INVESTIGATING SPINNER DOLPHIN PRESENCE ALONG THE WINDWARD COAST OF HAWAI‘I ISLAND THROUGH BOTH PASSIVE ACOUSTIC MONITORING (PAM) AND BOAT-BASED VISUAL TECHNIQUES

Alvarez, Petrisha

INVESTIGATING SPINNER DOLPHIN PRESENCE ALONG THE WINDWARD COAST OF HAWAI‘I ISLAND THROUGH BOTH PASSIVE ACOUSTIC MONITORING (PAM) AND BOAT-BASED VISUAL TECHNIQUES

Files

Alvarez_hilo.hawaii_1418O_10230.pdf (3.16 MB)

Date

2022-12

Authors

Alvarez, Petrisha

Advisor

Pack, Adam A.

Department

Tropical Conservation Biology & Environmental Science

Abstract

Spinner dolphins (Stenella longirostris) in Hawaiian waters are known for their predictable diel patterns of feeding offshore during nighttime hours and returning to bays and shallow coastlines to rest during daytime hours. They have been studied extensively since the late 1970s, largely along the leeward coasts of the main Hawaiian islands. In stark contrast, little effort has been dedicated to studies of spinner dolphins occurring along the windward coasts, although dorsal fin photo-identification mark-recapture studies and aerial surveys have confirmed the species’ presence. Here, I provide the first comprehensive study of spinner dolphins along the windward coast of Hawai‘i Island utilizing long-term passive acoustic monitoring (PAM) complemented by boat-based visual and acoustic surveys. Four bottom-moored Soundtrap underwater acoustic recorders were positioned along the Hilo coastline from Onomea Bay to Kealoha beach park (12.6 km²) within 16-25 m, recording 24 hour/day on a 10% duty cycle capturing 30s every 5 minutes from July 1- December 31, 2020. Throughout this period, 10 boat-based surveys were conducted bimonthly spanning 15 km² over the recording locations to confirm species identification and determine behavioral states. During these surveys, the dorsal fins of individual dolphins were photographed for the purposes of identification, and dipped hydrophone recordings of vocalizations were made to compare with spinner dolphin acoustic signaling recorded on the Soundtraps. Single species pods of spinner dolphins were encountered on 60% of boat surveys, with a mean group size of 73.2 (SE = ± 10.2) (mean minimum = 50.6 ± 6.9 SE; mean maximum = 92.7 ± 11.3 SE) comprised of mixed age classes, including calves of the year (mean = 3.2 ± 0.5 SE). Spinner dolphins were observed in Hilo Bay for as long as 4.5 hours. Behavioral states included milling and traveling with individual behaviors including those that had an aerial and/or percussive characteristics, nursing, and intercourse. The analysis of +8,000 hours of acoustic data captured from Soundtraps revealed the presence of spinner dolphins across the study area for 150 days of the 180 day recording period (83%). Spinner dolphin vocalizations were recorded on each Soundtrap with a greater detection rate per day along the Northern coast at Onomea Bay and Honoli‘i beach as compared to the Eastern locations along Keaukaha. Significantly greater acoustic presence was detected during the morning hours between 0600 – 1155 h, compared to the afternoon between 1200 - 1755 h. Overall, these findings reveal significant habitat usage by spinner dolphins of all age classes for resting and socialization purposes along the windward coast of Hawai‘i Island. To the extent that individual spinner dolphins using windward coastlines are distinct from those using leeward areas, future abundance estimates of spinner dolphins off Hawai‘i Island should consider both leeward and windward coastlines.

Keywords

Ecology, Acoustics

URI

http://hdl.handle.net/10790/7321

Extent

56 pages

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