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A study was conducted aboard the Cornish inshore gillnet fishery to determine if pingers could reduce porpoise and bottlenose dolphin bycatch, whether habituation to the pingers would reduce their effectiveness and how quickly porpoises and dolphins recolonise a pingered site after their removal. Vessels were equipped with passive acoustic monitoring systems that can recognize and log each animals click and AQUAmark 100 pingers were spaced 200 m apart on the nets. Control nets with no pingers were also used. There was a significant difference in the number of porpoise clicks between nets with and without pingers. There was no significant difference in the proportion of loud clicks logged when pingers were active and so the extent of displacement by pingers cannot be determined. In addition, there was no evidence of habituation to the pingers. It appears that porpoises take at least 7 hours to recolonise a pingered site. There were too few encounters with dolphins to determine the ability of pingers to reduce their bycatch or to determine how long recolonization will take.