the ciliated foot is prominent and larvae crawl on bottom. When water is 

 changed many larvae remain attached by byssus and must be washed off with a 

 strong jet of water. At this stage a stagnant culture is no longer suitable 

 because larvae clump and smother. Spat are transferred to trays with nylon 

 mesh base in running seawater. Coarsely strained water enriched with 50 

 cells of Isoahrysis and 5 of Tetraselmis (presumably per ul) . If trays are 

 being used at capacity, algal culture must be added continuously. The system 

 is closed, water is renewed twice a week, and spat are washed to remove 

 detritus and break up clumps. As they grow, spat are moved to progressively 

 larger mesh. Growth of individuals varies widely, and it is best to sort 

 selectively and grow size groups separately. At about 1 mm shell length spat 

 are removed and placed in running seawater at ambient temp outside the lab. 

 At this stage a 450-liter unit needs 5 liters of algal culture/day. In about 

 63 days about 28% of fertilized eggs survived to mean size of 1.2 mm. Trays 

 and pipes require constant maintenance. A million quahogs 2.5 mm long require 

 2,058 1 of seawater/hr ; a million at 15 mm require 144,000 1/hr. After 

 planting in the natural environment predation becomes a serious problem. 

 Carcinus maenas was the principal predator. Size of crab determines size of 

 clam that can be eaten. One method of protection is to cover clams with mesh 

 of suitable size. Clams of about 10 mm length at the beginning of the growing 

 season were over 20 mm at end of growing season, clams about 40 mm at beginning 

 of season were somewhat less than 55 mm at end of season. Mortality during 

 growing season varied from about 25% to 2%. Survival of larger clams was on 

 the average higher and much less variable. In Conwy the growing season is May 

 to September. Best growth is obtained in warmer waters on the south and 

 southeast coasts of England. Seed planted in the natural environment at 10 mm 

 long increases in length in the following 4 yrs by 10-23.2, 23.2-35.6, 

 35.6-47.2, and 47.2-58 mm, respectively. Mortalities were 12.1%, 15, 17.8, 

 and 20.5%, respectively (cumulative). Quahogs were planted at a density 

 of 2xl06/acre. Estimated survival on a commercial scale probably would 

 be lower, perhaps 50%. A million quahogs of 55-60 mm would weigh 30 metric 

 tons. Quahogs are excellent material for hatchery culture. They thrive in 

 soft muddy bottom unsuitable for culture of other shellfish. The major 

 problem is protection of seed against predation. Miscellaneous references 

 to hard clam in other chapters were: filtering rates of quahog are more 

 affected by low temp than oysters and mussels; quahog requires higher temp 

 for spawning than does 0. edulis; growth of M. meraenaria for 28 days after 

 metamorphosis is somewhat less than 1.5 mm at about 17 °C, and about 2.5 mm 

 at about 2 5.5°C; meat weight, expressed as ratio of dry meat weight/dry 

 shell weight x 100, declined from about 7 at 11°C to about 6 at 24°C. Chapter 

 4 contains information on value of various phytoplankton organisms as food for 

 0. edulis larvae, but much of this discussion will be of interest in 

 connection with clam culture, as will most other chapters. - J.L.M. 



]932 



Walne, P. R. 1974. 



Shellfish culture. Chapter 19 in Sea Fisheries Research. F. R. Harden Jones 

 (edj . John Wiley and Sons, New York: 379-398. 



At Conwy Fisheries Experiment Station, adult Meraenaria meraenaria are ripened 

 in running seawater at 21°C for about 40 days in winter and less in spring 

 and summer. A standard 75-liter bin is stocked with about 400,000 larvae 24 

 hr after fertilization. About 40% will metamorphose 21 days later at 22°C at 

 a mean size of about 0.25 mm. Quahog larvae do not attach permanently at end 

 of the free-swimming period. When most larvae have metamorphosed they are 

 transferred to trays with plastic mesh base, immersed in seawater. The water 

 is enriched with 50 cells of Isoahrysis or 5 cells of Tetraselmis per 

 microliter and circulated continually through the trays. With 750,000 

 quahaugs distributed among 8 trays, each with area of 400 cm2 , growth and 

 survival are good in a closed system with total volume 450 1. Clams reach 1 

 mm after about 40 days at 22° and are transferred to trays in running water 

 at ambient temperature outside the laboratory. About 28% of original eggs 

 survive to 1 mm. Standard trays have a base area 900 cm 2 and a flow of 1.2 

 1/min. A million quahaugs at 10 mm require in summer about 36,000 1/hr from 

 which they extract about 75% of the particulate matter. Thus, they should be 

 transferred to the natural environment as soon as possible. Unless protected 



538 



