Two Years, Eight Field Seasons, 173 distinct spinner dolphins
Written by Liah McPherson
It feels like just yesterday that I began my research on Oʻahu’s spinner dolphins. Somehow, two years have flown by and I have less than a year left of my master’s program. Since January of 2021, I have completed eight field seasons studying spinner dolphins off Oʻahu’s Waiʻanae Coast. I want to say up top that none of this would have been possible without lots of support from the MMRP team and many others from HIMB and the larger community. It takes a village (or an island) to estimate dolphin abundance. :)
A spinner dolphin calf surfacing during one of the last fieldwork days of the project. Photo taken under NMFS permit 21476.
In the past two years we’ve learned that spinner dolphins might move around the island more frequently than originally thought. MMRP post-doc Claire Lacey and our NOAA collaborators have seen multiple dolphins from my Waiʻanae Coast catalog on the North, South and East shores of Oʻahu. For that reason, I’ve been moving ahead with multi-state open robust design models to estimate abundance, emigration and survival for the dolphins using habit off the Waiʻanae coast. This model “allows” dolphins to enter and leave the study site during a field season – it’s a type of open population model.
The second chapter of my thesis is focused on looking at the age-structure of this population of spinner dolphins using drone-based photogrammetry – an important metric for population health. MMRP PhD candidate, Fabien Vivier, and I are working to create reproducible methods of measuring spinner dolphin groups. Developing this chapter will be my primary focus next semester.
There's plenty of work I need to do before sharing real results – including the potential addition of a final Winter field season – but I have a few figures to start describing the population included below.
Since the beginning of my field project, I’ve identified 173 distinct spinner dolphins that use the waters offshore Waiʻanae. The following figure (Figure 1) is called a discovery curve, and it helps us visualize the number to distinct animals in the population. When a discovery curve begins to “level off”, the implication is that we have identified most of the animals in the population. Of course, the curve will always be increasing somewhat, as previously unmarked animals gain marks, and if new animals enter the population. I’m eager to see how much more the discovery curve levels off with the addition of data from October.
Figure 1. Cumulative discovery curve of marked individuals identified between December 1st, 2020 and July 22nd, 2022. Each color indicates a different field season.
The most common "hotspots" where we found spinner dolphins along the coastline align with descriptions from previous research and with local knowledge (Figure 2). These locations provide protection from rough sea conditions, often boast clear water, and have sandy substrate – making it easier for the dolphins to remain aware of their surrounds and look out for predators as they rest.
Figure 2. Map of spinner dolphin sighting locations and group sizes offshore Oʻahu's Waiʻanae Coast during this two year project.
During our surveys, we opportunistically collected data on other species we encountered, such as humpback whales, bottlenose dolphins, pilot whales, pantropical spotted dolphins and pygmy killer whales.
Spotted dolphins, pygmy killer whales, pilot whales, bottlenose dolphins, and humpback whales seen during 2022's spinner dolphin fieldwork.
There's still so much work to be done for spinner dolphins around Oʻahu – both with my project, and in the future. I'm hopeful this work will provide valuable insights to the health and trajectory of this population, and inform research on these animals for years to come.
It's bittersweet to be nearing the end of the fieldwork portion of my project. I'll certainly miss my field days on the West Side, which have undoubtedly molded me into a better captain, photographer, drone pilot and scientist. But I think I'm even more excited to synthesize everything and produce results which could make an impact on the lives and conservation of these animals.
This research is conducted under NMFS permit 21476, with support from NOAA's Pacific Islands Fisheries Science Center, our partner on the project.
My favorite fieldwork view. Heading south from Kaʻena Point.