Rapid weight loss in free ranging pygmy killer whales (Feresa attenuata) and the implications for anthropogenic disturbance of odontocetes

Abstract

Understanding the impacts of foraging disruptions to odontocete body condition is fundamental to quantifying biological effects of human disturbance and environmental changes on cetacean populations. Here, reductions in body volume of free-ranging pygmy killer whales (Feresa attenuata) were calculated using repeated measurements of the same individuals obtained through Unoccupied Aerial System (UAS)-photogrammetry during a prolonged disruption in foraging activity arising from a 21-day stranding event. Stranded individuals were used to verify UAS-derived volume and length estimates through 3D-imaging, water displacement, and post-mortem measurements. We show that (a) UAS estimates of length were within 1.5% of actual body length and UAS volume estimates were within 10-13% of actual volume, (b) foraging disruption resulted in a daily decrease of 2% of total body mass/day, and (c) pygmy killer whales can lose up to 27% of their total body weight within 17 days. These findings highlight the use of UAS as a promising new method to remotely monitor changes in body condition and animal health, which can be used to determine the potential effects of anthropogenic disturbance and environmental change on free-ranging odontocetes.

Figure 1

The timeline of two stranding events managed by NOAA Fisheries involving 17 pygmy killer whales (Feresa attenuata) that took place in the nearshore waters of Maʻalaea Bay, Maui between August and October 2019.

Figure 2

Pygmy killer whale (A) volume (m³) ± SD and (B) body mass index (BMI; m³/m²) ± SD as a function of observation days during a 21-day stranding event that took place from September 13 to October 3, 2019 in the nearshore waters of Maʻalaea Bay, Maui, where Day 1 corresponds to September 16, 2019. Note: UAS measurements were collected over the last 17 days of the stranding event.

Figure 3

Adipocyte images (white is lipid—filled adipocyte; red is intervacuolar space) from blubber samples obtained during post-mortem examination of a non-fasting (A) and two fasting (B,C) male pygmy killer whales that stranded on August 29 (A) and September 24, 2019 (B,C) in Maʻalaea Bay, Maui.

Figure 4

Modified overview of the Population Consequences of Disturbances (PCoD) framework with insert showing where UAS-photogrammetry can be used to quantify changes in body volume and make inferences about energy loss and animal health. Figure has been modified from.

Figure 5

Example of an (A) aerial photograph taken from an unoccupied aerial system showing required body orientation at the surface to be used in analysis and (B) diagram noting the positions used to calculated total length and the 0–90% body volume. The diagram was made using Microsoft PowerPoint 2010 <https://www.microsoft.com/en-us/microsoft-365/powerpoint>.

Figure 6

Workflow to collect UAS-derived measurements of body volume and length (A) and determine UAS measurement consistency and accuracy through stranding data (B) by conducting post-mortem examinations and 3D scans (C–E). *3D scan was conducted on a false killer whale (Pseudorca crassidens).