Written by Claire Lacey
This work is a collaboration with NOAA / PIFSC
Regular readers may remember from a previous blog post - Counting dolphins - that this summer has seen the spinner team at MMRP get underway with the line transect surveys for dolphins around O`ahu. During the past few months, our time has been split between the leeward (west) and windward (east) coasts of the island. These two areas are very different, so its' been interesting to see the differences between the two - both in terms of dolphins, landscape, weather and levels of anthropogenic activity.
The leeward side - the side of the island which is sheltered from the prevailing wind - is the traditional resting habitat for island-associated spinner dolphins. They feed offshore at night, and then come in to sheltered waters during the day to rest. The dolphins typically favor areas with clear waters and sandy substrate, which - it is thought - helps them to detect predators (e.g. sharks) more easily. O`ahu is no exception, and spinner dolphins are regularly seen up and down this coastline, which is also - not coincidentally - the study site for Liah McPherson's work examining the size and age structure of this population of dolphins.
By contrast, spinner dolphin behavior, and indeed occurrence, on the windward side of the island chain is much less well documented. In part, because this coast is much less accessible. As the name suggests, the windward side of the island (any island) is the side exposed to the prevailing weather conditions, and so flat calm days can be few and far between. Whilst we can go out to sea on days when the weather isn't so nice, large surf, high waves and a choppy sea-surface can make it very difficult to see animals. If we miss lots of animals which actually are present, there is a knock-on effect in our estimate of the population size. So, we only survey on calm days - of which there are fewer on the windward side. When they do come though, they are beautiful.
Whilst we have certainly seen fewer groups of spinner dolphins off of the windward side than Liah would get on one of her west coast photo ID trips, we have still seen them on about 60% of our east-coast survey days. What has been particularly interesting, is that we have seen a few individuals that we recognize from Liah's photo ID earlier in the year - particularly from the April season. This means that at least some animals are using the habitat on both sides of the island. It's much too early to tell yet what proportion of animals this might apply to, or even how often they go "on tour" to the east side, but it's been interesting to observe. The map below shows the locations of three dolphins when they were first seen on the west coast, and again when they were most recently sighted during our east coast surveys.
Click the location markers to see the photos of the dorsal fins which allow us to recognize these individuals. If you have suggestions for nicknames for any of them, feel free to get in touch and let us know too!
As well as spinner dolphins, we've also seen bottlenose dolphins on both the east and west sides of the island, as well as a single encounter with a pantropical spotted dolphin on the east coast. It will be interesting to see how - if at all- the species composition changes as we move into winter.
Keep an eye on the spinner dolphin Facebook page for more information about spinner related fieldwork as it happens. If you’re interested, give the page a follow and feel free to say hello!
For more information about MMRPs spinner dolphin work, please also see our main project page.