Spatio-temporal variations in gillnet catch rates in the vicinity of North Sea oil platforms

Use of decommissioned North Sea oil platforms as artificial reefs may provide an alternative to their being removed. Three fishing experiments with gillnet fleets lasting 3-5 days and using different mesh sizes were conducted in the vicinity of two platforms to study their potential as sites for commercial fishing. In each case, five fleets were set in a star-shaped pattern centred around the platform and running up to a distance 1400 m. Catches consisted mainly of cod (Gadus morhua) and saithe (Pollachius virens), and in one case also of ling (Molva molva). In two experiments, catch rates of nets set within a distance of 110-165 m of the platform were three to four times higher than those of nets set at greater distances. In the third experiment, the highest catches were taken 150-300 m from the platform, also by a factor of four compared to catches taken at greater distances. However, patterns differed among species, areas, and seasons. Catch rates varied considerably between fishing days, and in only one experiment did catch rates (specifically of ling) decline consistently throughout the fishing period, suggesting gradual depletion. Although the study provided evidence of pronounced aggregations of fish close to the platforms, responses are complex and results are inconclusive regarding species-specific temporal and spatial patterns.