Increasing the Reflective Properties of Gillnets

A research project that is currently underway studies the potential for increasing the sound reflective properties of fishing nets to enable porpoise to identify a net from greater distances, and under harsh weather conditions. The use of sound reflective nets could eliminate the concerns associated with the use of acoustic alarms.

During the summers of 1998 and 2000, eight commercial demersal gillnet fishers participated in an experiment near Grand Manan, New Brunswick. This area has had high incidental catch of harbour porpoise. Two types of nylon mono-filament gillnet were used; one type was made of 100% nylon (used regularly worldwide), the other type contained fine barium sulphate particles that enhance sound reflection. All were placed in similar locations and water depths with similar soaking times. In 1998, 46 reflective nets and 103 control nets were deployed. In 2000, 185 reflective nets and 364 control nets were deployed. In both years, no harbour porpoise were caught in the reflective nets. Twelve porpoise and eight porpoise were caught in control nets in 1998 and 2000, respectively.

It has been determined that harbour porpoise can detect a regular nylon mesh at a distance of 3-6 m. However, sonar returns from targets such as entangled fish may mask the weaker echoes from the nets. The barium sulphate nets were found to be three times more reflective than standard nylon, and provide a stronger target that may enable the porpoise to avoid entanglement. The reflective nets are 3% stiffer than regular nets, but this stiffness may help to avoid entanglement. The nets are slightly more expensive (10-20%), but last approximately 30% longer than regular nets.

The new reflective netting had no effect on the commercial fish catch, and actually decreased the number of seabirds caught in the nets as well. The by-catch of harbour porpoises in reflective nets is significantly reduced, and could serve as an alternative to acoustic alarms.