OBSERVATIONS ON THE LOCAL DISTRIBUTION, BIOLOGY AND ECOLOGY OF PALOURDE <i>TAPES DECUSSATUS</i> (L.) OF RELEVANCE TO ITS EXPLOITATION IN THE REPUBLIC OF IRELAND

Abstract

This is an account of the changing status of palourde Tapes decussatus, which occurs in patches associated with a distinctive granulometry that are isolated along the Irish littoral. The species briefly entered the official marine landings statistics in 1975 in anticipation of a high harvest value to coastal communities. The 'clam boom', which peaked in 1975-1976 was, however, quickly followed by collapse. Later capture statistics were confused with those of a cultured species, the Manila clam Tapes semidecussatus, and these statistics are elucidated. decussatus is near the limit of its north-south range in Ireland, where the species is in retreat. This paper investigates nine beds in Counties Donegal, Galway and Kerry. Some had been examined thirty years previously. The density, year class frequency and growth rates of the species were measured in each to ascertain the reasons for its brief commercial performance and to assess its current status. The age of individu als ranged from 0 to 16+ years. Clam beds had a density of 17-116g m-2; age at full recruitment was seven years in commercial samples. decussatus has a eulittoral range on the sea shore and is associated with sheltered coastal topography. The species lives within a poorly sorted mixed granulometry of mud, sand and gravel, which is described in quantitative terms from some sample locations and compared with two other common inshore facies: Spisula sand and fine sand associ ated with Ensis siliqua. decussatus was often found in close proximity to fresh water inflows and it can survive eutrophic conditions and toxic algal blooms. Despite its low densities, T. decussatus was the quantitatively dominant bivalve wherever it occurred. The species responds to favourable growing conditions, but throughout Ireland site-specific growth rates were very similar. There was no evidence of exceptional annual spatfalls having contributed to the populations that were exam ined. It is proposed that the population structures described are of a ^-selected species that is long lived and replicates by low but regular annual recruitments. The majority of the biomass resides in the older age classes and these are targeted for harvest; once they have been intensively exploited, T. decussatus populations are very slow to recover.