Fisheries observers and skippers tested the effectiveness of the Dolphin Dissuasive Device (DDD) as a bycatch mitigation tool for porpoises and dolphins. This study was conducted aboard fishing vessels in the Cornish over-12m static net sector fishery in the waters of Southwest England. DDD's were placed at the end nets. The DDD's caught significantly fewer porpoises but no significant difference in dolphin bycatch was observed. When bycatch rates were separated between short (up to 4 km) fleets and long (larger than 4 km) fleets, there was no significant difference in porpoise bycatch in the long fleet but there was in the short fleet.