By Liah McPherson
We are pleased to announce a new publication in Scientific Reports entitled: Quantifying the abundance and survival rates of island-associated spinner dolphins using a multi-state open robust design model
McPherson, L., Badger, J., Fertitta, K., Gordanier, M., Nemeth, C., Bejder, L. Quantifying the abundance and survival rates of island-associated spinner dolphins using a multi-state open robust design model. Sci Rep 14, 14764 (2024). https://doi.org/10.1038/s41598-024-64220-3
Oʻahu has the highest human density in the Hawaiian Islands – and spinner dolphins along the Waiʻanae coast often rest just a few hundred meters from shore. These dolphins are a captivating presence in the nearshore waters of the Hawaiian Islands, however, beneath their charm lies a tale of vulnerability, driven by their constrained behavioral patterns, genetic isolation, and susceptibility to anthropogenic activity. Our recent study, published in Scientific Reports, aims to quantify the abundance and survival rates of these dolphins off the Waiʻanae Coast of Oʻahu, providing critical insights for their conservation.
Why It Matters
Spinner dolphins are critical to the marine ecosystem, serving as indicators of ocean health and contributing to ecological balance. They also have high cultural and economic value off Oʻahu. Their predictable behavior and habitat use make them particularly susceptible to threats from humans in the forms of pollution, adverse fisheries interactions, introduced pathogens, and direct disturbance from recreational activity. These perturbations can have lasting impacts, therefore, understanding their abundance and survival rates are vital for implementing effective conservation strategies and informing NOAA stock assessment reports.
Our Study
We estimated the abundance (seasonal and super-population), apparent survival, and emigration patterns of spinner dolphins off Waiʻanae by conducting design-based photo-identification surveys. Collecting data over two consecutive years, we completed eight seasonal field efforts, each with six surveys of the study area. This extensive effort allowed us to employ advanced modeling techniques to derive accurate estimates.
Key Findings
Abundance Estimates:
We identified 165 unique individuals over the course of the study. Here are three distinct dolphins we saw frequently! From left to right, their nicknames are "Dipper", "Chopper" and "Doublescoop". Nicknames help us remember individuals and their associated ID numbers. We can estimate toal abundance based on the proportion of distinct animals we observe, and how frequently we resight them.
Our seasonal abundance estimates varied, with the lowest numbers of dolphins observed during winter months. The number of dolphins estimated during the eight field seasons ranged from 140 (Winter 2022) to 373 (Summer 2022) individuals. We hypothesize that such variations are due to seasonal differences in environmental conditions off Waiʻanae, and the dolphins' reproductive biology.
We estimated the overall super-population size, representing the total number of individual dolphins using the study area during the entire study period, to be 633 dolphins.
Survival and Emigration:
We found the apparent survival rate of the dolphins to be 0.95, indicating strong low permenant emigration from the study area. High survival rates are typical of long lived mammal species.
Our study identified a Markovian pattern of temporary emigration, suggesting that while dolphins often left the study area, they were likely to return, highlighting their fidelity to the coastline.
Additional research on circum- and inter-island dolphin movements around and between Oʻahu and the Maui Nui region may shed light on both seasonal movement patterns and overall abundance for the Oʻahu/4-Islands stock.
Conclusion
This work represents the first systematic mark-recapture effort to assess the abundance
and survival rates of these vulnerable dolphins, offering valuable insights for their conservation. Further research on the movement patterns of individual spinner dolphins – around Oʻahu and to other islands – may provide insight into total stock size, seasonal trends in abundance and best practices for long-term monitoring.
For more detailed insights and data, you can access the full study here.
This research was possible due to the support of our collaborators at NOAA-PIFSC and the help of many MMRP lab members and interns. Mahalo!
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