EsTUARiNE Garden 



PLANTING CLAMS 



By Ccf Flimlin 



Baby clams 



BY NANCY DAVIS 



A juvenile clam lays nestled 

 in its bed among the roots 

 of seagrass in Back Sound. 

 But danger lurks nearby. A 

 blue crab makes its way toward the 

 young clam. Helpless to move, the 

 clam finds itself in the crab's strong 

 claws. The crab picks up the clam, 

 handles it for a moment, and then, 

 using its claws, begins to chip away at 

 the clam's shell. Within minutes, the 

 crab pops the meat of the baby clam 

 into its mouth. 



The crab continues to forage in the 

 clam bed, attempting to satiate his 

 appetite. Just one clam won't be 

 enough. In a short while, the crab 

 leaves a path of destruction. 



Nature is not kind to young clams. 

 The mortality rate among three-milli- 

 meter seed clams is about 95 percent, 

 says Sea Grant researcher Charles 

 "Pete" Peterson. Although the blue 



crab is a major predator, it is not alone 

 in its appetite for clams; mud crabs, 

 stone crabs, shrimp and snapping 

 shrimp, bottom-feeding fish, moon 

 snails and whelks also take their toll. 

 Such high mortahty rates are common 

 among species in the natural environ- 

 ment, but they pose problems for clam 

 culturists. 



Clam gardening is a form of aqua- 

 culture in which seed clams are pur- 

 chased from a hatchery, then placed 

 on estuarine bottom leased from the 

 state. The clams are protected until 

 they reach a marketable size. 



Peterson says, "The only remaining 

 unsolved problem in the technology of 

 'clam gardening' . . . is predator pre- 

 vention." 



In 1983 and 1984, Peterson approach- 

 ed the clam mortality problem from 

 several angles. He wanted to know at 

 what size a clam becomes invulnerable 

 to predators, and if clam gardening 

 could be made economically viable 

 through manipulation of seed clam 

 size, season of planting and the use of 

 anti-predation structures. 



Peterson began by bringing his sub- 

 jects into the laboratory. He served 

 various sizes of clams to crabs to find 

 out at what size the clams become 

 invulnerable. He found that crabs eas- 

 ily make a light meal of small clams, 

 crushing the shells with their claws. 

 When offered a larger clam, the crabs 

 simply chipped away at the edges of 

 its shell until they reached the meat. 

 Although the crustaceans will handle a 

 large clam, they may eventually reject 

 it in favor of a more manageable meal. 



In a previous study, Peterson found 

 that a clam's environment determines 

 its rate of growth; the mollusks grow 

 faster in some areas than in others. For 

 example, growth rates for clams at 

 sites in the North River and Newport 

 River were three times higher than at 

 sites in Bogue Sound. 



In turn, growth rates affect the rate 

 of predation. In areas with slow 



growth rates, it takes longer for clams 

 to become less susceptible to their nat- 

 ural enemies. 



Peterson measured growth rates for 

 clams in 10 sites. Then he gathered 

 data on clam mortality in those loca- 

 tions, and tested some theories. 



Peterson knew that for a period dur- 

 ing the winter, hard crabs remained 

 relatively inactive. At the same time, 

 unlike clams growing in northern 

 waters, the mollusks continue to grow 

 here during colder months. Peterson 

 hypothesized that seed clams planted 

 in the winter would grow large enough 

 by spring to be relatively invulnerable 

 to crab predation. 



At his test sites, Peterson planted 

 three sizes of seed clams and grew 

 them through the winter, with little 

 loss to predation. The next step will be 

 a cost analysis. Since price varies for 

 the different sizes. Sea Grant econo- 

 mist Jim Easley will weigh those costs 

 against the rate of survivorship. Cultur- 

 ists need to know which size seed clam 

 will give them the best return for their 

 money. 



Peterson tested another theory: 

 could caging out predators improve 

 hard clam recruitment enough to ren- 

 der it an economically viable alterna- 

 tive to clam hatcheries? To find out, he 

 erected exclosures in a variety of habi- 

 tats. In particular, he tested a light 

 mesh webbing designed to exclude 

 predators from clam beds. The web- 

 bing proved effective, but before 

 Peterson recommends its use, he wants 

 Easley to evaluate its cost effective- 

 ness. 



In another Sea Grant project, marine 

 advisory agents Gary Van Housen, 

 Bob Hines and Randy Rouse and fisher- 

 men Gary Culpepper and Mark Hooper 

 are testing the possibility of using crab 

 shedding tanks during the off-season 

 for holding seed clams. Fishermen 

 could buy the smaller, less expensive 

 clams and hold them until they reach a 

 sufficient size for planting. 



