New publication: Abundance estimates of spinner dolphins around O‘ahu

Written by Claire Lacey

We would like to share a newly published paper:

Circum-Island Line-Transect Abundance Estimates of Spinner Dolphins Around O‘ahu, Hawai‘i.

Authors: Claire Lacey, Marie C. Hill, Amanda L. Bradford, Erin M. Oleson, Fabien Vivier, Aude F. Pacini, Philip S. Hammond, Lars Bejder.

Marine Mammal Science, 2025; 0:e70055  https://doi.org/10.1111/mms.70055

Available to read 

Spinner dolphin spinning – © Deb Besser, taken under permit #21476

Spinner dolphins – Stenella longirostris – are one of Hawaiʻi’s most familiar marine mammal species, and can be regularly seen around the coastline of the main Hawaiian Islands, where, they show a distinct behavioral pattern, feeding offshore at night and then spending the day in protected, often shallow coastal areas. They use day time to socialize and rest. This behavioral pattern means they are often found in areas that are popular with people too, which can leave them vulnerable to disturbance.

Knowledge of abundance and distribution is one of the most important cornerstones of conservation – with many efforts specifically relying on knowing how many animals there are in a population, and how they are distributed throughout their habitat. If this information is out of date, or unavailable, it can limit the effectiveness of these efforts.  Whilst there are previous estimates of how many spinner dolphins use parts of this coastline, this is the first time the number of animals using the whole of the Oʻahu coastline has been estimated.  

Surveying the island

Between October 2020 and May 2022, field teams from MMRP and NOAA / PIFSC conducted small-boat line-transect surveys around Oʻahu (You can read more about the fieldwork that went into these surveys in our previous blogs Counting Dolphins, Oʻahu line transect surveys and It takes a village).  Over 73 survey days, the teams logged 3,098 kilometers (1925 miles) of transects—roughly the distance from Oʻahu to Alaska!

Survey transects achieved around the coastline of Oʻahu. The red rectangle on the south coast represents a restricted entry anchorage zone in which we were unable to survey.

Innovative Tools: Drone Data Meets Boat Surveys

Counting dolphins isn’t entirely straightforward. They are marine mammals, and while they have to surface regularly to breathe, they also spend time underwater, out of sight of observers in the boat. To address this, we combined traditional line transect surveys with a new method using unoccupied aerial system (UAS or drone) data, to help account for the proportion of animals visible at the surface. This correction ensures our population estimates are more accurate. We also checked for responsive movement.  If the dolphins are changing their behavior due to the presence of our survey boat – that would bias our results. Fortunately, we found no evidence of such behavior during our study.

So… how many spinners are there?

The result: an estimated 594 spinner dolphins around Oʻahu (95% confidence interval: 360–980, CV = 0.26). This is the first ever circum-island abundance estimate for spinner dolphins around Oʻahu—a critical baseline for future monitoring and management.

Check out the paper for the full story!

These surveys were funded by PIFSC and PIRO through a Hawai‘I Pacific Islands CESU Cooperative Joint Venture Agreement with the Marine Mammal Research Program at the University of Hawai‘i.

We would like to gratefully acknowledge the considerable contribution made by the field teams—both boat captains and observers - that participated in the data collection efforts that underpin this paper