Modified dredges were used on Aequipecten opercularis (queen scallop) fishing grounds off the Isle of Man in the north Irish Sea to determine seasonal variability in swimming behaviour in queen scallops and its effect on dredge fisheries. Scallops, which evaded dredge capture by swimming up into the water column, were captured by a specially designed net deployed above the dredge gear. The gear was used over a 20-month period and the number and size distribution of queen scallops captured in both the net and dredges were recorded. A subsample of captured scallops was maintained in running seawater tanks and used to assess two components of swimming behaviour, the time taken to respond to stimuli and the number of valve adductions carried out until exhaustion. There was considerable seasonal variability in the proportion of queen scallops that avoided dredge capture by swimming. Very low numbers of queen scallops were found in the net during the winter and spring. Net capture increased in June, and was high throughout the summer and autumn when up to 42% of scallops greater than 55 mm in height, captured in dredges, swam over the dredge mouths. There was a strong positive correlation between seawater temperature and proportion of captured scallops found in the net. Swimming experiments in the laboratory indicated that the observed seasonal variability in net capture was probably a result of changes in the time taken to respond to stimuli, rather than changes in the ability to perform a large number of valve adductions. There was no relationship between reproductive state and the proportion of actively swimming scallops. Analyses of size distributions indicated limited swimming activity in the largest scallops but no effect of epifaunal load was found on the ability of scallops to avoid dredge capture.