Measuring Calf Age Structure in a Population of Humpback Whales in Maui, HI Based on Calf Dorsal Fin Curvature

Abstract

Humpback whales travel thousands of miles to Hawaii each year to breed and give birth to their calves. To examine calf age structure within the humpback whale population visiting/inhabiting the ‘Au’Au Channel, between Maui and Lanai, we observed mother and calf pairs in January 2022. Relative age of humpback calves can be determined by the degree to which the dorsal fin is furled. In utero, the fetal dorsal fin is furled, and after birth the presence of fetal folds and the angle of the dorsal fin can be used as indicators of calf age. We performed focal-follows of mother-calf pairs for 15 minutes, noting behaviors displayed of mom, calf, and escort(s) if present. During focal-follows, we positioned the boat behind calves to capture images of their dorsal fins. The images of the calf’s dorsal fin were taken from directly behind, as close to the six o’clock position as possible. The angle of the fin relative to the horizontal axis was measured in Adobe Photoshop. We used this technique to describe the age structure of the calf population and to test whether calf age influenced geographical location or speed of travel. Our research was conducted under NOAA NMFS permit #22750-01.

Presentation