I first joined Blair Irvine and Randy Wells in 1974 in their pioneering study of dolphin tagging and radio tracking in Sarasota Bay. In addition to my involvement in the Sarasota Dolphin Research Program, I study spotted and spinner dolphins and yellowfin tuna that are the center of the tuna-dolphin controversy in the eastern tropical Pacific Ocean (ETP). Purse-seine fishermen have used the tuna-dolphin association to help them find and catch the tuna since the late 1950s, but at a cost of a high incidental mortality of dolphins in the early years of the fishery. Since then, research, techniques for releasing dolphins from the nets, and international agreements that put observers on every boat of the international fleet have brought the mortality down to a tiny, sustainable fraction of what it was, but we still know very little about these dolphins that spend their lives far out at sea. Capturing dolphins at sea, radio-tagging and releasing them safely, and then tracking them is difficult, risky, and expensive, but that was what I and my co-author Susan Chivers (another Sarasota Bay capture-release veteran), along with many colleagues, attempted to do to learn more about their movements, diving behavior, and association with tuna.
        Before heading to sea, many of our dolphin-handling and tagging techniques were tested in Sarasota Bay, and many of our team gained experience in capture-release there. However, some things just can’t be simulated in a shallow Florida bay. In the open ocean, dolphins were captured by a 200 ft-long tuna purse-seiner that set a 500-ft deep, mile-long net that encircled up to 500 dolphins at a time. Swimmers then grabbed the dolphins while inside the net, put them into a small raft, and tagged them and released them with the rest of the herd.

Spotted dolphins in tuna purse seine net.

        Over the years, we have radio-tracked at sea 20 spotted dolphins (Stenella attenuata) for up to 6 days. Nine of the dolphins also carried time-depth recorders that provided 477 hours of dive data. The movement data suggested that the dolphins sought out areas with higher biological productivity, such as along the continental slope or along areas where the thermocline abruptly shallows. By tracking and tagging multiple dolphins we found that the dolphin herds were dynamic, changing in size and membership throughout the course of a day. The diving data revealed that spotted dolphins are nocturnal feeders, diving deeper and longer at night as they track the daily vertical migration of fishes and squids, while daytime was primarily spent traveling (see graph below). We also found that the association with tuna was not a permanent one, suggesting the possibility of catching large yellowfin tuna without the presence of dolphins.
        The study was funded by NOAA’s Fisheries Service, the Inter-American Tropical Tuna Commission, with publication support from Dolphin Biology Research Institute and is being published in Marine Mammal Science.

Dive pattern of tagged spotted dolphin.