Determining optimal pinger spacing for harbour porpoise bycatch mitigation

A trial was conducted in the Danish North Sea hake gillnet fishery in July to September 2006 to determine whether the spacing of the Aquatec AQUAmark100 pinger could be increased without reducing the effectiveness of the pinger in mitigating harbour porpoise bycatch. The trial was designed as a controlled experiment where nets without pingers formed the control group (41 hauls) and nets with pingers spaced at 455 m (24 hauls) and 585 m (43 hauls), respectively, formed the 2 experimental groups. Nets without pingers had a bycatch frequency of 0.54 incidents per haul, nets with pingers spaced at 585 m had a bycatch frequency of 0.12 incidents per haul, and nets with pingers spaced at 455 m had a bycatch frequency of 0. The bycatch frequencies for the 2 experimental groups were both significantly different from the bycatch frequencies of the control group (p < 0.0001). These results show that the spacing of the Aquatec AQUAmark100 pinger can be increased without reducing the effectiveness of the pinger in mitigating harbour porpoise bycatch, thereby reducing some of the disadvantages of widespread pinger deployment. The results also stress the importance of basing implementation regulation on solid evidence and led the Danish Fisheries Directorate in 2007 to allow the use of the AQUAmark100 pinger with a spacing of up to 455 m under derogation to the European Union's Council Regulation No. 812/2004.