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All dolphins are not alike - that creates problems

As mentioned in the last blog, there is a large variation in size, appearance and behavior among the 42 species of currently recognized Oceanic dolphins. This unfortunately becomes a huge problem, when humans treat all these species the same. Comparing the common bottlenose dolphin with the spinner dolphin highlights some of the differences and points out some of the problems.

Spinner dolphins mostly feed in the Deep Scattering Layer (DSL), a layer of marine organisms with a high concentration that hide at depths of 4-700m during the day and migrate closer towards the surface every night. Spinner dolphins can only reach these organisms, including lantern fish, squid and shrimp, at night when they come closer to the surface.

Because these prey items are so small, only about 2-10 cm (1-4 inches) long, each dolphin must catch about 1,000 individuals every night, corresponding to an average of about 1.5 prey items/minute for the 11 hours these schools are available to the dolphins. This, however, does not take into account that the dolphins constantly need to get back to the surface to breathe about every 5-10 minutes or so.

The dolphins would not be able to do this on their own, but rather work together in large, highly coordinated groups of up to 28 individual dolphins. They swim in pairs at slightly different depths, circling the prey and corralling them into very dense groups. The pairs of dolphins then coordinate their feeding runs through the school to capture the prey. All this mostly takes place some 1-200m (3-600 ft) below the surface.

A coordinated feeding bout can be seen in this clip, where 20 spinner dolphins, each represented by a yellow square, are seen to first herd and then circle a school of prey, before repeatedly swimming through the concentrated prey to feed (from the research of Dr. Kelly Benoit-Bird, courtesy of Oregon State University.

As a consequence of this highly coordinated nighttime activity, at great depths, for a large part of every night, spinner dolphins rest during the day. They are normally a pelagic species, found far offshore, in tropical and subtropical waters. Here the spinner dolphin schools often join schools of spotted dolphins in the morning – spotted dolphins are active during the day. This way the spinners can relax and rest, relying on the spotters to keep an eye out for predators and other dangers. The main exception to this association is around the many Islands in the tropical and subtropical Pacific Ocean where, like in Hawaiʻi, the spinner dolphins instead find calm, protected bays, often on the leeward side of the islands and generally with a sandy bottom at a depth of around 10-20 meters (30-60 ft), where they rest for several hours during the morning and early to mid afternoon.

Bottlenose dolphins, on the other hand, feed during the day and spend about 10-20% of daylight hours feeding. Their prey and prey size can vary a lot. A common size for fish is around 25 cm, but they may feed on very large prey. In fact some of the fish they feed on are so large that they have to work on them for a while, like rubbing them on the bottom and slapping them against the surface, to make them soft enough to be able to swallow. These dolphins may feed alone or in small groups of a few animals, depending on the prey and situation.

So approaching these species during the day will have very different consequences. While bottlenose dolphins will be disturbed in their daily activities, they would still be fully alert and able to swim away. They would also have lots of time available to feed earlier or later in the day and their rest would not be disturbed. For spinner dolphins resting in a protected bay, however, the situation would be completely different. They would be prevented from resting and forced to leave a long-established resting area used by generations of individuals. A disturbed rest, we know from ourselves, can have far reaching consequences, like having an impact on the highly complex feeding operations the following night.

What is more, it has been documented how the population of spinner dolphins along the Kona Coast of the Island of Hawaiʻi, changed their behavior when approaching a resting bay so as to not be easily detected. We also observed how they used less optimal resting areas in response to years of human disturbance. You can see some of the highlights from our report to the National Marine Fisheries Service here and can also download the full report to NMFS if interested. Fortunately, as a result of a natural disaster, we also documented how the spinner dolphins quickly returned to their traditional resting areas and resting pattern as soon as people left them alone (See graph here).

The situation in Hawaiʻi is now much better, since NOAA instituted a ban on approaching Hawaiian spinner dolphins closer than 50 yards (46 m). However, as we noted during our recent visit to the Big island, there are still people that approach the dolphins much too close in the resting bays. The difference is that this behavior can now be reported to the authorities who can enforce the ban and issue fines.


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