Accurate stock assessments are essential for effective marine mammal conservation under the U.S. Marine Mammal Protection Act. Current assessments for short-finned pilot whales (Globicephala macrorhynchus) in Hawaiʻi group all individuals into one stock, potentially obscuring declines in smaller, demographically independent populations. Pilot whales associated with Kauaʻi and Niʻihau are of particular concern due to their limited ranges and high exposure to long-term, large-scale naval activities. To generate population-specific abundance estimates, spatial capture-recapture models were applied to long-term photo-identification data collected between 2011 and 2023. Small boat surveys followed non-systematic tracklines, identifying 237 distinct individuals, including 161 island-associated and 56 oceanic whales. Most individuals (68%) were sighted only once, while ten (4.2%) were sighted five or more times. Analyses included high-quality photos of whales with distinctive dorsal fin markings, scaling density estimates to the total population based on the proportion of marked individuals. Spatial encounter histories were analyzed with a spatial capture-recapture modeling approach. Top-ranked models included study effects of primary period on density and detection, with detection probability showing a quadratic relationship with depth, indicating strong habitat influences. Mean densities across two-year blocks were 2.5 (SD ± 1.9) and 2.2 (SD ± 1.9) marked whales per 100 km² for island-associated and oceanic populations, respectively. When scaled for the proportion of marked individuals, these corresponded to mean population sizes of 587 (SD ± 494) and 446 (SD ± 430) individuals in an area of 14,000 km². Detection probability peaked along the 1,500-3,000 m depth contour, consistent with known foraging habitats. 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.
Citation:
Fertitta, K., S.D. Mahaffy, P. Patton, R.W. Baird, and L. Bejder. 2026. Abundance of Short-Finned Pilot Whales Near Kaua’i Island with Spatial Capture-Recapture. Presentation at the 26th Biennial Conference on the Biology of Marine Mammals, San Juan, Puerto Rice, 19-23 October 2026.