Abstract
False killer whales Pseudorca crassidens and short-finned pilot whales Globicephala macrorhynchus are known to interact with long-line fishing gear in Hawaiian waters, causing eco- nomic loss and leading to whale injuries and deaths. The main Hawaiian Islands' insular popula- tion of false killer whales is listed as endangered and the offshore population is considered 'strate- gic' under the Marine Mammal Protection Act due to relatively high bycatch levels. Discriminating between these species acoustically is problematic due to similarity in the spectral content of their echolocation clicks. We used passive acoustic monitoring along with data from satellite tags to dis- tinguish signals from these 2 species. Acoustic encounters recorded with autonomous instruments offshore of the islands of Hawai'i and Kaua'i were matched with concurrent and nearby location information obtained from satellite tagged individuals. Two patterns of echolocation clicks were established for the 2 species. The overall spectral click parameters were highly similar (22 kHz peak and 25 kHz center frequency), but false killer whales had shorter duration and broader bandwidth clicks than short-finned pilot whales (225 µs, 8 kHz (�3 dB bandwidth) and 545 µs, 4 kHz, respectively). Also, short-finned pilot whale clicks showed distinct spectral peaks at 12 and 18 kHz. Automated classification techniques using Gaussian mixture models had a 6.5% median error rate. Based on these findings for echolocation clicks and prior published work on whistle classification, acoustic encounters of false killer whales and short-finned pilot whales on auto - nomous instruments should be identifiable to species level, leading to better long-term monitoring with the goal of mitigating bycatch.
Highlights
False killer whales Pseudorca crassidens and shortfinned pilot whales Globicephala macrorhynchus are known to interact with pelagic long-line fisheries in Hawaiian waters, causing economic loss to the fishing industry and at least occasionally resulting in injury or death to the animals (Forney et al 2011)
Whistles are primarily used for communication, whereas echolocation clicks are used for detection, localization, and target classification in spatial orientation and foraging (e.g. Herman & Tavolga 1980, Au 1993)
Satellite-tagged false killer whales and shortfinned pilot whales utilized the areas within the vicinity of the high-frequency acoustic recording packages (HARPs) on a number of occasions and were matched with acoustic encounters containing click energy below 20 kHz (Tables 2 & 3, Figs. 2 & 3)
Summary
False killer whales Pseudorca crassidens and shortfinned pilot whales Globicephala macrorhynchus are known to interact with pelagic long-line fisheries in Hawaiian waters, causing economic loss to the fishing industry and at least occasionally resulting in injury or death to the animals (Forney et al 2011). Since both of these species are known to echolocate, passive acoustic monitoring techniques are being. This is true during periods when animals are echolocating without whistling and when combining both vocal production types in the classification decision
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