Rigs-to-reefs in the North Sea: hydroacoustic quantification of fish in the vicinity of a "semi-cold" platform

We describe an experiment intended to estimate the extent to which decommissioned platforms in the North Sea attract fish. The work was carried out at a "semi-cold" platform in the Norwegian Sector between May and September 1998. Hydroacoustic quantification and trawling carried out by a research vessel running transects from the platform out to a distance of five nautical miles did not demonstrate higher concentrations of fish near the platform. However, for practical and security reasons the vessel was not permitted to operate closer than 50-100 m to the platform. To measure concentrations of fish close to the sides of the jacket, acoustic transducers were hung from three sides of the platform. Large differences in fish density were observed, depending on the side of the platform concerned, time of day, and season. During daytime, mackerel were schooling around the platform, while demersal fish tended to be found under the platform and close to the bottom. At night, the demersal fish spread throughout the water column in such a way that their distribution was suitable for acoustic biomass estimation. Estimated densities of cod and saithe were 0.13, 0.27, and 0.24 kg m(-2) in May, July, and September, respectively, with corresponding biomass estimates at the platform of 7, 14, and 108 t. An additional 2.3 and 1.2 t of mackerel was present in July and September, respectively.