Making nets more acoustically visible : Floats as passive mitigation measures to reduce bycatch of harbour porpoise (Phocoena phocoena) in gillnets

Sammanfattning: Incidental capture in fishing gear is today affecting at least 112 marine mammal species worldwide and for many smaller toothed whale species (Odontocete), gillnets are considered to be the largest problem. Although some effective mitigation measures exists, these are often context specific and many might be hard to implement due to reasons such as high costs and maintenance. There is a pressing need to develop low-cost solutions to reduce small toothed whale bycatch and one such possible solution is the use of passive acoustic reflectors, making the net more visible to toothed whale echolocation. A systematic literature review was performed to identify previously tested passive acoustic reflectors and their effectiveness. 20 different modification types were identified and two groups were found to have the highest potential: hard plastic floats and acrylic glass spheres. A field trial was performed to further evaluate hard plastic floats as a potential, low-cost, mitigation measure. The aim of the field trial was to investigate harbour porpoise (Phocoena phocoena) presence and click behaviour around rope panels equipped with and without floats. Harbour porpoise echolocation clicks was recorded using click detectors (C-PODs). Detection positive minutes per hour (DPM/h) was used as a measure of presence and buzz-clicks, a type of clicks produced with a very short interval between them (inter click interval ≤15ms) used for example when exploring or hunting, was used as a proxy for click behaviour. Results indicate that harbour porpoise presence and click behaviour is affected by floats with a spacing of 2m and 6m compared to control without floats. Placement of panels could however not be excluded to have an impact on the results and further studies are required to confirm the effect of floats on harbour porpoise presence and click behaviour.