Abstract
The Gormanstown razor clam bed, which measured 21km2 in extent in 1998, provided more than half the razor clams harvested in Europe for three years, 1999–2001. Exploitation of the bed commenced in late 1997. As a result of hydraulic dredging, the sediments have higher sorting coefficients and some larger grades were added in the form of shell fragments in the intervening years. The macrobenthos of the bed was monitored over a period of seven years, beginning one year after exploitation commenced. Throughout that time the dominant species, Ensis siliqua , which had accounted for up to 90% of the biomass in the first year of the study, declined to approximately 50% in 2005. Ensis siliqua displays the characteristics of a K-selected species: heavy standing biomass, type one survivorship curve and slow replication. Coinciding with exploitation, the bed was invaded by other deposit and suspension-feeding bivalves, notably Pharus legumen and Lutraria lutraria, whose population expansion can be traced to the early dredge fishery. The ratio of Ensis to Lutraria sampled weights was 124:1 in 1998, but it steadily advanced to 1.2:1 in 2005. Thus Lutraria , another suspension feeder, has displaced the razor clam. The Shannon-Wiener index of diversity rose as exploitation of the bed progressed, and in 2005 (eight years after dredging commenced), it had not returned to its 1998 level. However, the trend over the period was downwards, suggesting that a new stability involving a different species composition may be achieved. An age length key applied to length–frequency distributions of E. siliqua indicated that no age class older than age eleven has accounted for 10% of the population since 2001, although that was usually the case in the four preceding years.
Published Version
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