Population density and growth of the newly introduced Atlantic rock crab Cancer irroratus Say, 1817 (Decapoda, Brachyura) in Iceland: a four-year mark-recapture study

The Atlantic rock crab Cancer irroratus was discovered in Icelandic waters in August 2006 and spread rapidly along the coastline. The species has reproduced successfully in Iceland, and there have been strong indications that C. irroratus has become common on soft-bottom coastal habitats in south-western Iceland. To be able to evaluate the effect of such non-indigenous species on the ecosystem, population density estimates are required. In the years 2011 to 2014, a mark-recapture study was carried out on C. irroratus in a small inlet in the Kollafjörður fjord, south-western Iceland. In total, 6475 C. irroratus were captured and of them 4670 were tagged with sequentially numbered T-bar anchor tags. Of the crabs tagged, 155 were recaptured over the four years (3.3%), 34 had moulted and one of them presumably twice over the four-year period. The size range of recaptured crabs was 7.3–12.8 cm carapace width and the growth increment per moult ranged from 1.2 to 2.6 cm, with an average of 2 cm. Based on the mark-recapture analysis, the population at the study area in Kollafjörður was estimated to be about 107,000 individuals.