Allometry and morphometry of blue whales photographed in the Gulf of California: insights into subspecies taxonomy in the Eastern North Pacific

ABSTRACT: The Eastern North Pacific (ENP) blue whale population is considered the most recovered worldwide. Despite this, its subspecies taxonomy is yet to be resolved. A previous morphological analysis using vertical aerial photogrammetry on blue whales from the ENP suggested that they are morphologically similar to the Indian Ocean pygmy blue whale Balaenoptera musculus brevicauda. This subspecies has been discriminated from the Antarctic blue whale B. musculus intermedia by a shorter total length and a proportionally shorter caudal peduncle (from dorsal fin to notch of flukes), a difficult morphological characteristic to measure at sea. In this study, we present allometric and morphometric analyses of the caudal peduncle proportion in relation to total length based on photogrammetric data from whales observed in the Gulf of California, Mexico (a winter ground for the ENP population), to compare with caudal peduncle measurements available from both Southern Hemisphere subspecies. Throughout allometric analysis, the caudal peduncle showed negative growth as the individual got older, indicating that this body part differs according to age-class. Our morphometric analysis indicated that the caudal peduncle proportion of these blue whales was significantly larger than that of the Indian Ocean pygmy blue whale. The integration of our results with the previous photogrammetry study suggests that more than one morphotype inhabits the ENP, and highlights gaps in our knowledge on the taxonomy of these whales. The inexpensive photogrammetric technique used in this study could be easily combined with other studies to better understand and protect these endangered whale populations.