appreciably reduced by eelgrass near but not on the beds. But eelgrass may 

 favor setting of young. Growth rate is proportional to amount of water 

 circulating. Probably the most important factor determining more rapid growth 

 in down river locations than locations near head of tide is the greater 

 circulation. Crowding on the bottom in Bideford River did not influence 

 growth, except in wire containers held above bottom, where fouling interfered. 

 In Bideford River distribution of settled clams was not limited by salinity or 

 tamp. Set was found to within a few hundred yards of head of tide. In the 



laboratory clams survived exposure to low salinity and even fresh water for 

 days at ordinary summer temp. Highest temp recorded at Bideford in 1939 

 and 1940 in the intertidal zone was 35 °C . Clams survived this temp in the 

 laboratory for at least 8 hr in fresh and 25 hr in seawater . Max temp for 

 survival, at temp increments of 1° to 1 l/2°C/hr, was between 39.5 and 4 0°C. 

 Larger clams were affected by high temp sooner than smaller, but smaller 

 clams were affected by lowered salinity sooner than larger. Clams survived 

 high temp in increased salinity as well as in ordinary seawater. In Tatama- 

 gouche Bay distribution of small clams appeared to be determined by location 

 of settlement of larvae. A few occurred just below high water, but abundance 

 increased to a max near half -tide level, and fell off suddenly farther out. 

 No small clams were found out as far as low water. General distribution is 

 limited by temp which must be reached before spawning will occur. In 1938, 

 '39 and '40 spawning in Bideford River did not begin until temp reached at 

 least 23°C, then continued as a gradual process to about 1 Sept. On shores 

 of Tatamagouche Bay exposure is not important in limiting distribution of 

 small quahaugs after setting. No evidence of mortality was seen except by 

 drilling, in collections made to within 100 ft of high water. In Bideford 

 River clams can survive exposure to air in summer for at least 60% of the 

 time, thus inhabiting intertidal zone from low water to above half -tide level, 

 but winter survival is hazardous in any part of the intertidal zone. Presence 

 of eelgrass favors establishment and survival of young clams on very soft 

 bottom following setting. Unusually intense storms in cold weather may kill 

 many clams in the intertidal zone, probably because they are hibernating and 

 cannot regain the surface if buried, or bury if exposed. In warm weather 

 clams can regain the surface if buried to a depth of several inches below 

 normal position, but they are very inactive in horizontal movement. It is 

 improbable that distribution would be influenced by voluntary horizontal 

 movement. In Tatamagouche Bay mortality was very high from drilling by 

 Polinioes heros, which is the only predator there. No serious losses were 

 seen in Bideford River from drilling, although P. heros was present downriver. 

 Disease has been reported frequently. Large numbers died in Malpeque Bay 

 apparently from disease in 1915. In 1940 disease was reported in N.S. and in 

 P.E.I. No disease had been reported in Bideford River and Malpeque Bay since 

 1915. The disease has not been identified. Growth was slow. In the most 

 favorable places 5 to 6 yrs were required for minimum legal length of 1 3/4 

 inches. Marked reduction in abundance about 1912 can be attributed to 

 removals in excess of rate of production and growth. Present distribution of 

 quahaugs in Bideford River was limited by scarcity of breeding stock. 

 Planting of medium to large clams (several hundred) in one area led to 

 increased set nearby. Clams mature at early age in Canada. Active spermatozoa 

 form in some in 2nd year and mature ova in third year and older. In 

 experimental studies to test effects of crowding, clams were placed 6 in apart, 

 2 1/2 in apart, and touching each other. Growth was not affected. Laboratory 

 studies showed that clams would survive for long periods at extremes of temp 

 and salinity greater than found in natural habitat. Clams buried 6 in deep 

 by a storm all died. - J.L.M. 



995 



Kerswill, C. J. 1941. 



The growth of quahogs in Canada. Fish. Res. Bd. Can. Prog. Rep. Atl. 30: 3-4. 



Hard clams of Prince Edward Island averaged about 5 mm long after their first 

 season. Spawning occurred when water temperature was about 23°C in July and 

 August. Since growth occurs only when water temperature is above 10°C and 

 water is colder than this after about September 15, the growing period in the 

 first season is short. In Canada it takes 5 to 6 years for clams to reach 

 marketable size of 1 3/4 inches long. Growth was greatest in areas of 

 unobstructed bottom where water can circulate unimpeded. In the intertidal 

 zone, clam growth increased toward low water. - D.L. 



280 



