Unexpectedly low calf numbers within the spinner dolphin population off Hawaiʻi Island were revealed in a study led by scientists with the (MMRP) at the University of Hawaiʻi at Mānoa along with national and international collaborators. Their research, published in , determined that calves make up only about 9% of the population, a stark contrast to the 17% expected in a healthy, stable population, raising urgent concerns about the long-term survival of this isolated population.
Viewing spinner dolphins is an attraction of the tourism industry in Hawaiʻi, however, those activities put pressure on the marine species. Previous research has shown that this population of dolphins experiences the highest levels of disturbance exposure recorded for dolphins worldwide.
“The evidence suggests chronic human disturbance—especially tourism and recreational-related interactions during critical dolphin resting periods—as a likely contributing factor affecting reproductive success and calf survival,” said lead author Fabien Vivier, who was a doctoral student at the in the 糖心视频 Mānoa at the time of this work. “The findings support stronger conservation actions, such as time-area closures for human activities, alongside continued long-term monitoring—actions that have long been called for.”
Drone surveys reveal population trends
To reach these conclusions, scientists utilized drone photography to measure the body length of free-swimming dolphins. By combining these aerial measurements with age-length data from stranded spinner dolphins collected by Kristi West, study co-author and researcher with the 糖心视频 Health and Stranding Lab at the 糖心视频 Mānoa , the team created the first age-length growth curve for this specific population. This allowed them to accurately categorize individuals into calves (under 2 years), juveniles (2–9 years), and adults (over 9 years).
Age structure is an early indicator of population health and long-term viability. The documented low proportion of calves suggests the spinner dolphin population off Hawaiʻi Island may not be replacing itself fast enough, which could lead to future population decline.
Highly valuable and vulnerable population
In Hawaiʻi, spinner dolphins are culturally, ecologically and economically important. Research from 2013 estimated that each individual dolphin is valued at more than $3.3 million over its lifetime and collectively, dolphin tourism generates more than $100 million annually for the state’s economy.
With spinner dolphins following a predictable daily cycle—foraging offshore at night and returning to shallow coastal bays during the day to rest and socialize—they are especially vulnerable to tourism activities in these nearshore habitats.
“This study provides one of the first population-level demographic signals suggesting those long-term disturbances may now be affecting reproduction and calf survival,” said Professor Lars Bejder, co-author on the study and MMRP director.
A call for stronger protections
While a 2021 federal rule requires people and vessels to remain at least 50 yards away from the dolphins, experts from the U.S. Marine Mammal Commission and the Pacific Scientific Review Group have argued that this measure is insufficient. The study authors are calling for stronger conservation actions, specifically time-area closures that would restrict human access to critical resting bays during the day.
“Understanding these impacts is critical for protecting an isolated local population before declines become irreversible,” Randall Wells, study co-author and director of Brookfield Zoo Chicago’s . “We hope this work provides the science-based evidence needed to develop effective protections for spinner dolphins while helping balance marine tourism, cultural stewardship, and long-term ecosystem health in Hawaiʻi.”
Future research and monitoring will depend on available funding, with scientists aiming to track long-term changes in age structure, reproduction, and population and further refine non-invasive drone-based methods.