Growth statistics of an exploited razor clam (Ensis siliqua) bed at Gormanstown, Co Meath, Ireland

Abstract

A clam bed of 21 km2 was discovered at Gormanstown on the East coast of Ireland in 1997. Between July 1998 and August 1999, dredge samples were collected from vessels fishing there and these provided material for biological analysis. E. siliqua occurs with an inshore invertebrate fauna; occasional occurrences of large numbers of Donax vittaus, Pharus legumen and Lanice conchilega were associated with razor clams of less than 6 cm long. As harvesting proceeded smaller individuals appeared in the samples; these may have immigrated onto the bed from the periphery of high densities of larger clams. On the clam bed the animals ranged from 0+ to 19+ years old. Ensis siliqua grew at a slower rate on the west coast of the Irish Sea than on the coast of North Wales. Males grew slightly faster and longer than females and males were more numerous in the landings. The clams are estimated to commence maturation at approximately 4 years of age. They have a similar gonadal cycle to those in Portugal and a similar spawning period from mid-May to the end of July or early August. Condition reflects gonad development, reaching a peak in May. Harvesting by hydraulic dredging causes breakages which are higher in quarters 1 and 4 than in quarters 2 and 3. Bruised razor clams do not eliminate sand in desanding tanks and so they are rejected by processors. The total biomass of the Gormanstown bed is estimated at approximately 1500 tonnes of which 2/3 had been removed by July 1999, two years after exploitation commenced.