DISTRIBUTION, SHELL GROWTH AND PREDATION OF THE NEW-ZEALAND OYSTER, TIOSTREA (=OSTREA) LUTARIA HUTTON, IN THE MENAI STRAIT, NORTH WALES

A population of the New Zealand oyster, Tiostrea lutaria, at Tal-y-foe l in the Menai Strait was surveyed during June-July 1992. Oyster densi ty was highest in the immediate vicinity of the Ministry of Agricultur e, Fisheries and Food (MAFF) experimental shellfish beds where this sp ecies had been introduced in 1963, but a few isolated oysters occurred up to 0.5 km from this locality. Intertidal and subtidal populations showed clear differences in size composition. Analysis of size frequen cy distributions using the method of Bhattacharya (1967) indicated tha t these populations could be broadly resolved into two (intertidal) an d four (subtidal) overlapping size classes. The age of individual oyst ers was determined from annual growth lines in acetate peel replicas o f polished and etched sections of the shell. Although growth rates of intertidal and subtidal oysters were similar during the first few year s of growth these populations exhibited different Von Bertalanffy grow th constants (K = 0.597 +/- 0.398 & 0.299 +/- 0.068, respectively) and attained a different asymptotic length (L(infinity) = 79.89 +/- 17.77 & 108.48 +/- 9.56 mm, respectively). The maximum age of subtidal oyst ers (8 yrs) was also greater than that of intertidal oysters (5 yrs). Laboratory predation experiments showed that whilst crabs, Carcinus ma enas and Cancer pagurus, fed voraciously on the Pacific oyster Crassos trea gigas, they were reluctant to feed on T. lutaria, particularly wh en both species were presented simultaneously. Video recordings of for aging crabs suggested that this reluctance to feed on T. lutaria was d ue to mechanical difficulties associated with prey handling.