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

Abstract

The 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.

First live sighting of Deraniyagala's beaked whale (Mesoplodon hotaula) or ginkgo‐toothed beaked whale (Mesoplodon ginkgodens) in the western Pacific (South China Sea) with preliminary data on coloration, natural markings, and surfacing patterns

“…This work provided for the first time detailed dorsal fin relative measurements for a beaked whale species through the methods from Yahn et al . (2019). Therefore, the data provided here can be compared with further data from other Mesoplodon species in future research.…”

“…Following Yahn et al . (2019) methods, fin and body measurements were obtained from at‐sea photographs of the sighted whales. Seven lateral and three vertical lengths were measured in pixels and compared as relative measurements (Table 1).…”

“…The videos collected were used to estimate the average number of respiration per minute per whale (respiratory frequency). Following Yahn et al (2019) methods, fin and body measurements were obtained from at-sea photographs of the sighted whales. Seven lateral and three vertical lengths were measured in pixels and compared as relative measurements (Table 1).…”

Sexually dimorphic characteristics of short‐finned pilot whales, false killer whales, pygmy killer whales, and melon‐headed whales assessed using fin and body morphometrics from photographs taken at sea

“…Age classification was determined using recurring calf presence for females throughout their sighting histories, adult body size matching physically mature adult conspecifics, or duration of sighting history exceeding 10 years (Yahn et al, 2019). Ten years of age does not fully encompass the range of sexual maturity in all species (short-finned T A B L E 1 Demographics of data indicating the number of individuals sexed using genetic sampling (g), recurring calf association (c), and postanal keel morphometry (m) for each species.…”

“…Some species of dolphins can be sexed using behavior-related physical differences, such as competitive male scarring in common bottlenose dolphins (Tursiops truncatus) (Currey et al, 2008;Tolley et al, 1995) and pigmentation loss in Australian humpback dolphins (Sousa sahulensis; Brown et al, 2016), but adults of many species cannot be reliably sexed at sea. Delphinid fin and body morphometric analysis has been a useful tool in discriminating between similar-appearing species (Rone & Pace, 2012;Yahn et al, 2019) and ecotypes (Costa et al, 2021;Simões-Lopes et al, 2019). Adults of some delphinid species have sexually dimorphic differences in fin and body proportions that can be used to determine sex, including spinner dolphins (Stenella longirostris; Perrin et al, 1991), Fraser's dolphins (Lagenodelphis hosei; Jefferson et al, 1997), and North Pacific Shiho and tropical Atlantic Naisa short-finned pilot whales (Globicephala macrorhynchus; Kritzler, 1952;Yonekura et al, 1980), or can be indicators of sex in species such as striped dolphins (Stenella coeruleoalba; Carlini et al, 2014).…”