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Dolphin Watch — Chapter 5

Chapter 5

* How Do We "Track" Dolphins
That Aren't Freeze Branded?

* The Dorsal Fin
* Using ID Numbers
* A "Library" of Images


Meet Dan Odell
Interview With Dan
How Do We "Track" Dolphins That Aren't Freeze Branded?
In Chapter 2, we explored the prior three-year study of bottlenose dolphins in the Indian River Lagoon that started in 1979. In that study, we aimed primarily to test the use of freeze branding and to employ freeze branding to determine the home ranges of Indian River Lagoon dolphins. In our new study, we are tracking movement patterns of individual dolphins, but not using freeze branding.

Dolphin number 56, whose very interesting movements we focused upon in Chapter 4, was a freeze-branded dolphin. These freeze-branded dolphins are easy for almost anyone to identify. However, to freeze brand a dolphin you have to catch it and that takes boats, people and money. So how do we identify dolphins that aren't freeze branded?

The Dorsal Fin
The weather conditions haven't been very cooperative for our study so far this spring due to high winds, rain, and choppy waters. But we managed to get out on the water once and you've seen a few of the photographs of dolphin dorsal fins that we took. Some had notches and scratches. Unlike a freeze brand which doesn't change much, the edges of a dolphin's dorsal fin, especially the back edge, change throughout the dolphin's life. Most of the nicks and scratches are caused by other dolphins, but sometimes a dolphin gets too close to a boat propeller and the dorsal fin gets cut.

Dolphin Nicking Another's Dorsal Fin

When a dolphin is born, its dorsal fin is smooth and doesn't have any nicks or scratches. These appear as the dolphin grows and interacts with other dolphins. Unless you spend a lot of time watching a small group of dolphins, it's very difficult to follow a calf as it grows up. In most studies like the one we are doing in the IRL, the first few years of a dolphin's life are "lost" because they are so hard to identify.

The first step in starting — or actually "restarting" — a dolphin photo-identification study, like the one we are conducting in the lagoon, is to take as many photos of dolphin dorsal fins as we can. We do this in order to build a photo catalog. Usually only the best photos are kept in the catalog, but when you're just starting out you have to work with any photos that you get.

Using ID Numbers
As we accumulate photos of dorsal fins that have distinct nicks and scratches, we give each one a unique ID number. Some animals with very distinct dorsal fins may even get a name because it is easier for humans to work with names than it ragged dorsal finis with numbers. Could you imagine talking to your friends using their Social Security numbers rather than their names? "Hello 555-45-1212, this is 666-25-1967."

Each photograph of a distinctive dorsal fin that we take will be given an ID number and cataloged according to the location and size of the nicks. Scratches tend to disappear fairly quickly while nicks are permanent (but they may get bigger). For example, the first distinct dolphin dorsal fin that we identify this year will be given the ID number "97-1". The second one will be "97-2". We will catalog the nicks by the location on the dolphin's body and size. We mark things like front or back of the dorsal fin; upper third, middle third, bottom third; and small, medium or large nick.

A "Library" of Images
In time a computer program that draws and stores an outline of a dorsal fin will be used to catalog and identify the fins. All new photos will be scanned by a computer program that analyzes each image and compares it with the "library" of previously scanned photos. The computer will dorsal fin present us with one or more "matches." Then, by looking carefully at these matches, we rely on the human eye to make the final determination of whether or not the new photo matches any in the library. If it does, it will be assigned the appropriate ID number. If it doesn't match, it will be given a new ID number and added to the photo ID catalog.

As we accumulate photos and make matches, we will be able to plot the locations of individual dolphins over the course of a year or more and see what kinds of habitats (shallow vs. deep water, sea grass vs. sandy bottom) they prefer and how extensive their movements are. Over time we will be able to see how (or if) a dolphin's habits change with the season or as the dolphin gets older. Maybe we'll even learn why dolphin number 56 went to South Carolina from the Indian River Lagoon!

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