Abundance of ocean quahog, Arctica islandica, assessed by underwater photography and a hydraulic dredge

Abundance of ocean quahog (Arctica islandica) was estimated with underwater photography and a hydraulic dredge at 15 m depth in Oenundarfjoerdur, NW Iceland. Abundance estimates based on counts of siphons from the underwater photographs were much higher than from analysis of the dredge catches. A total of seven taxa were identified from photographs, of which only ocean quahogs and brittle stars were found in sufficient abundance for further analysis. The large scale spatial distribution of ocean quahogs and brittle stars was contagious, where the data corresponded well with a negative binomial distribution but differed significantly from a Poisson distribution. The Morisita index of dispersion was used to analyze distribution patterns within each photograph. These analyses revealed that even on small spatial scales the distribution of ocean quahog could be contagious.