Abundance of short-finned pilot whales near Kauaʻi island, Hawaiʻi

Abstract

Accurate stock assessments that reflect true population structure are essential for effective marine mammal conservation under the U.S. Marine Mammal Protection Act. Current abundance estimates for short-finned pilot whales (Globicephala macrorhynchus) in Hawaiʻi group all individuals together, potentially obscuring declines in smaller, demographically independent populations (DIPs). Whales associated with Kauaʻi and Niʻihau are of particular concern due to their limited ranges and high exposure to anthropogenic noise. To generate population-specific abundance estimates, spatial capture-recapture models were applied to long-term photo-identification data collected by Cascadia Research Collective between 2011 and 2023. Surveys followed non-systematic boat tracklines, identifying 237 distinct individuals, including both island-associated and pelagic whales. Spatial encounter histories were built using a 4 × 4 km grid and analyzed with a spatial capture-recapture modeling approach . Models incorporated depth, distance to shore, time, and population structure as covariates. Top-ranked models included study period effects on density and detection, with detection probability showing a quadratic relationship with depth, indicating strong habitat influences. Mean densities across two-year periods were 2.5 and 2.2 marked whales per 100 km² for island-associated and pelagic populations, respectively. When scaled to total abundance, these corresponded to mean population sizes of 587 (SD ± 494) and 446 (SD ± 430) individuals. Detection probability peaked along the 1,500-3,000 m depth contour, consistent with known foraging habitats. These findings provide the first spatially explicit, population-level abundance estimates for short-finned pilot whales near Kauaʻi and Niʻihau, offering a framework for monitoring localized population trends and demonstrating how non-systematic survey data can inform marine mammal management and conservation.