How to tell them apart? Discriminating tropical blackfish species using fin and body measurements from photographs taken at sea

Abstract

AbstractThe misidentification of species and populations is a hindrance to effective cetacean management. We devised a method of species identification using 10 fin and body measurements obtainable from at‐sea photographs, and demonstrated its ability to distinguish four species of Hawaiian “blackfish”: pygmy killer, melon‐headed, short‐finned pilot, and false killer whales. Measurements from photos of 382 known individuals were converted into 14 ratios and reduced using nonmetric multidimensional scaling and principal component analysis. The first three components of the PCA explained 81% of variance among species. Ratios indicated by PCA and NMDS were tested using analysis of variance, and results identified three fin ratios that had distinct means across all four species: height/base, depth (from anterior insertion) at trailing edge apex/depth at topmost point, and width (from leading edge) at posterior point/base. Dual analysis of adults and all age classes showed near‐identical sources of variance, 90% similarity in interspecies ratio relationships, and overlapping ratio means and ranges. Results suggest similar ontogenetic growth across these four species, and confirm the efficacy of this discrimination technique for all age classes. This study established a reliable means of distinguishing these cetacean species, which will improve the efficacy of management in areas with sympatric distributions.