Assessing the importance of net colour as a seabird bycatch mitigation measure in gillnet fishing

Gillnets are used widely in fisheries throughout the world and known to cause the death of thousands of seabirds each year. Currently few practical or technical options are available to fishers for preventing seabird mortalities. The ability of little penguins (Eudyptula minor) to differentiate between different coloured netting materials was tested under controlled conditions to ascertain if changes in gillnet colour could facilitate a potential mitigation measure by improving visibility of nets. The study involved a repeated-measures design with penguins exposed to variously coloured mono-filament threads creating a gillnet mimic. The gillnet mimic was made up of gillnet material configured as a series of vertical lines 25mm apart stretched tightly across a stainless steel frame that measured 1160mmx1540mm and divided into two equal panel areas. The panels were placed in a large tank within an enclosure that housed 25 penguins. Penguins were able to readily access the tank and swim freely. The frame was always introduced into the tank with one panel containing a gillnet mimic, and the other panel left empty as a control. Gillnet filament colours tested were clear, green and orange. Orange coloured monofilament lines resulted in lower collision rates (5.5%), while clear and green monofilament lines resulted in higher rates of collision (35.9% and 30.8%, respectively). These results suggest that orange-coloured lines were more apparent to the birds. Constructing nets of orange-coloured material may be effective in reducing bycatch in gillnets set in shallow waters and high light levels where seabirds are able to identify fine colour differences. Further testing under experimental conditions, accompanied with at-sea trials to verify effectiveness in varied light conditions is warranted, together with an assessment of the effect of gillnet colour on catch efficiency of target species.