Preliminary study of cetacean depredation on pelagic longline fisheries using passive acoustic monitoring off Reunion Island

Depredation can be defined as the predation of caught fish or bait by free-ranging animals. Since the 1900s, depredation of Reunion’s longline fishery by toothed whales is known to contribute significantly to reduced commercial catch (sometimes destroying 100% of the catch). Describing depredation by cetaceans is a key driver in helping implement non-destructive adaptive fishing solutions. With fishing mainly occurring at night and over long distances, passive acoustic monitoring is a promising method. A preliminary study was launched to determine the technical feasibility accompanied by acoustic analysis of associated with depredation. Over two months (November-December 2014), 3 autonomous hydrophones (HTI-96-MIN) were attached at the extremities and central section of a 30 km longline for 9 fishing operations, 30 miles off Reunion Island. A total of 387 hrs of sound were recorded and analyzed. Biological sounds (clicks and whistles) and physical sounds were quantified over time with two automatic-methods in relation to recorder locations. Whistle samples allowed species identification using a semi-automatic method (ROCCA classifier). Catch data were correlated with cetaceans’ presence. Engagement and support from local fishers resulted in a final protocol demonstrating good quality acoustic measurements with reduced physical noise. Whistles and clicks represented 34% of all detections (~12% for clicks). Cetacean sounds were detected during all trials with variable detection rates (between 2.5 to 66% of the recorded duration). Distances between hydrophones enabled the drawing of possible trajectories of groups along longlines. On four fishing trials, cetaceans were detected immediately after the line deployment. Six different species of toothed whales were identified with a predominance of false killer whales (Pseudorca crassidens). The presence of the pilot whales (Globicephala macrorhynchus) was probably underestimated due to its similarity with Pseudorca emissions and the paucity of studied samples. Since few signs of depredation were visible on catches, no obvious correlation was determined between the presence of cetaceans and depredation rates. Further investigations are thus required to build on these preliminary results.