New publication: Drone photogrammetry reveals age-structure shifts with potential consequences for the viability of spinner dolphins in Hawaiʻi.

We are pleased to share a new publication exploring the age structure and population health of spinner dolphins of the Leeward coast of Hawaiʻi Island, using drone-based photogrammetry. This study is entitled Drone photogrammetry reveals age-structure shifts with potential consequences for the viability of spinner dolphins in Hawaiʻi.

Mother and calf spinner dolphins.

Spinner dolphins in Hawaiʻi follow a unique daily routine: they forage offshore at night and return to shallow coastal bays during the day to rest and socialize. Because these resting habitats are also popular areas for tourism and recreation, local spinner dolphins have experienced over four decades of chronic human disturbance. Since 2005, NOAA Fisheries has expressed concerns that increasing human activity around spinner dolphins could affect the long-term health of the population. In 2021, a federal rule was implemented requiring people and vessels to remain at least 50 yards from spinner dolphins throughout the Main Hawaiian Islands. However, additional protections proposed for key resting bays, such as time-area closures during peak resting periods, were not adopted. Both the U.S. Marine Mammal Commission and NOAA’s Pacific Scientific Review Group have since recommended stronger protections for these important resting habitats.

In this study, we found that the population contains significantly fewer calves than expected for a healthy, stable dolphin population, raising concerns about long-term population viability and the potential impacts of repeated disturbance on reproduction and calf survival.

To conduct this work, we used drones to collect non-invasive aerial images of free-swimming spinner dolphins off Hawaiʻi Island during surveys in September 2021 and 2022. By combining body measurements obtained from drone imagery with age and morphometric data collected from stranded spinner dolphins in Hawaiʻi (Fig. 1), we estimated the proportion of calves, juveniles, and adults in the population and compare it to what would be expected in a stable population. We also developed the first age-length growth curve for this population. 

Our findings highlight the importance of continued monitoring and effective conservation measures to help protect spinner dolphins in Hawaiʻi’ as well as their critical resting habitats.

Figure 1: Schematic overview of the integrated workflow used to quantify the age-structure of spinner dolphins along the leeward coast of Hawaiʻi Island

Abstract:

Evaluating age-structure in long-lived species is crucial for understanding population dynamics. In mammal populations, the proportion of calves, juveniles, and adults serves as an early indicator of population trajectory, where stability usually reflects viability, and deviations may signal growth or decline. Surveys conducted in 2021 and 2022 integrated unoccupied aerial system (UAS, drone) photogrammetry to assess the age-structure of the Hawaiʻi Island population of Gray’s spinner dolphins Stenella longirostris longirostris chronically exposed to human disturbances for over 4 decades. Age-length data from stranded dolphins allowed us to estimate total body length (TL) of dolphins from photographs using the relationship between TL and blowhole-to-dorsal-fin distance and to develop the first age-length growth curve for this population. We then used TL-estimates to classify dolphins as calves (0–2 yr), juveniles (≥2–9 yr), or adults (9+ yr). Calves comprised only 9.3% of the population, significantly lower than the 17.2% expected under stable conditions for spinner dolphins (p < 0.001) and below values reported for bottlenose dolphins Tursiops spp. (12.4–18%;). This suggests limited recruitment, raising concerns about long-term viability. Sampling and ecological factors (e.g. prey availability) did not explain the low calf percentage, suggesting that other factors, such as chronic anthropogenic disturbances like tourism, are likely affecting this population’s reproductive rates. These findings support the need for effective management interventions, such as time-area closures in critical dolphin resting habitats, which may help mitigate disturbances. Long-term monitoring of this population is needed to evaluate trends in age-structure and abundance and the efficacy of management interventions.

Drone view of a large group of approximately 110 individuals, from which body measurements were collected to estimate age classes and characterize the overall group structure.

Hōʻuluʻulu (Abstract in the Hawaiian Language by Cam Nemeth):

He mea nui nō ke ana ʻana i ke ʻano o nā makahiki ma ka lehulehu i nā holoholona ola lōʻihi i mea e maopopo ai ke ʻano o ka lehulehu. I nā lehulehu mammal, ʻo ka pakeneka o nā keiki, maikaʻi i ka mau, a he hōʻailona o ka hoʻoulu ʻana a i ʻole ka hōʻemi ʻana paha ka hanuʻu ʻana. I 2021 a 2022, i hoʻohana ʻia ai ka paʻi kiʻi ʻana o ka helekopa uila liʻi i mea e ana ai i ke ʻano o nā makahiki i ka lehulehu naiʻi Gray (Stenella longirostris longirostris) no Moku o Keawe, i pōʻino mau nō i ka hoʻoluhi kānaka. Ma muli o ka ʻike makahiki-lōʻihi mai nā naiʻa i ili, ua hiki iā mākou ke koho i ka lōʻihi kino o nā naiʻa ma nā kiʻi mai ka pilina o ka lōʻihi kino holoʻokoʻa me ke kāwā i waena o ka puhi me ke kualā, a ke hoʻokumu i ka piʻo ulu mua loa no kēia lehulehu. A laila, ua hoʻohana mākou i nā koho lōʻihi kino no ka hoʻonohonoho ʻana i nā naiʻa ma ke ʻano he keiki (0-2 makahiki), ʻōpio (≥2–9 makahiki), a i ʻole he makua (9+ makahiki). Hōʻike mākou, emi loa aku nō ka pakeneka o nā keiki (9.3%) i mua o ka pakeneka i kuhi ʻia no nā naiʻa ma kekahi kaiāulu maikaʻi (17.2%, p-value < 0.001), a emi aku ma mua o ke kīkoʻo helu (12.4–18%) i hōʻike no ka naiʻa bottlenose (Tursiops spp.). Hoʻopuka manaʻo kēia e ulu ʻole ana ka lehulehu, a kuhi i ko ka lehulehu pōʻino. ʻAʻole i wehewehe ʻia ka pakeneka emi o nā keiki e kā mākou ʻano hana a i ʻole ka hana o nā naiʻa, e kuhi ana i nā mea ʻokoʻa, e like me ka hoʻoluhi kānaka mau a me ka hana no nā malihini, e hoʻololi i ka nui o ka hānau ʻana ma kēia lehulehu. Kākoʻo nō kēia ʻike nei i ka mālama ʻana i ka naiʻa, e like me kaupalena ʻana i kekahi mau wahi nui loa no kā ka naiʻa hoʻomaha ʻana, e hōʻemi paha ana i nā hoʻoluhi. Pono ʻia ke kilo wā-lōʻihi ʻana i kēia lehulehu nei i mea e ana ai i ke ʻano hanuʻu a me ka holomua pono o ka mālama ʻana.

Citation:

Vivier F, Lusseau D, Hofmann NJ, West K, McPherson L, Nemeth C, Wells RS, van Aswegen M, Manlik O, Chivers S, Pacini A, Booth C, Bejder L (in press) Drone photogrammetry reveals age-structure shifts with potential consequences for the viability of spinner dolphins in Hawaiʻi. Endang Species Res :0-0 https://doi.org/10.3354/esr01512

Permits:

Field research was conducted under National Marine Fisheries Service (NMFS) permit numbers 21476, with all activities approved by the University of Hawaii Institutional Animal Care and Use Committee. Stranding response, necropsy, and sample collection were conducted under NMFS permit numbers 18786 and 24359.

Group of spinner dolphins fast traveling.

Funding:

We thank the Office of Naval Research (N00014-20-12624), NOAA Fisheries-PIFSC (NA19NMF4720181), the US Department of Defense’s Defense University Research Instrumentation Program (N00014-19-2612), Dolphin Quest, the Omidyar Ohana Fund and the Hawaiʻi Community Foundation for funding fieldwork operations. Graduate Assistantships for F. Vivier were funded by the Office of Naval Research (N00014-20-12624) and Dolphin Quest. The stranding response was supported by the NOAA National Marine Fisheries Service (NMFS) John H. Prescott Marine Mammal Rescue Assistance Grant program over many years. Earthwatch Institute, Dolphin Quest, Inc., and Brookfield Zoo Chicago funded much of the health assessment research in Sarasota Bay from the Sarasota Dolphin Research Program that provided the morphometric data.

Special thanks:

We thank E. Leunissen for providing an upgraded version of G.U.I. Whalength. We would like to acknowledge Doug DeMaster for his comments on an early version of the manuscript, Jay Barlow for his insightful comments on age-structure, and Len Thomas for his earlier contributions to refining the overall approach. We also thank NOAA’s Pacific Scientific Review Group for their early reviewing of the manuscript. We thank the staff, students, volunteers, veterinarians, and collaborators of the Sarasota Dolphin Research Program for making Sarasota data collection possible. We thank Claire Lacey for providing the map on dolphin group sightings. We would like to acknowledge Captain Zodiac for their contribution to the data collection process.

Team members Fabien Vivier and Kyleigh Fertitta conducting drone operations off Kona.