The distribution and population structure of the bivalve Arctica islandica L. in the North Sea: what possible factors are involved?

Abstract

The present paper summarises observations on the distribution, abundance and population structure of the bivalve Arctica islandica in the North Sea between 1970 and 2000, and demonstrates that Arctica has a widespread distribution in the North Sea north of 53°30′N. Along its southern and eastern borders the distribution seems to be limited to depths beyond 30 m. A comparison between distribution patterns of Arctica in 1972–1994 and in 1996–2000 suggests slight changes along its southernmost border in the Oyster Ground. In the south-eastern North Sea, the average density of Arctica (>10 mm) was 7 individuals per 100 m 2 and the population was dominated by full-grown specimens exceeding 50 mm shell height. The highest abundance of spat, juveniles and adults was found in the deeper central section of the Oyster Ground that is stratified during summer. There, the mean density was 21 individuals (>10 mm) per 100 m 2. These densities were much lower than in the northern North Sea (Fladen Ground), where abundance was one to two orders of magnitude higher and peaked at 28 600 individuals per 100 m 2. In the Fladen Ground, the population structure was bimodally shaped and dominated by juveniles. In the Oyster Ground, the skewed size class distribution suggests that the recruitment to larger size classes is hampered. An insufficiently dense stock of reproducing adults generating less dense spatfalls, possibly in combination with limited survival of spat and juveniles, prevents successful recruitment. Although natural processes may contribute to the skewed population structure, intensive bottom trawling is thought to have a major effect as well. It is therefore questionable whether under present-day conditions the population of Arctica in the SE North Sea can be considered sustainable in the long term.