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Clearly-defined winter-growth lines are present in acetate-peel replicas of shell sections of the umbo region of the European flat oyster, Ostrea edulis . They are formed annually during March and April and can be reliably used to determine age. The clarity of the annual winter-growth lines is variable: the first growth line is weak, ill defined, and composed of a number of finer lines, but the second and subsequent winter-growth lines are clearly defined. The pattern of annual lines in oysters from the River Blackwater, on the east coast of England, are clearer than those from the two south-coast sites studied, the River Fal and Solent. Analysis of the annual growth lines has been used to provide first-ever estimates of the age and growth for these wild populations. Samples from three wild stocks consisted of mainly 2- to 6-year-old oysters with maximum ages estimated at 14 years (River Blackwater) and 10 years (River Fal and Solent). River Blackwater oysters reach a size of 60 mm in 2–3 years and attain a greater size (Von Bertalanfly growth parameters K = 0.41 ± 0.18: L inf = 93.37 ± 8.88) than stocks in the River Fal (K = 0.35 ± 0.09: L inf = 80.29 ± 6.40) and Solent (K = 0.46 ± 0.17: L inf = 72.09 ± 7.76), which grow to 60 mm in 5 years.
C. A. Richardson (Mon,) studied this question.