The age structure of a population of the hard-shell clam, Mercenaria mercenaria from Southampton Water, England, derived from acetate peel replicas of shell sections

Annual lines in the internal shell structure of Mercenaria mercenaria were used to age clams from North Wales (cultivated stock) and Southampton Water (wild stock). A comparison was made between the number of lines visible under low-power ( × 6 magnification) in shell sections and the number observed microscopically (× 10 to × 25 magnification) in acetate peel replicas of sections. The number of lines in peels from the shells of nine cultivated clams from North Wales coincided exactly with the known number of years (22) since deposit; the estimated age of the other four shells from the same sample differed by only one year from expectation. In contrast, counting lines under × 6 magnification could underestimate age by up to 7 years. The ages of one-hundred-and-thirtyfour wild clams from Southampton Water were determined using acetate peel replicas. The age frequency distribution in the samples collected in 1983 and 1988 revealed certain prominent year classes in the population. The years 1969, 1976, 1977, and 1984 were occasions when there was above average settlement and/or subsequent survival. With the exception of 1977, successful settlement was found to correspond with above average water temperatures. The reasons why some years are more successful than others for larval settlement and recruitment of the clams are discussed. It is suggested that a combination of below average freshwater flow into Southampton Water as well as above average water temperatures may have been the important factors influencing larval settlement and survival of clams during 1976 and 1984.