Abundance and Survival Rates of the Hawai’i Island Associated Spinner Dolphin (Stenella longirostris) Stock

Abstract

Reliable population estimates are critical to implement effective management strategies. The Hawai’i Island spinner dolphin (Stenella longirostris) is a genetically distinct stock that displays a rigid daily behavioural pattern, foraging offshore at night and resting in sheltered bays during the day. Consequently, they are exposed to frequent human interactions and disturbance. We estimated population parameters of this spinner dolphin stock using a systematic sampling design and capture–recapture models. From September 2010 to August 2011, boat-based photo-identification surveys were undertaken monthly over 132 days (>1,150 hours of effort; >100,000 dorsal fin images) in the four main resting bays along the Kona Coast, Hawai’i Island. All images were graded according to photographic quality and distinctiveness. Over 32,000 images were included in the analyses, from which 607 distinctive individuals were catalogued and 214 were highly distinctive. Two independent estimates of the proportion of highly distinctive individuals in the population were not significantly different (p = 0.68). Individual heterogeneity and time variation in capture probabilities were strongly indicated for these data; therefore capture–recapture models allowing for these variations were used. The estimated annual apparent survival rate (product of true survival and permanent emigration) was 0.97 SE±0.05. Open and closed capture–recapture models for the highly distinctive individuals photographed at least once each month produced similar abundance estimates. An estimate of 221±4.3 SE highly distinctive spinner dolphins, resulted in a total abundance of 631±60.1 SE, (95% CI 524–761) spinner dolphins in the Hawai’i Island stock, which is lower than previous estimates. When this abundance estimate is considered alongside the rigid daily behavioural pattern, genetic distinctiveness, and the ease of human access to spinner dolphins in their preferred resting habitats, this Hawai’i Island stock is likely more vulnerable to negative impacts from human disturbance than previously believed.

Acoustic response of Hawaiian spinner dolphins to human disturbance

“…Visual vessel based surveys were carried out on a monthly schedule to generate a robust estimation of dolphin abundance spending two days in Bay 1 and Bay 3 and four days in Bay 2 and Bay 4 each month (see Tyne et al (2014) and Tyne et al (2016) for more information). Weather permitting, information was collected at the top of the hour during hourly vessel scans between 07:00 and 16:00, including the presence/absence of dolphins, an estimation of the group size and the number of motorized vessels including the research vessel in the bay.…”

“…In contrast, in this study we moved under the surface with passive acoustic monitors to assess the potential acoustic response of Hawaiian spinner dolphins to the presence of human activities in four well-studied resting bays (e.g. Norris et al (1994); Östman-Lind (2009); Tyne et al (2014); Tyne et al (2016)). Given the importance of sound and these bays for these dolphins, assessing the potential acoustic response to human presence in these areas is crucial.…”

An Overview of Increasing Incidents of Bottlenose Dolphin Harassment in the Gulf of Mexico and Possible Solutions

“…Scientific research has revealed the real impact that human activities are having on local spinner dolphin populations (Courbis and Timmel, 2009;Tyne et al, 2014). A proposed rule has been in the pipeline since an ANPR was issued by NMFS in December 2005 (NOAA, 2005).…”

Evaluating monitoring methods for cetaceans

“…In Hawaii, spinner dolphins live in small (Tyne et al 2014) , isolated stocks with restricted ranges (Andrews et al, 2010) and have evolved a specialised behavioural ecology (Norris and Dohl 1980). They forage cooperatively offshore at night, and return to sheltered bays to socialise and rest during the day (Norris and Dohl 1980;Norris et al 1994;Benoit-Bird and Au 2009;Tyne et al 2015) during which time the bays are also used extensively by people for tourism, recreational and subsistence purposes (Heenehan et al 2015).…”

“…Here, data from a rigorous photo-identification study designed to estimate abundance were used to provide a second consecutive annual abundance estimate for the Hawaii Island spinner dolphin stock (see Tyne et al 2014 for the first estimate) and evaluate the power of different sampling strategies to detect change in abundance. Five scenarios with different levels of sampling effort, based on the systematic approach employed in Tyne et al (2014), were evaluated in terms of their efficacy to detect trends in abundance by varying sampling effort, the rate of change in abundance, precision, power and the interval between annual abundance estimates.…”