report abundance in terms of the number of 

 hard clams caught in a unit area of bottom, 

 e.g., number per 100 square feet. As the 

 area covered by the jet dredge in the 20- 

 minute sampling tow was known and the hard 

 clams caught in each sampling tow were 

 actually counted, it was possible to convert 

 all samples to nunnbers per 1 00 square feet. 

 The average population densities (table 1) 

 for the three areas were 2.2, 1.9, and 1.7 

 bushels per tow. These are equivalent to 

 0.7, 0.6, and 0.5 hard clams per 100 square 

 feet respectively. The two greatest abund- 

 ances found were 13.7 and 34.8 bushels 

 per tow, or 4 and 11 clams per 100 square 

 feet. 



In Chincoteague Bay, Maryland, Wells 

 (1957) obtained a population density for 

 clams 38 mm. (1 1/2 inches) or more in 

 length as high as 68 clams per 100 square 

 feet or over 6 times the highest density 

 observed from Nantucket Sound. Even higher 

 densities were obtained in Narragansett 

 Bay, Rhode Island, when a survey was made 

 of clams 15 mm. and larger (Louis D. 

 Stringer, personal communication). Al- 

 though the average density of 156 clams 

 per 100 square feet obtained fronn the 

 Providence River is 14 times greater than 

 the largest density obtained for Nantucket 

 Sound, one particularly large sample yielded 

 1,760 clams per 100 square feet. The Nar- 

 ragansett Bay density figures include clams 

 15 to 66 mm. in length and are not strictly 

 comparable with Nantucket Sound densities. 

 However, if only clams above 66 mm. are 

 considered, the average density in Nar- 

 ragansett Bay was 21 per 100 square feet, 

 a figure nearly twice as large as the highest 

 single sannple obtained in Nantucket Sound. 



The absence of clams less than 60 mm. 

 during this survey is of considerable con- 

 cern with regard to the future of this 

 fishery. Admittedly the evidence that small 

 clams do not exist in Nantucket Sound is 

 inconclusive since the whole area was not 

 explored with gear what would effectively 

 capture them. Yet if we suppose that clam 

 sets equal to an average density of 1 clam 

 per 100 square feet occurred and survived 

 for any 1 year in the past 5, nearly 10 clams 

 less than 60 mm. should have been caught 

 in the 196 clam-shell bucket samples. In- 

 stead, none was caught, even though bucket 

 samples were taken fronn bottom that had 

 produced one of the largest catches of hard 

 clams, i.e., the one tow yielding 4 clams 



per 100 square feet. This would lead to the 

 assumption that the occurrence or survival 

 of young clams must be lower than 1 clam 

 per 100 square feet. 



One of the important reasons for the 

 hard clam survey was to discover any areas 

 of commercial abundance not already known 

 to the fishermen. No new commercial areas 

 were found. The various general areas 

 where hard clams could be found were 

 known. However, the exact locations of 

 heavy concentrations were not completely 

 known and defining these areas acconn- 

 plished an important purpose of the survey. 



Two theories could account for the 

 occurrence of the present population of 

 hard clams in Nantucket Sound. First, these 

 clanns could have occurred because of suc- 

 cessful setting at infrequent intervals, 

 resulting in dominant year classes. Sec- 

 ond, the present abundance may be the ac- 

 cumulation over nnany years of a very few 

 offspring which set and survive each 

 year. 



Since female hard clams have a high 

 reproductive potential and can produce an 

 average of 25 million eggs each spawning 

 season (Davis and Chanley 1 956), relatively 

 few adults are needed to produce large 

 numbers of offspring when particularly fa- 

 vorable environmental conditions occur. 

 In general, particularly favorable conditions 

 do not occur each year, but when they do 

 the resulting offspring are so numerous in 

 comparison to the numbers produced in 

 other years that they result in a dominant 

 year class. A dominant year class is 

 characterized by a relatively large number 

 of individuals that are the same age and 

 nearly the same size. 



Of the physical, chemical, and biolog- 

 ical conditions, water temperature has a 

 particularly linniting effect upon hard clam 

 reproduction. Spawning does not take place 

 at temperatures below 69° F. (Loosanoff and 

 Davis 1950). Turner (1957) reports that 

 bottom water in the middle of Nantucket 

 Sound rarely reaches this temperature even 

 in midsummer. Therefore, conditions for 

 the production of a dominant year class in 

 Nantucket Sound may occur only in those 

 rare years when the water tennperature 

 goes above 69° F. and when all other en- 

 vironmental conditions are suitable for 

 the survival of offspring. 



10 



