A control group was fed Isochrysis or Tetraselmis. In all cases algae were fed 

 to bivalves on the basis of cell numbers, yet estimates of cell volume for most 

 species were obtained by a Coulter counter or measurement of centrifuged packed 

 cell volume. Differences in growth between Chlamydomonas and Tetraselmis were 

 not due to amino-acid composition, as no substantial differences in amino-acid 

 composition were noted. Skeletonema oostatum and Pyramimonas grossii (less than 

 100 cells/ul) showed better growth than control, but based only on one 

 experiment each. Clams fed Tetraselmis sp. showed maximum growth at 15-20 

 cells/ul, while growth rate was not very sensitive to different cell 

 concentrations of Isochrysis galbana. Substantial growth was noted with 

 Olisthodisaus sp. (1.-0-7.0 cells/ul), Cryptomonas sp. (75 cells/ul), 

 Cricosphaera carterae (5.0-7.5 cells/uD, Dicrateria inornata (200 cells/ul), 

 and Monoahrysis lutherii (100 cells/ul). - W.J.B. 



1928 



Walne, P. R. 1970. 



The growth of relaid American hard clams (quahogs) in England and Wales. Min. 

 Agric. Fish. Food, Fish. Exp. Sta. , Conway, N. Wales, Shellf. Inf. Leaflet 16, 

 8 p. , 5 figs. 



Venus (Mereenaria) mercenaria is not native to Europe, but attempts have been 

 made to introduce it into Britain, France, Holland, and Belgium. All soon died 

 out, with the exception of 1 or 2 in France, presumably because the areas were 

 unsuitable or water temps were not high enough for reproduction. The origin of 

 the reproducing population in Southampton Water in southern England is not 

 known. The standing stock is presently several hundred tons of market-sized 

 clams. In the Shellfish Culture Unit at Conway about 42% of fertilized eggs 

 have been reared to spat stage and about 25% to 10 mm shell length. 

 Thereafter, when planted in the natural environment, small clams are 

 vulnerable to predation by shore crabs (Carcinus maenas). When quahogs reach 

 2 to 25 mm long they are resistant to even the biggest crabs, but they do not 

 reach this size until 2 yrs, and it is costly and inconvenient to protect them 

 for so long. For growth experiments in the natural environment each clam was 

 numbered. From March to September 1964, a standard clam 40 mm long grew by 6 

 to about 17 mm (final length 46 to 57 mm). At 5 sites, growth was above 

 average and clams reached an average length of 54.2 mm by fall. At 4 sites, 

 average length in fall was only 45.9 mm. Muddy shores in quiet creeks and 

 estuaries were preferable to exposed areas or bottoms in which the substrate 

 was too hard. Growth experiments in 1966-1968 showed that the relationship 

 between length in year n and in year n+1 could be described by a straight 

 line. The relationship was: 10 mm at planting = 23.2 mm after 1 yr; 2 yrs 

 35.6 mm; 3 yrs 47.2 mm; and 4 yrs 58.0 mm. If favorable sites were selected, 

 better growth than this average relationship could be obtained. Annual 

 survival, expressed as a percentage, fell into 2 clear groups: clams 9 to 13 

 mm long at planting had an average mortality of 12.1%, clams 17 to 21 mm at 

 planting died at the rate of only 3.3% per yr. Seed 10 mm long at planting 

 will be 50 to 60 mm long at harvesting, 3 to 4 growing seasons later. If clams 

 are protected by netting, about 80% will survive to market size. To allow for 

 accidents and other eventualities, 50% mortality from planting to harvest 

 probably is more realistic. Experimental densities of clams were 2 million/ 

 acre. If growth and mortality under commercial operation were the same as in 

 experiments, the harvest would be one million clams, 55 to 60 mm long per acre. 

 These would weigh about 60 g. Thus, a million would weigh 30 metric tons. 

 The. natural population in Southampton Water contains 25 to 30 tons/acre. 

 - J.L.M. 



1929 



Walne, P. R. 1972. 



The influence of current speed, body size and water temperature on the 

 filtration rate of five species of bivalves. J. Mar. Biol. Assn. U.K. 

 52(2): 345-374. 



Experiments were based on removal of particulate matter by clams. It was 

 assumed that filtering was 100% efficient. In a closed system Mercenaria 

 mercenaria of shell length 4-5 mm were fed different concentrations of 

 Isochrysis galbana, Phaeodactylum tricornutum, and Dunaliella tertiolecta. 



536 



