EFFECTS OF LARGE PREDATORS ON 



THE FIELD CULTURE OF THE HARD CLAM, 



MERCENARIA MERCENARIA^ 



Individuals in the clam industry have used fences 

 to keep the cownose ray, Rhinoptera bonasus, out 

 of planted areas (Lewis^; Burton ). Tiller et al. 

 (1952) indicated losses due to skates in planted 

 holding areas and stated that "One man reported 

 the loss of 600 bushels of small clams in two nights 



during 1948 " Merriner and Smith'* stated 



that cownose ray predation is a serious problem on 

 oyster and clam grounds in Chesapeake Bay. From 

 these observations it is clear that such large pred- 

 ators could be a significant deterrent to the culture 

 of clams from Delaware Bay southward along the 

 Atlantic coast. 



The present study continues a program de- 

 signed to evaluate methods of protecting areas 

 seeded with young Mercenaria mercenaria. The 

 initial portion of the study outlined the interactive 

 effects of pens, gravel, current baffles and crab 

 traps on the first year's growth and survival 

 (Kraeuter and Castagna 1977). The results of the 

 second year study on the interactive effects of 

 these manipulations are recorded below. The data 

 indicate effectiveness of efforts to prevent preda- 

 tion on clams surviving the first year's plantings. 



of metal framed current baffles and crushed gran- 

 ite gravel. Current baffles 0.6 m high were con- 

 structed to decrease the scouring effects of cur- 

 rents. Since average tidal amplitudes are 1.2 m, 

 the baffles did not prevent entrance offish or crabs 

 into the plots. Each baffle was about 1.5 m long 

 and 12 baffles were set in an array forming four 

 squares (Figure 1). Clam seed (about 2 mm) was 

 planted in all sites at 3,000/m2. 



Clams were sampled in each site with a 7.4 cm 

 diameter corer. A 0.6 m^ grid was placed over each 

 treatment and 10 random samples were removed 

 in July 1977. This is a continuation of the previous 

 year's sampling. For final sampling (October- 

 November 1977), all sites were harvested using a 

 suction sampler with an attached mesh bag. Four 

 quadrats corresponding to the squares formed by 

 placing current baffles in squares were sampled as 

 discrete units (Figure 1). Where no baffles were 

 utilized for the treatment, squares were marked 

 by stakes and sampled as though the baffles had 

 been present. All clams removed from the plots 

 were brought to the laboratory, counted, and the 

 percent commercial size (1 in (25.4 mm) thick New 

 York legal limit) was determined. The data 

 (counts) were transformed by logj^ and compared 

 by a factorial analysis of variance design 

 (ANOVA). 



Methods 



Results and Discussion 



Details of the experimental design were pre- 

 sented in the previous paper (Kraeuter and Cas- 

 tagna 1977) and are briefly discussed below. 



Four contiguous intertidal sites were marked by 

 pushing stakes into the muddy substrate and two 

 of the four sites were enclosed by 10 mm mesh 

 plastic net 2.3 m high stretched around the 38 m 

 circumference. The two remaining sites were left 

 open (Figure 1 ). Crab traps were placed within one 

 of the penned and one of the unpenned (no net) 

 sites to assess the predatory effects of the blue 

 crab, Callinectes sapidus. In addition, within each 

 site, areas to be seeded were marked and desig- 

 nated to be treated with or without combinations 



'Contribution No. 924 from Virginia Institute of Marine 

 Science. 



^J. H. Lewis, seafood shipper and packer, Saxis, VA 23427, 

 pers. commun. Nov. 1976. 



^L. L. Burton, seafood shipper and packer, Burton's Seafood, 

 Chincoteague, VA 23336, pers. commun. Sept. 1976. 



"Merriner, J. V, and J. W. Smith. 1979. Gear feasibility 

 study for the cownose ray, Rhinoptera bonasus. Va. Inst. Mar. 

 Sci., Spec. Rep. Appl. Mar. Sci. Ocean Eng. 227, 27 p. 



Results from the first year sampling (through 

 September 1976) indicated that baffles and gravel 

 in combination were superior to any other ti'eat- 

 ment. Plots were sited in an area where preda- 

 ceous echinoderms were not present, and although 

 pens were not effective in preventing crab preda- 

 tion, no discernable damage could be attributed to 

 other predators (Kraeuter and Castagna 1977). 



The statistical summaries (Table 1) are a con- 

 tinuation of the table presented by Kraeuter and 

 Castagna (1977), and, as in that paper, it is impor- 

 tant to emphasize that the sampling results from 

 one period to the next were not independent. The 

 final data represent the cumulative effects of all 

 environmental and biotic interactions on clams 

 planted in fall 1975. 



The July 1977 results mirrored those of earlier 

 sampling periods (Kraeuter and Castagna 1977) 

 with the exception that the pen x trap and pen x 

 baffle X trap interactions were significant at the 

 0.05 level. This was due, in part, to the higher level 

 of predation in the penned area without traps (18 



538 



FISHERY BULLETIN: VOL. 78, NO. 2. 1980. 



