The cockle Cerastoderma edule (L.) on the South Bull, Dublin Bay: population parameters and fishery potential

Abstract

The history of the Dublin Bay cockle fishery is reviewed briefly with emphasis on the period 1893-1913. Prior to 1900 some 80 tonnes of cockles are said to have been landed annually. In the subsequent decade the fishery declined and in recent times has been defunct. The population of cockles on the South Bull, Dublin Bay, was investigated in 1971 and 1972. Cockles were distributed throughout the beach, but occurred in greater density in the mid-shore region. The mean density, however, was low (9-13 cockles per sq.m) and the maximum recorded was only 51 per sq.m; this is consistent with the relatively exposed nature of the beach. The population was dominated by 0+ and 1+ age groups, though cockles as old as 9+ were collected. The age structure was consistent with regular annual recruitment, and the mean mortality figure for cockles in their second, third, fourth and fifth years was Z=0.76. The mean lengths at the end of the first four winters were 6.5, 22, 28.5 and 32 mm. The value of L∞ was 40 mm, and K = 0.6. The relationships between shell lengths and the weights of the shell, dry meat, wet meat, and shell + wet meat are described. The potential of this cockle population for commercial exploitation is considered, although in view of the low population density the catch per unit effort would probably be too small to be commercially viable at present. Yield curves indicate that a minimum legal length of 24 mm would be appropriate for protection of the breeding stock while allowing the optimum yield to be obtained. The potential sustained yield of the beach was estimated at 4 tonnes/sq.km annually at a fishing mortality of F=0.10.