than wide in the northern, although this difference overlaps. The southern 

 species quickly gapes and spoils when removed from the water, which 

 restricts its marketability. Northern species will remain tightly closed 

 out of water for 2 weeks or more, especially under refrigeration. In both 

 species, ability to remain closed depends upon physiological condition, temp 

 and humidity. In the several commercial hatcheries now operating, clams and 

 their larvae are fed living food. Growth experiments in Fla. were conducted 

 in natural environment by covering with wire to prevent predation. Growth of 

 individual clams varied greatly. Best shell growth was in spring and fall 

 for both species. Northern species grew all winter, but shell growth 

 virtually stopped when water temp rose above 86°F. Southern species grew all 

 summer but growth ceased when temp dropped below about 55°F. Annual growth 

 of southern quahog was almost double that of northern. The 2 species 

 hybridize readily in the laboratory and the hybrid is fertile. Fi , F2 and F3 

 hybrids are growing in Fla. F]_ and F2 hybrids grew as well as southern parent 

 or better. Fi hybrids have intermediate shell characteristics, F2 and F3 

 hybrids have segregated shell characteristics. F^ hybrids were able to remain 

 closed out of water almost as long as northern parent. It takes about 2 yrs 

 for northern quahog to reach half shell market size in Fla. waters. Fifty 

 clams/ft2 was about maximum density for satisfactory growth. Southern quahog 

 and hybrids reach market size in 1 to 1 1/2 yrs. One advantage of hard clam 

 for mariculture is that small clams bring a higher price than older clams, 

 which allows maximum turnover rate for crops. Quahogs have no known epidemic 

 diseases, but disease might be expected when they are grown under- mariculture 

 conditions. Some hazards need to be solved. Several animals are voracious 

 predators, especially blue crab {Callineotes sap-idus ) . In cages, blue crabs 

 can destroy 2-inch clams. Other predators are other crabs, whelks and moon 

 shells. Smaller clams are most vulnerable. A small pilot quahog farm of- 

 3,500 ft 2 had a maximum of over 100,000 clams at densities of 10 to 75/ft . 

 Protection from predators was given by a 6 foot high fence of 1/4 to 1/2 inch 

 mesh fencing of wire or nylon webbing , A few predators entered and had to be 

 removed at times. Seed clams less than 1/2 inch long were not satisfactory 

 because small predators could enter the screen. Smaller mesh screen would 

 interfere with water circulation and be more vulnerable to storm damage. 

 Fouling on meshes increased storm damage. Larger clams had less than 10% 

 mortality from predators. Rearing clams to 1/2 inch size or larger requires 

 large quantities of food and space and tremendous labor. Clams as small as 

 1/25 inch have been planted successfully in the field, but they were given 

 protection that would not be practical commercially. Experimental results 

 suggest that clams could be grown at a density of at least a million clams/ 

 acre. Northern quahog would reach half-shell size in about 2 yrs, F]_ hybrids 

 in 1 yr. At a market price of 3C each, gross return/acre would be $30,000 

 provided that minimal densities could be maintained and prices remained firm. 

 Unexpected mortalities or poor growth from environmental variations could be 

 problems. An area with adequate currents would be necessary for food supply 

 and removal of wastes. Salinity must remain above minimum levels. Other 

 costs were estimated: fences cost about $2,000/acre installed; wire lasted 

 about 3 yrs, and netting about 2 yrs, with minor repair; beds of crushed 

 shell or stones, like those developed in Va. to reduce predation, if they 

 worked in Fla., would add to costs. A hatchery would be essential. Costs 

 were not estimated, but some logistic problems and needs are discussed. 

 Labor would be another significant cost. One man with a helper probably 

 could operate a small hatchery to supply enough seed clams for several acres, 

 and take care of some other needs, like surveillance, minor repairs, sampling 

 for growth, and preventing poaching. Additional labor would be needed for 

 preparing beds for planting and for harvesting the crop. Mariculture of clams 

 offers as much potential as any other organism now being considered, but it is 

 still largely experimental. The principal problem probably is in rearing 

 young clams to a size relatively resistant to predation. One interesting and 

 challenging problem is further domestication through selection and breeding 

 for desirable traits. Recommended facilities and conditions are: 1) a well 

 equipped hatchery; 2) use Fi hybrids for seed clams; 3) supply and maintain 

 adequate predator control; 4) planting densities not over 50 clams/ft2; 5) 

 preliminary trial plantings for evaluations; and 6) proper site selection: 

 a) salinity above 25°/oo, b) no extremes in salinity and temp, c) substrate 

 of mud/sand, d) areas protected from continual wave action, and e) no 

 pollution. - J.L.M. 



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