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Dolphin Watch — Chapter 2

Chapter 2

* Studying Dead Dolphins
* Counting Dolphin Years
* Studying Live Dolphins
* Catching Dolphins
* Observing Dolphins

Meet Dan Odell
Interview With Dan

Last week I mentioned that my current "Indian River Lagoon [IRL] Dolphin Project" is going to follow up on an earlier field study. Let me tell you about that other study now.

Studying Dead Dolphins
I first studied the bottlenose dolphins in the IRL in 1974 in cooperation with Sea World of Florida. Money for dolphin studies is always in short supply, and one of the easiest (and smelliest!) ways to learn about them is to pick up the dolphins that are found dead on the beaches. These are called stranded or beached animals. Dolphins die from many kinds of natural diseases. Some also die as a result of human activities (they become entangled in nets, for example).

I picked up dead dolphins in south Florida from my base at the University of Miami, and Sea World picked up dead IRL dolphins from its base in Orlando. Since dolphins were protected by federal law, we all worked under permits issued to us by the U.S. government. We would weigh and measure each dead dolphin and perform a necropsy (an examination of tissue after death) to collect tissue samples, search for parasites, and examine its stomach contents. When we were done we cleaned the skull and collected the teeth.

Counting Dolphin Years
Tooth DiagramIs it possible to know how old a dolphin is by examining its teeth? Dolphins are born with one set of teeth that continue to grow a little bit each year. (Bottlenose have 20 to 25 teeth in each of 4 rows.) The teeth don't grow larger at the top (or crown), but in the root — the part of the tooth that is in the jaw beneath the gum. We can tell how old a dolphin is by cutting a thin section from the root of the tooth and looking at it under a microscope. We use a saw with a diamond blade to cut the tooth. The pulp cavity (the inside of the root of the tooth) fills in a little bit each year as the dolphin gets older. Just like we can count the growth layers inside a tree trunk, we can count the growth layers inside a dolphin's tooth. Scientists believe that a dolphin adds one growth layer each year. From this information we can tell how long dolphins live and calculate their average life span. Using the dolphins' ages and other data collected from the necropsy, we can calculate growth rates and determine when dolphins become sexually mature.

Studying Live Dolphins
But we can't learn everything we need to know about dolphin biology from studying dead dolphins. We have to study live dolphins too. The federal law that protects dolphins also allows people to collect some alive for public display in oceanariums and for research, and the Indian River Lagoon was one place where dolphins were collected. But in order to be sure that too many dolphins weren't collected, the government had to learn about the dolphins first. In 1979 the National Marine Fisheries Service issued a research contract to the Hubbs-Sea World Research Institute to study the bottlenose dolphins in the Indian River Lagoon.

The government wanted to know if the dolphins spent their entire lives in the IRL, and they wanted us to test a method known as freeze-branding to mark individual dolphins. Freeze-branding is used to mark many different animals. It is a cold process compared with the hot brands often used on cattle. We use a brass branding "iron" and cool it in liquid nitrogen. The process briefly freezes the animal's skin and kills the cells that make pigment. The end result is a white mark in the shape of the branding "iron."

Catching Dolphins
fast boat We started our two-year research project in August 1979 near Titusville, Florida, and caught 25 dolphins in two weeks. The dolphins were surrounded with a long net let out from the back of a fast boat. When the dolphins were surrounded we gradually closed the net until we could place the animals in canvas stretchers and lift them aboard the boat. We were careful to avoid setting our nets around dolphin herds that contained calves. Sometimes the dolphins escaped before we could close the net and sometimes they went under it. Only rarely did they jump over the net.

Each dolphin was weighed, measured, and given a physical examination by Sea World veterinarians. The veterinarians collected blood samples and pulled a tooth (after the dolphin's jaw was anesthetized). The last thing we did was to freeze-brand each dolphin on both sides of the dorsal fin and on both sides of the base of the dorsal fin. The whole process took 20 to 30 minutes per dolphin before they were released back into the water.


In 1980 we marked an additional 50 dolphins. In 1981 we recaptured some branded dolphins in order to get a close-up look at their brands. Catching and releasing the dolphins didn't take nearly as long as it took to find a branded dolphin to recapture!

Observing Dolphins
In the first three months of the study in 1979 we made over 400 dolphin-herd sightings. During the two years of the study we made over 1,200 herd sightings and took hundreds of photographs. We resighted most of the branded dolphins at least once and many of them were seen on several occasions. During our entire study we never saw a marked dolphin outside of the Indian River Lagoon. In the years since the study, several of the marked dolphins have been found dead (from natural causes) and boaters continue to see some of the branded dolphins. Number 56 was seen in December 1996 — more than 15 years after the dolphin was branded!

In the coming weeks we hope to encounter some of the other dolphins branded 17 years ago as well as dolphins that weren't even born then. This time we will only be photographing the dolphins, and collecting data on their movements, feeding, and socializing patterns. We hope to be able to learn even more about the behavior of these animals — things that can only be learned from a long-term study.

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