A preliminary account of fisheries for the surf clam Spisula solida (L) (Mactracea) in Ireland

Abstract

Surf clams from seven stocklets on the west and south coasts are examined to provide a preliminary account of the fishery in Ireland. Spisula solida is the species harvested in every case. Most of the material was collected by commercial (box or hydraulic) dredge. A quantitative account of a surf clam bed is based on sampling by Day grab in Waterford Harbour. Ageing was by external shell sculpture. A limited comparison of this method with ageing by internal shell structure confirmed the method was usable and the results are presented on this basis with, however, reservations on the reliability of the approach. S. solida in Ireland had Linf of c 43 cm. Material gathered in Clifden, Co Galway, had an Linf of 35 mm. The Clifden stocklet was heavily fished when the material was gathered and a low value for Linf is attributed to the Lee phenomenon. A growth curve is constructed for only one stocklet, that in Waterford Harbour. Growth was slower than for S. solida in the North Sea, a possible consequence of heavy fishing also. The clam bed in Waterford Harbour was a low elevation bank of coarse (Spisula) sand. The area of the bed had become reduced during the preceding year by the invasion of silt grades displaced by earthworks upstream; these were injected into the coarse material to form a perimeter of the clam patch. Within the bed, the highest biomass of S. solida was 600 g/sq.m. Representation of age frequencies within samples suggests that heavy spatfalls of S. solida occur at irregular intervals and this complicates the calculation of F values from a catch curve. A yield per recruit curve is prepared for the clam patch in Waterford Harbour.