Species misidentification is a common problem for some groups of cetaceans, and may lead to inaccurate descriptions of behavior, habitat use, group size, abundance, or acoustic characteristics, among other things. Four tropical species of “blackfish” overlap broadly in their ranges and are often confused at sea owing to their similar gray-black coloration, bulbous head without a beak, and falcate dorsal fin: pygmy killer whales, melon-headed whales, short-finned pilot whales, and false killer whales. Adult pilot whales have a distinct fin size and shape, but misidentification can occur prior to maturity before distinguishable characteristics develop. Some blackfish species can be identified through full-body documentation of unique cape or mouthline pigmentation, but no method currently exists to determine species from nondescript photographs using dorsal fin or body dimensions. This study identified ten two-dimensional, lateral measurements obtainable from typical photos at sea that quantify the distance between reference points along the fin’s contour and the dorsal ridge between the blowhole and fin origin, which can be used to discriminate among species. We measured 285 age-classified, known individuals photographed around the main Hawaiian Islands spanning 31 years of sighting history. Results showed that both mature and immature pilot whales had significantly higher ratios of fin base to dorsal ridge length than other blackfish, and mature animals had significantly greater horizontal distance between reference points than other species. False killer whales had significantly smaller fin base to dorsal ridge length ratios than other blackfish, and significantly shorter fin height to base length than melon-headed and pygmy killer whales. Conversely, melon-headed whales were distinguishable from pygmy killer whales by a significantly wider dorsal fin at multiple reference points. This study established a reliable method for discriminating between nondescript blackfish encountered at sea, which will improve the accuracy of species identification for sightings supported by photographic documentation.
Yahn, S., R. Baird, S. Mahaffy, D. Webster. 2017. How to tell them apart? Blackfish species discrimination using fin and body morphometrics obtainable from photos at sea. Abstract (Proceedings) 22nd Biennial on the Biology of Marine Mammals, Halifax, Nova Scotia, October 22-27, 2017.Download PDF
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