Scientist at Work Blog: A Parade of Spinning Dolphins

Scott Baker, associate director of the Marine Mammal Institute at Oregon State University, writes from Samoa, where he studies the formation of local communities among dolphins and their genetic isolation from one another.

Friday, Aug. 17

Today?s survey was our most successful so far. With the continuing easterly trade winds, we decided to change tactics, searching along the shore and the relatively protected western point of Savai?i, rather than searching offshore.

We quickly found a group of more than 40 spinner dolphins, one of the most abundant and widely distributed dolphins in the tropical Pacific. In the open ocean of the eastern tropical Pacific, spinner dolphins can be found in groups of a thousand or more, often associated with schools of tuna. Since the early 1960s, purse-seine fisheries exploited this close association by ?setting? on dolphins and catching the valuable tuna swimming below. Unfortunately, the dolphins were also encircled by the huge nets, resulting in high levels of mortality known as fisheries bycatch. Hundreds of thousands of spinner dolphins were killed before regulations and modification of fisheries methods were introduced in the 1970s.

Around islands of the Pacific, however, spinner dolphins are found in much smaller groups and have a strong local fidelity to specific inshore habitat, regularly appearing in the same sheltered bays or openings in fringing reefs. Long-term studies in the Big Island of Hawaii and Moorea in the Society Islands, near Tahiti, have shown that spinner dolphins feed in deep offshore waters at night and move into relatively protected coastal waters during the day to socialize, rest and shelter from pelagic predators. This predictable pattern of daily return and the acrobatics of the spinning behavior, leaping and twisting high in the air, make spinner dolphins easy to find even from a distance.

As the first group of spinner dolphins moved past us into Fagalele Bay, we sighted a second and then a third. All three groups appeared to be arriving from offshore of the western point of Savai?i and then traveling slowly east within a few hundred yards of the coast, presumably to one or more of the sheltered bays along northwest Savai?i. Although it is impossible to make an exact count, we estimated there were at least 40 individuals in each group, for a total of more than 120 dolphins.

After more than an hour of watching spinner dolphins parade by, we turned back west onto our survey track and sighted what we thought at first was a fourth group of spinners. From the size of the shape of the dorsal fins, we quickly realized these were bottlenose dolphins. These were big, burly dolphins, nine feet or more in length, with blunt snouts and dark coloration ? quite different in appearance from Flipper, the famous representative of this species from the Gulf of Mexico. This group of 15 individuals charged through the water with incredible energy and formidable purpose. Fortunately, they were interested in our boat and moved in to ride the bow, where we are able to collect a few genetic samples.

Surprisingly, we know relatively little about communities of bottlenose dolphins around islands of the South Pacific. Some are occasional visitors from larger, pelagic groups, while others are likely to be island residents, using a combination of shallow coastal and nearshore deep-water habitat. The genetic analysis of the biopsy samples will give us some insight into which form, or ecotype, these are ? pelagic or insular ? and how they might be related to other communities of bottlenose dolphins we have sampled in the South Pacific.

It is easy to forget that these charismatic species are also top predators and play an important role in the marine ecosystem of these islands. With a better understanding of which communities of dolphins are insular and which are pelagic, which are connected and which are isolated, we will be better able to design protected areas of sufficient scale to maintain viable populations and ecosystem function.

Source: http://feeds.nytimes.com/click.phdo?i=2ae0f82bdf5ac6bae1518bf9d442e0c5

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