Abundance and demographic parameters of spinner dolphins (Stenella longirostris subsp.) off Oʻahu's Waiʻanae Coast

Abstract

The occurrence and behavior of spinner dolphins (Stenella longirostris subsp.) off Waiʻanae, Oʻahu are well-described, yet a data deficit on their abundance and trajectory hinders effective conservation. This study employed multi-state open robust design (MSORD) modelling to estimate seasonal dolphin abundance, apparent survival, and temporary emigration, and further-developed a method for age-structure quantification via unoccupied aerial system (UAS; drone) photogrammetry. Dolphin photo-identification and UAS data were collected between April 2021 and January 2023 off the Waiʻanae Coast. Abundance varied seasonally, ranging from 140 (± 36.8 SE) to 373 (± 60.0) individuals and was lowest during Winter. Apparent survival was high and constant (0.95 ± 0.02 SE) and temporary emigration was Markovian. Seasonal environmental variability and spinner dolphin reproductive biology may drive patterns of abundance and temporary emigration. A crude estimate of total abundance for spinner dolphins documented off Waiʻanae (N = 550) is comparable to a recent island-wide abundance estimate, suggesting that the Waiʻanae Coast is a candidate site for long-term monitoring of Oʻahu spinner dolphins. UAS data analysis showed that the total length of spinner dolphins can be effectively estimated by measurements of the blowhole to dorsal fin distance. As age-length growth curves do not exist for the subspecies of spinner dolphin in Hawaiʻi, age was estimated using a growth curve for a related subspecies. Additional life-history information about Hawaiian spinner dolphins will allow both the use of a more accurate growth curve, and an assessment of population health. This work provides the first systematic estimates of abundance and demographic parameters for spinner dolphins off Waiʻanae, imparting valuable information for monitoring and management decisions.