Aerial photogrammetry and size structure analysis of sperm whales (Physeter macrocephalus) at Kaikōura, New Zealand

Abstract

Length measurement is a fundamental metric in the study of animals, and an important challenge in cetacean science. Whales are among the most difficult animals to measure alive, and a variety of techniques have been applied to this task, most relying on extrapolation from measurement of part of the body to total length. Kaikōura, New Zealand is one of the few places worldwide where sperm whales can be found only a few miles offshore, providing an ideal setting for the development of measurement techniques. My research used aerial photogrammetry from an unmanned aerial vehicle (UAV) to test the accuracy of estimating length from interpulse intervals (IPIs) in the whales’ echolocation clicks, and develop a new, recalibrated, equation. I used the new equation to estimate lengths from acoustic recordings of sperm whales at Kaikōura dating back to 1996, to investigate temporal and seasonal trends in size, and investigate the size structure of this population. This equation was also used to update estimates of individual growth, and examine trends in arrival and departure sizes. Nineteen whales were measured using aerial photogrammetry and via IPI. Both measurement methods produced repeatable results that were more precise than previously published studies (mean CV = 1.5% and 0.7%, respectively). The refined regression equation presented here is especially valuable because aerial photogrammetry allows direct measurement of the entire length of individual whales, and is of proven accuracy. In this sense, it is the “gold standard”. Applying the new regression equation to IPIs measured from 507 recordings dating back to 1996 revealed a significant increase in mean length of whales in summer during recent years (2014 – 2019), with no trend in mean length during winters. The increasing size during summer months is driven by the same individuals returning, each time having grown slightly larger, and contributing to an aging population during this period. Growth rates were measured and updated for 37 whales that had been recorded repeatedly between 1996 and 2019. Some whales showed evidence of growth levelling off, indicating that some individuals are reaching physical maturity at Kaikōura. This is the first research that has measured sperm whales using UAV photogrammetry. The revised equation for estimating total length via IPI should be preferred for future length estimation of sperm whales in high latitudes.