 0.314 ns
***P < 0.001; ns = not significant.
374
The greatest mean daily increment in length was observed in May, when the lowest density replicates (740) in the raceway averaged 158.1 pm per clam per day (Fig. 2). At the high density in the raceway maximum length increase occurred 1 month earlier (April). Clams in the field controls exhibited fairly uniform growth rates throughout the study (Fig. 2). Greatest increase in biomass occurred 1 month later than the greatest increase in length (Fig. 3).
PO0
*.-. ,
MARCH
I
I
APRIL
MAY
I
JUNE
\\ -.. ..... . ..‘0 . I
JULY
AUGUST
Fig. 2. Averagegrowth rates (pm/day)
of seed clams planted at densities of 740 and 19 980 seed/m2 in raceways and field controls (February-August 1981).
’\
,A ,’ d’ IMARCHI
APRIL
I t4Ay
I JuNE
I JULY
‘\ \ GROWTH RATE ,c (length1 . IAUGUSTI
Fig. 3. Water temperature (“C) over study period and average growth rate (pm/day and &l/day) of seed clams planted at low density (740 seed/m’) in raceway.
375
Growth in the raceways was analyzed in relation to water flow rates. Flow rate was kept constant at 8 l/min but the volume of water available per volume of clams (effective flow rate) was different for each sample compartment and decreased as the clams grew. Total growth (change in biomass) was significantly correlated with the minimum effective flow rate (r = 0.941) (Fig. 4). This is reflected in the significant differences in growth observed at the different positions and densities in the raceways (Table II). 10
1
0,
I
0
I
Effective
Flow Ratelvolume
r 10
water/volume
I
100
clams/minute)
Fig. 4. Correlation between effective water flow rate (1 water per 1 clams per minute)and mean total increase in biomass (ml/clam) for seed clams grown in raceways for 6 months (February-August 1981).
Chlorophyll a was measured as an indicator of food availability. Available chlorophyll a ranged from 2.13 to 6.34 pg/l during the study period, averaging 4.13 pg/l. The difference in chlorophyll removal by clams at the four densities was highly significant (P < 0.001) with the highest density removing consistently more than the lowest density. Clams at the highest density removed as much as 83% of the available chlorophyll (average 49%), while those at the lowest density had a maximum removal of 46% (average 17%). Although the amount of chlorophyll removed was directly related to the number of clams, less chlorophyll was removed per clam as the density increased. The amount of chlorophyll removed per clam per day was highly correlated with the rate of biomass increase (r = 0.73, P < 0.05). Water temperature over the experimental period ranged from a minimum temperature of 73°C in February to a maximum of 31.5”C in July (Fig. 3). Maximum rate of increase in length occurred when temperatures were be tween 20 and 24°C while maximum biomass increase occurred when temperatures were between 24 and 29°C. Prolonged high temperatures (> 30°C) were associated with decreased growth rates and increased mortality. DISCUSSION
Growth rates obtained in this study were compared with the data of Eldridge et al. (1979) for hatchery-reared seed clams grown in field units in
376
South Carolina. From May to December of the first year of their study, seed clams (13 mm) averaged 1.8 mm/month increase in length. This is virtually identical to the average rate of growth in the raceways (1.89 mm/month) but lower than the rate for the lowest density replicates in the raceway, which averaged 2.17 mm/month. The maximum monthly increment reported by Eldridge et al. (1979) occurred in the spring of the second year of that study, when clams grew 3.77 mm between April and May. In the present study, clams at the two lower densities grew at a rate of more than 4 mm/ month during May and June. This study confirms the reports by previous investigators that growth rate is reduced at high densities. Menzel and Sims (1962) found reduced growth at densities of 800/m2. Eldridge et al. (1979) found that growth was limited at a density of 1159/m2. In our study there was no significant difference in growth between clams planted at 740,222O and 6660 clams/m2 in raceways, but growth was reduced at a density of 19 980/m2. Since biomass is a more realistic measure of density than number of clams, all the above densities were transformed to biomass/m2, using an empirically determined regression of volume (biomass) on length. The limiting density reported by Menzel and Sims (1962) was equivalent to 8.2 l/m’, while that of Eldridge et al. (1979) was approximately 8.9 l/m*. These figures agree with the results of the present study in which clams grew well in raceways at densities of almost 9 l/m2. Although total mean growth in the raceways and the field controls was similar (Table I), growth in the field was less variable throughout the study (Fig. 2). Clams in the raceway grew much faster than those in the field units in the spring, but clams in the field controls continued to grow during the summer when there was little growth in the raceway. Total mean growth at the low density was significantly greater in the raceway than in the field, but there was no difference in growth at the high density in the two locations. These differences in growth in the raceways and the field units suggest that conditions in the two locations are not as similar as might be expected. One factor which obviously differed between the two locations was water flow. In the raceways flow was constantly unidirectional and the rate of flow was controlled. Mean growth in the raceways was high at effective flow rates as low as 3 1 water per 1 clams per min, but an effective flow rate of 8-9 1 per 1 clams per min was necessary for maximum growth. Rhodes et al (1981) reported flow requirements of 5 1 water per 1 animals per min for surf clams and 6.5 1 water per 1 animals per min for bay scallops. Effective flow rate was significantly correlated with total mean growth for the individual replicates (Fig. 4). At all densities the samples nearest the inflow (maximum effective flow) grew significantly faster than those adjacent to the outflow (minimum effective flow) (Table II). This contributed significantly to the high variability in growth rates exhibited among raceway replicates. Rhodes et al. (1981) reported that, for surf clams and bay scallops,
377
growth in raceways was controlled by food supply. Kirby-Smith (1972) suggested that for bay scallops to grow well in raceways, at least 60% of the incoming chlorophyll should remain in the effluent. In this study, the average growth rate was high even when as much as 75% of the available chlorophyll was removed; but, at these removal rates, growth at the outflow end of the raceway was significantly reduced. Rhodes et al. (1981) reported that 1 pg/l of chlorophyll should remain in the effluent in order to assure adequate food supply. In this study effluent levels of less than 1 @g/l were associated with low growth but, even at higher effluent levels, clams in the compartments nearest the outflow grew more slowly than those nearest the inflow. Summer growth was low in all compartments despite effluent levels of more than 3 pg/l, suggesting that food supply was not the major factor limiting growth during this period. The slow growth during the summer coincided with periods when water temperatures were 30°C or higher. Ansell (1968) concluded, from his comparisons of the data of many investigators, that maximum growth of it4. mercenaria occurred at temperatures of 2O”C, with reduced growth at temperatures of 20-30°C and no growth above 31°C. While it is concurred that little growth occurs at temperatures exceeding 3O”C, the fastest growth occurred when water temperatures were between 20 and 28°C (Fig. 3). At temperatures exceeding 29”C, it was observed that clams often exhibited partial gaping and appeared to cease feeding. This may explain both the low growth and the low chlorophyll removal rates during the summer. In conclusion, growth rates for seed clams in raceways in South Carolina were somewhat higher than have been reported by other workers for field plantings. Although the density (number clams/m2) which could be maintained in the raceways was higher than previous reports from field studies, the biomass which could be supported was similar. At low densities, clams grew significantly faster in raceways than iri field controls. Even at the highest density (19 980 clams/m2), initial growth in the raceways was excellent and a size suitable for field planting (8.5 mm mean length) was attained in only 2 months. Density appeared to become a limiting factor as biomass approached 9 l/m2. Food supply appeared to be a major factor controlling growth rates in the raceways. When temperatures were less than 3O”C, the average rate of growth in the raceways was high as long as 1 @g/l of chlorophyll remained in the effluent. However, even when the effluent contained more than 1 pg/l of chlorophyll, growth rate declined in proportion to the distance from the inflow. The clams at the outlet end of the raceway were apparently food limited at all times. At temperatures exceeding 3O”C, growth rates in the raceways were extremely low, despite apparently adequate food supply. At these high temperatures, clams in the adjacent creek grew faster than those in the raceway. ACKNOWLEDGEMENTS
The authors would like to thank Trident Seafarms, Inc., for its cooperation in this study, particularly for providing seed clams and allowing the use
378
of research space in its nursery. This research was supported by NOAA Office of Sea Grant, U.S. Department of Commerce. Ms. Hadley received financial support from the National Science Foundation through a graduate student fellowship, and from the Charleston Natural History Society through its Chamberlain Research Grant. This paper is adapted from portions of a thesis submitted by N.H. Hadley to the Charleston Higher Education Consortium in partial fulfillment of the requirements for the degree of Master of Science.
REFERENCES Ansell, A.D., 1968. The rate of growth of the hard clam Mercenaria mercenaria (L) throughout the geographical range. J. Cons., Cons. Int. Explor. Mer., 31 (3): 364-409. Castagna, M.A. and Kraeuter, J.N., 1977. Mercemvia culture using stone aggregate for predator protection. Proc. Natl. Shellfish. Assoc., 67: l-6. Eldridge, P.J., Waltz, W., Gracy, R.C. and Hunt, H.H., 1976. Growth and mortality rates of hatchery seed clams Mercenaria mercenaria in protected trays in waters of South Carolina. Proc. Natl. Shellfsh. Assoc., 66: 13-22. Eldridge, P.J., Eversole, A.G. and Whetstone, J.M., 1979. Comparative survival and growth rates of hard clams, Mercenaria mercenaria, planted in trays subtidally and intertidally at varying densities in a South Carolina estuary. Proc. Natl. Shellfish. Assoc., 69: 30-39. Kirby-Smith, W.W., 1972. Growth of the bay scallop: the influence of experimental water currents. J. Exp. Mar. Biol. Ecol., 8: 7-18. Manzi, J.J. and Whetstone, J.M., 1981. Intensive hard clam mariculture: a primer for South Carolina watermen. South Carolina Sea Grant Consortium, Mar. Adv. Publ. 81-01, Charleston, SC. Manzi, J.J., Burrell, Jr., V.G. and Carson, W.Z., 1980. A mariculture demonstration project for an alternative hard clam fishery in South Carolina: preliminary results. Proc. 11th Annu. Meet. World Maricult. Sot., 11: 79-89. Menzel, R.W. and Sims, H.W., 1962. Experimental farming of hard clams, Mercenaria mercenaria, in Florida. Proc. Natl. Shellfish. Assoc., 53: 103-110. Rhodes, E.W., Goldberg, R. and Widman, J.C., 1981. The role of raceways in mariculture systems for the bay scallop, Argopecten h-radians irradians, and the surf clam, Spisula solidissima. In: C. Claus, N. DePauw and E. Jaspers (Editors), Nursery Culture of Bivalve Molluscs. Eur. Maricult. Sot. Spec. Publ. No. 2, pp. 227-251. Strickland, J.D.H. and Parsons, T.R., 1968. A practical handbook of seawater analysis. Fish. Res. Board Can., Bull. 167.
369
GROWTHOFSEEDCLAMS, MERCENARIAMERCENARIA, AT VARIOUS DENSITIES IN A COMMERCIAL SCALE NURSERY SYSTEM
NANCY H. HADLEY and JOHN J. MANZI* Grice Marine Biological (U.S.A.) *Marine Resources
Laboratory,
216 Fort Johnson Road, Charleston,
Research Institute,
P.O. Box 12559,
Charleston,
Contribution No. 60 from Grice Marine Biological Laboratory Carolina Marine Resources Center. (Accepted
SC 29412
SC 29412
(U.S.A.)
and NO. 175 from South
7 May 1983)
ABSTRACT Hadley, N.H. and Manzi, J.J., 1984. Growth of seed clams, Mercenaria mercenaria, various densities in a commercial scale nursery system. Aquaculture, 36: 369-378.
at
Hatchery-raised Mercenaria mercenaria (mean size = 3.9 mm) were placed in commercial nursery raceways at densities approximating 740, 2220, 6660 and 19 980 seed/m’. Each density wss replicated eight times in the nursery and the highest and lowest densities were replicated four times in adjacent subtidal field controls. All replicates were monitored monthly from February to August 1981 to determine growth and survival. Temperature and salinity were measured daily and inflow and outflow water were sarnpled monthly to determine chlorophyll a concentrations. Results indicated that growth was significantly affected by planting density in both raceways and field controls. Although total mean growth for the raceway and the field was similar, a number of observations indicated that different factors influence growth in the two locations. Growth in the raceways was inversely proportional to both distance from inflows and planting density. Greatest growth was observed in the lowest density nearest the inflow and slowest growth was observed in the highest density nearest the outflow. Growth rates were analyzed in relationship to effective water flow rate (volume water/volume clams/min), effective density (number clams/unit water) and chlorophyll a stripping rates.
INTRODUCTION
The hard clam, hfeFCenUFia mercenaria, is an important commercial shellfish species on the east coast of the United States. Decreasing natural harvests and increasing value have led to interest in mariculture of this species. Because the space and food requirements increase exponentially as clams grow, it has proven most economical to grow them in a natural environment at controlled densities (Castagna and Kraeuter, 1977). However, mortality of seed clams less than 10 mm long (greatest anterior-posterior dimension) is high in field plantings (Eldridge et al., 1976; Manzi et al.,
0044-8486/84/$03.00
o 1984
Elsevier Science Publishers B.V.
370
1980). Commercial hatcheries generally supply seed clams at sizes appreciably below this minimum (Manzi and Whetstone, 1981), thus necessitating an intermediate system for growing small seed to a size suitable for planting in the field. The South Carolina Marine Resources Research Institute, in cooperation with private industry (Trident Seafarms Company, Charleston, SC), has established a pilot nursery to provide intermediate growth of seed clams in raceways. This report summarizes a study performed between February and August 1981 to examine the growth of small seed clams at various densities in raceways in South Carolina, using field plantings for comparison. Relationships between seed density, water flow rate, chlorophyll a removal rates, and seed growth were examined. MATERIALS
AND METHODS
This study was conducted in the clam nursery facilities of Trident Seafarms Company, located on Folly Island, SC (Fig. 1). Two experimental raceways constructed of epoxy resin coated wood were subdivided into 16 compartments (0.07 m2), arranged in rows of four compartments each. Water was pumped from the Folly River, a barrier island river, to a header tank and gravity-fed through a PVC manifold to the raceways at a flow rate of approximately 8 1 per min per row. Field units providing the same bottom area as the raceway compartments were constructed of vinyl-coated wire (5-cm mesh) lined with- fiberglass window screening. In February 1981, seed clams averaging 3.9 mm were planted in the raceways at densities corresponding to 740, 2220, 6660, and 19980 clams/m2. Each density was replicated eight times in the raceways. The highest and lowest densities were replicated four times at a shallow station in Folly River, adjacent to the nursery. All compartments were monitored monthly from February to August 1981 to determine growth and survival. Direct counts were made of all replicates and replacements added as necessary to maintain the original densities. Length was measured by ocular micrometer or Vernier calipers across the longest anterior-posterior axis, and recorded to the nearest 0.1 mm. Packed volumes were determined using appropriate graduated cylinders. Temperature and salinity were measured daily. Water samples were taken at monthly intervals from the inlet and outlet of each raceway row for chlorophyll a determinations. Chlorophyll a was determined by standard fluorometric methods (Strickland and Parsons, 1968), using a Turner fluorometer Model 111. RESULTS
Growth rate, as reflected by changes in size and volume, was significantly affected by planting density in both raceway and field control (Table I).
371
:v- f
CHARLESTON
I
‘.
STOUO
HARBOR
FOLLY ISLAND INLET
Fig. 1. Map showing location of experimental facilities.
Raceway populations exhibited mean increases in length over the study period of 13.03, 12.92, 10.60 and 8.79 mm at densities of 740, 2220, 6660 and 19 980 clams/m’, respectively. Analysis of variance followed by Student -Neuman-Keuls- test showed that the highest density (19 980) grew significantly slower than the two lowest densities (740 and 2220). Growth at 6660 clams/m2 did not differ significantly from growth at the lower densi-
372 TABLE I Mean total increase in length (mm) and volume (~1) of seed clams (initial size i=3.9 mm, 24 ~1) in raceway (R) and control (C) replicatesover the 6 month study period Density (seed/ml) 740 C
R Length
Z Volume
Z
2220
6660
19 980
R
R
R
C
increase (mm) 16.70 13.28 9.30 8.13 16.17 14.07 13.47 13.09 i3.03
11.44 11.03 11.56
14.42 14.11 10.77 9.45 15.65 13.63 12.89 12.37
13.26 9.48 9.48 9.66 13.19 11.00 9.96 8.83
10.33 9.50 9.50 8.47 10.26 8.56 7.47 6.22
10.41 10.21
11.43
12.92
10.60
8.79
10.35
increase (fil)
2935 1803 856 581 3101 2060 1916 1676 1866
1405 1226 1539 -1421
2203 2003 1165 730 2268’ 1910 1586 1315
1610 797 674 707 1559 1253 939 679
749 735 567 875 619 444 311
1005 1022
1648
1027
652
1005
923
ties or from growth at the highest density. Clams in the field controls grew 11.43 mm (low density) and 10.35 mm (high density) over the study period. This difference was significant at the 95% confidence level. Increase in biomass was similarly affected by planting density. Growth in the raceway was inversely proportional to distance from the water inflow (Table II). Highest growth was recorded in the lowest density nearest the inflow; lowest growth was observed in the highest density nearest the outflow. Because of the linear flow arrangement, the compartments further from the inflow experienced higher effective densities (number of clams per unit water). Regression of mean total length increase on effective density (log-transformed) was highly significant (P < 0.001). As a result of the significant relationship between growth rate and position within the raceway, or effective density, there was high with-in group variance for the raceway data, making parametric statistical comparison with
313 TABLE II Effect of distance from the inflow (cm) on mean total increase in length (mm) of seed clams (initiaI size = 3.9 mm) grown in raceways at four densities from February to August 1981 (190 days) Approximate distance from inflow (cm)
Density (clams/m*)
x
740
2220
6660
19 980
O-26
16.70 16.17
14.52 15.65
13.26 13.19
10.33 10.26
13.76
29-55
13.28 14.07
14.11 13.63
9.48 11.00
9.50 8.56
11.70
58-84
9.30 13.47
10.77 12.89
9.48 9.96
9.50 7.47
10.36
87-113
8.13 13.09
9.45 12.37
9.56 8.83
3.47 6.22
9.52
x
13.03
12.92
10.60
8.79
field controls (which had low within-group variance) difficult. The KruskalWallis non-parametric test demonstrated no significant difference between growth in the raceway and in the field controls. On the assumption that the conditions in the sample compartments nearest the inflow most closely resembled the natural environment, those were compared with the field controls of the same density, using one-way analysis of variance (Table III). At the low density, growth in the raceway was significantly greater than growth in the field controls (P < O.OOl), while at the high density there were no significant differences between growth in the two locations. TABLE III Statistical comparison of mean total increase in length (mm) of seed clams (initial size = 3.9 mm) grown at two densities in raceways (samples nearest inflow) and field control units Density (seed/m’) 19 980
740
x
Raceway
Control
Raceway
Control
16.70 16.17
11.44 11.03 11.68 11.56
10.33 10.26
10.41 10.21 10.42
16.44
11.43
10.30
10.35
F = 351.69***
F = 0.314 ns
***P < 0.001; ns = not significant.
374
The greatest mean daily increment in length was observed in May, when the lowest density replicates (740) in the raceway averaged 158.1 pm per clam per day (Fig. 2). At the high density in the raceway maximum length increase occurred 1 month earlier (April). Clams in the field controls exhibited fairly uniform growth rates throughout the study (Fig. 2). Greatest increase in biomass occurred 1 month later than the greatest increase in length (Fig. 3).
PO0
*.-. ,
MARCH
I
I
APRIL
MAY
I
JUNE
\\ -.. ..... . ..‘0 . I
JULY
AUGUST
Fig. 2. Averagegrowth rates (pm/day)
of seed clams planted at densities of 740 and 19 980 seed/m2 in raceways and field controls (February-August 1981).
’\
,A ,’ d’ IMARCHI
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Fig. 3. Water temperature (“C) over study period and average growth rate (pm/day and &l/day) of seed clams planted at low density (740 seed/m’) in raceway.
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Growth in the raceways was analyzed in relation to water flow rates. Flow rate was kept constant at 8 l/min but the volume of water available per volume of clams (effective flow rate) was different for each sample compartment and decreased as the clams grew. Total growth (change in biomass) was significantly correlated with the minimum effective flow rate (r = 0.941) (Fig. 4). This is reflected in the significant differences in growth observed at the different positions and densities in the raceways (Table II). 10
1
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Effective
Flow Ratelvolume
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100
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Fig. 4. Correlation between effective water flow rate (1 water per 1 clams per minute)and mean total increase in biomass (ml/clam) for seed clams grown in raceways for 6 months (February-August 1981).
Chlorophyll a was measured as an indicator of food availability. Available chlorophyll a ranged from 2.13 to 6.34 pg/l during the study period, averaging 4.13 pg/l. The difference in chlorophyll removal by clams at the four densities was highly significant (P < 0.001) with the highest density removing consistently more than the lowest density. Clams at the highest density removed as much as 83% of the available chlorophyll (average 49%), while those at the lowest density had a maximum removal of 46% (average 17%). Although the amount of chlorophyll removed was directly related to the number of clams, less chlorophyll was removed per clam as the density increased. The amount of chlorophyll removed per clam per day was highly correlated with the rate of biomass increase (r = 0.73, P < 0.05). Water temperature over the experimental period ranged from a minimum temperature of 73°C in February to a maximum of 31.5”C in July (Fig. 3). Maximum rate of increase in length occurred when temperatures were be tween 20 and 24°C while maximum biomass increase occurred when temperatures were between 24 and 29°C. Prolonged high temperatures (> 30°C) were associated with decreased growth rates and increased mortality. DISCUSSION
Growth rates obtained in this study were compared with the data of Eldridge et al. (1979) for hatchery-reared seed clams grown in field units in
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South Carolina. From May to December of the first year of their study, seed clams (13 mm) averaged 1.8 mm/month increase in length. This is virtually identical to the average rate of growth in the raceways (1.89 mm/month) but lower than the rate for the lowest density replicates in the raceway, which averaged 2.17 mm/month. The maximum monthly increment reported by Eldridge et al. (1979) occurred in the spring of the second year of that study, when clams grew 3.77 mm between April and May. In the present study, clams at the two lower densities grew at a rate of more than 4 mm/ month during May and June. This study confirms the reports by previous investigators that growth rate is reduced at high densities. Menzel and Sims (1962) found reduced growth at densities of 800/m2. Eldridge et al. (1979) found that growth was limited at a density of 1159/m2. In our study there was no significant difference in growth between clams planted at 740,222O and 6660 clams/m2 in raceways, but growth was reduced at a density of 19 980/m2. Since biomass is a more realistic measure of density than number of clams, all the above densities were transformed to biomass/m2, using an empirically determined regression of volume (biomass) on length. The limiting density reported by Menzel and Sims (1962) was equivalent to 8.2 l/m’, while that of Eldridge et al. (1979) was approximately 8.9 l/m*. These figures agree with the results of the present study in which clams grew well in raceways at densities of almost 9 l/m2. Although total mean growth in the raceways and the field controls was similar (Table I), growth in the field was less variable throughout the study (Fig. 2). Clams in the raceway grew much faster than those in the field units in the spring, but clams in the field controls continued to grow during the summer when there was little growth in the raceway. Total mean growth at the low density was significantly greater in the raceway than in the field, but there was no difference in growth at the high density in the two locations. These differences in growth in the raceways and the field units suggest that conditions in the two locations are not as similar as might be expected. One factor which obviously differed between the two locations was water flow. In the raceways flow was constantly unidirectional and the rate of flow was controlled. Mean growth in the raceways was high at effective flow rates as low as 3 1 water per 1 clams per min, but an effective flow rate of 8-9 1 per 1 clams per min was necessary for maximum growth. Rhodes et al (1981) reported flow requirements of 5 1 water per 1 animals per min for surf clams and 6.5 1 water per 1 animals per min for bay scallops. Effective flow rate was significantly correlated with total mean growth for the individual replicates (Fig. 4). At all densities the samples nearest the inflow (maximum effective flow) grew significantly faster than those adjacent to the outflow (minimum effective flow) (Table II). This contributed significantly to the high variability in growth rates exhibited among raceway replicates. Rhodes et al. (1981) reported that, for surf clams and bay scallops,
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growth in raceways was controlled by food supply. Kirby-Smith (1972) suggested that for bay scallops to grow well in raceways, at least 60% of the incoming chlorophyll should remain in the effluent. In this study, the average growth rate was high even when as much as 75% of the available chlorophyll was removed; but, at these removal rates, growth at the outflow end of the raceway was significantly reduced. Rhodes et al. (1981) reported that 1 pg/l of chlorophyll should remain in the effluent in order to assure adequate food supply. In this study effluent levels of less than 1 @g/l were associated with low growth but, even at higher effluent levels, clams in the compartments nearest the outflow grew more slowly than those nearest the inflow. Summer growth was low in all compartments despite effluent levels of more than 3 pg/l, suggesting that food supply was not the major factor limiting growth during this period. The slow growth during the summer coincided with periods when water temperatures were 30°C or higher. Ansell (1968) concluded, from his comparisons of the data of many investigators, that maximum growth of it4. mercenaria occurred at temperatures of 2O”C, with reduced growth at temperatures of 20-30°C and no growth above 31°C. While it is concurred that little growth occurs at temperatures exceeding 3O”C, the fastest growth occurred when water temperatures were between 20 and 28°C (Fig. 3). At temperatures exceeding 29”C, it was observed that clams often exhibited partial gaping and appeared to cease feeding. This may explain both the low growth and the low chlorophyll removal rates during the summer. In conclusion, growth rates for seed clams in raceways in South Carolina were somewhat higher than have been reported by other workers for field plantings. Although the density (number clams/m2) which could be maintained in the raceways was higher than previous reports from field studies, the biomass which could be supported was similar. At low densities, clams grew significantly faster in raceways than iri field controls. Even at the highest density (19 980 clams/m2), initial growth in the raceways was excellent and a size suitable for field planting (8.5 mm mean length) was attained in only 2 months. Density appeared to become a limiting factor as biomass approached 9 l/m2. Food supply appeared to be a major factor controlling growth rates in the raceways. When temperatures were less than 3O”C, the average rate of growth in the raceways was high as long as 1 @g/l of chlorophyll remained in the effluent. However, even when the effluent contained more than 1 pg/l of chlorophyll, growth rate declined in proportion to the distance from the inflow. The clams at the outlet end of the raceway were apparently food limited at all times. At temperatures exceeding 3O”C, growth rates in the raceways were extremely low, despite apparently adequate food supply. At these high temperatures, clams in the adjacent creek grew faster than those in the raceway. ACKNOWLEDGEMENTS
The authors would like to thank Trident Seafarms, Inc., for its cooperation in this study, particularly for providing seed clams and allowing the use
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of research space in its nursery. This research was supported by NOAA Office of Sea Grant, U.S. Department of Commerce. Ms. Hadley received financial support from the National Science Foundation through a graduate student fellowship, and from the Charleston Natural History Society through its Chamberlain Research Grant. This paper is adapted from portions of a thesis submitted by N.H. Hadley to the Charleston Higher Education Consortium in partial fulfillment of the requirements for the degree of Master of Science.
REFERENCES Ansell, A.D., 1968. The rate of growth of the hard clam Mercenaria mercenaria (L) throughout the geographical range. J. Cons., Cons. Int. Explor. Mer., 31 (3): 364-409. Castagna, M.A. and Kraeuter, J.N., 1977. Mercemvia culture using stone aggregate for predator protection. Proc. Natl. Shellfish. Assoc., 67: l-6. Eldridge, P.J., Waltz, W., Gracy, R.C. and Hunt, H.H., 1976. Growth and mortality rates of hatchery seed clams Mercenaria mercenaria in protected trays in waters of South Carolina. Proc. Natl. Shellfsh. Assoc., 66: 13-22. Eldridge, P.J., Eversole, A.G. and Whetstone, J.M., 1979. Comparative survival and growth rates of hard clams, Mercenaria mercenaria, planted in trays subtidally and intertidally at varying densities in a South Carolina estuary. Proc. Natl. Shellfish. Assoc., 69: 30-39. Kirby-Smith, W.W., 1972. Growth of the bay scallop: the influence of experimental water currents. J. Exp. Mar. Biol. Ecol., 8: 7-18. Manzi, J.J. and Whetstone, J.M., 1981. Intensive hard clam mariculture: a primer for South Carolina watermen. South Carolina Sea Grant Consortium, Mar. Adv. Publ. 81-01, Charleston, SC. Manzi, J.J., Burrell, Jr., V.G. and Carson, W.Z., 1980. A mariculture demonstration project for an alternative hard clam fishery in South Carolina: preliminary results. Proc. 11th Annu. Meet. World Maricult. Sot., 11: 79-89. Menzel, R.W. and Sims, H.W., 1962. Experimental farming of hard clams, Mercenaria mercenaria, in Florida. Proc. Natl. Shellfish. Assoc., 53: 103-110. Rhodes, E.W., Goldberg, R. and Widman, J.C., 1981. The role of raceways in mariculture systems for the bay scallop, Argopecten h-radians irradians, and the surf clam, Spisula solidissima. In: C. Claus, N. DePauw and E. Jaspers (Editors), Nursery Culture of Bivalve Molluscs. Eur. Maricult. Sot. Spec. Publ. No. 2, pp. 227-251. Strickland, J.D.H. and Parsons, T.R., 1968. A practical handbook of seawater analysis. Fish. Res. Board Can., Bull. 167.