by Irene Tejaratchi

Dolphins use sound to detect the size, shape, and speed of objects hundreds of yards away. Fascinating and complex, the dolphin’s natural sonar, called echolocation, is so precise it can determine the difference between a golf ball and a ping-pong ball based solely on density. Although humans have researched these intelligent marine mammals for decades, much of their acoustical world remains a mystery.

One of the keys to dolphin echolocation is water’s superb conduction of sound. Sound waves travel 4.5 times faster in water than they do in the air. Dolphins use this to their advantage, in ways that would make a superhero envious. Using nasal sacs in their heads, dolphins send out rapid clicks that pass through their bulbous forehead, or “melon.” The sound is focused, then beamed out in front of the dolphin. The sound wave speeds through the water, bounces off the object under investigation, and is reflected back to the dolphin. Fat-filled cavities in the dolphin’s lower jaw receive this information and auditory nerves conduct it to the middle ear and brain, where an acoustic picture is created.

Scientists say that dolphins may also use clicking to communicate with one another. Although dolphins do not possess vocal cords, they still “speak” using sounds such as whistles, squeaks, and trills. A mother dolphin may whistle to her newborn for days, apparently to imprint a signature whistle upon her baby that will enable it to recognize her. It is believed that dolphins use whistles to identify one another and possibly for other functions, such as communicating strategic alerts while hunting in a group, but scientists have yet to crack the code. Many doubt, however, that dolphins have a formal language akin to that of humans.

In the 1950s, researcher John C. Lilly helped pioneer the systematic study of dolphin vocalization. A strong advocate of interspecies communication, Lilly wrote several books about dolphins, inspired the film Day of the Dolphin (1973), and was a supporter of the Marine Mammal Protection Act of 1972. Another pioneer of dolphin research, Kenneth S. Norris, first obtained evidence of dolphin echolocation by blindfolding a bottlenose to test its ability to locate an object underwater.

Since the 1960s, American military scientists have studied dolphins, and have trained them to perform such tasks as attaching explosives and eavesdropping devices to enemy ships or submarines. In the mid-1980s, the U.S. Navy began training dolphins to search for mines using their echolocation. In 2003, dolphins were deployed for the first time in a real war situation to probe the seafloor for mines near the Iraqi port of Umm Qasr. For decades, animal activists have opposed the use of dolphins for entertainment or military activities, citing questionable training methods and the stress-related illnesses, such as ulcers, that the animals can manifest in such situations.

Dolphin advocates also object to the navy’s use of manmade sonar, which is used to scan and investigate the ocean depths, claiming that it is harming dolphins and other marine mammals. They point to incidents such as the beaching of four different whale species off the coast of the Bahamas in March 2000, following navy sonar exercises in the area. Marine mammals strand themselves for a variety of reasons, but investigations confirmed that navy sonar caused the Bahamas stranding. Researchers are not exactly sure how manmade sonar affects marine mammals. Some believe the intense sounds may scare or disorient them and cause them to rapidly flee to the water’s surface, resulting in a sort of decompression sickness that damages sensory organs and causes internal bleeding.

If technological sonar can be implicated in the death of dolphins, it would be a tragic irony, considering that the sonar is based in part upon nature and dolphins’ superior echolocation capability. Efforts to replicate dolphin echolocation continue to fall short, as humans have yet to achieve the complexity and precision that 50 million years of evolution has bestowed upon dolphins. Perhaps if scientists could understand dolphin-speak they’d have more luck, but for now the true nature of dolphin communication remains mysterious.

In making NATURE’s A Mystery in Alaska, filmmaker Shane Moore consulted with an array of scientists who have been studying different facets of the issue. One is Gary Thomas, a fisheries scientist who spent more than a decade examining how fish, their food, and their predators — including sea lions — interact in Prince William Sound in southeastern Alaska.

Thomas says his team’s work, which included surveying fish populations using sonar and documenting night-time behaviors of fish-eating wildlife with infrared cameras, has helped highlight the important role that herring play as a food source for sea lions, particularly in the brutal winter months when other food is scarce.

In 2003 NATURE spoke with Thomas, now a professor at the University of Miami, about his work.

How did you get involved in the mystery in Alaska?

I went up to Prince William Sound after the 1989 Exxon Valdez oil spill to study plankton, fish, and wildlife populations using an ecosystem approach. I’ve always worked on protecting wild fish stocks, but one reason we’ve had trouble sustainably managing stocks is that we simply don’t know how many fish we have. I was interested in developing some better ways, using acoustic and optical systems, to measure population sizes and relate them to changes occurring in the ecosystem.

What did you find?

One thing that fell out of it all was that herring are a key winter food fish for many fish and wildlife species. The herring populations aggregate in the sound over the winter, until spawning time in the spring. We used underwater sound to repeatedly monitor the fish populations, and in winter we found over 90 percent of the sound’s population of herring concentrated in less than one percent of the area in which you would find them in the summer. We also observed many predators around the herring, such as sea lions, humpback and killer whales, and birds. After a decade of surveys, it was clear that many fish predators [including sea lions] aggregate around the herring.

Beginning in 2000, we used [infrared cameras] to document that, during winter, the sea lions and other predators primarily hunt the herring at night. The herring swim up to within 30 feet of the surface at night, while most of your other fish species are out in much deeper water. So you’ve got one of the most desirable fat-rich forage fish in the Pacific Ocean coming in close to the shoreline — it makes sense that it is going to be a target for most of the fish eaters.

Gary Thomas has spent years studying the Alaskan ecosystem.

So the take-home message is that sustaining herring populations is key to sustaining sea lion populations?

Particularly in the winter, when food limitation is a problem and these animals are most stressed, herring is critical food to sea lions. Every place where we’ve looked at herring aggregations in the winter, we have found that these places are hotbeds of fish predators. It’s not the whole picture on sea lion foraging, but it’s a big part of it. The major problem is that herring is also an important commercial fish, so you need a reliable way to manage catches. Traditional methods used to predict herring abundance don’t work very well. The [population] predictions can be so far off, it’s like playing Russian roulette with your herring stocks to use them to set harvest limits. Sometimes the errors are so great that it allows the fisheries to take the bulk of the spawning stock, and sometimes they are only allowed to harvest a small part of what is available.

What’s the alternative?

Well, I think we’ve shown you can use [sonar-based] methods to make accurate population estimates in the late winter or early spring. And if you use those estimates to set your harvest, you could protect the spawning stock.

Has that idea caught on?

Some people are experimenting with it. But sometimes it takes a while for the management to catch up with the science.

What has been the effect of the herring fishing ban? And what comes next?

The [herring] stock is recovering but [only] after reaching precariously low levels of less than 5,000 metric tons two winters ago. The mammal and fish predators can easily take that much in a year, so recovery was in doubt. Last year was the first positive recruitment and this year looked even better.

If they allow these young fish (3 and 4 year olds) to mature and grow for a few years without a fishery, say until they are 6 and 7 years of age, they might be able to sustain a fishery again. But I would probably favor the elimination of purse seining [catching fish by using a type of encircling net]. Purse seiners are so effective at catching herring, and they handle such large quantities of both small and large fish, that they are difficult to manage. Also, the seiners tend to “high grade,” or repeatedly seine up small fish in order to catch the more illusive and valuable larger fish. High grading is extremely tough on the small fish and there is good evidence to suggest it can damage the population. [Once the ban is lifted,] I would have the fishers use gillnets, which are much more selective for size of fish and less likely to catch more than the quota.

You’ve recently moved to Florida — what are you working on now?

I’m looking to set up coastal fisheries monitoring programs around the world using the approach and techniques we developed in Alaska. We showed in Prince William Sound that we could learn a great deal about how the ecosystem [operates] by studying some major populations. Other people and places could benefit from that approach too.

In the 1950s, a blindfolded dolphin forever changed the way people think about these seagoing mammals. At the time, researchers weren’t sure why dolphins produced their remarkable range of clicks, trills, moans, and squeaks, which are documented on NATURE’s Dolphins: Close Encounters.

Dolphins use sound to identify objects. But some guessed that the animals might be using sonar, in which sound is bounced off a target, to “see” their prey and surroundings. In theory, the idea made perfect sense, because while light doesn’t penetrate very far into the sea, sounds can travel great distances. The problem was finding proof that dolphins actually used echolocation.

The late Kenneth S. Norris, one of the pioneers of dolphin research, found a way to obtain the evidence. Working with a captive bottlenose dolphin at California’s Marineland aquarium, Norris and his colleagues devised a way to blindfold the dolphin and test its ability to locate and identify objects. He discovered that dolphins can use sound to determine more than just distance and direction — they could even identify size, shape, and texture. One dolphin, for instance, could tell the difference between a copper sheet and one made of aluminum. More recent studies have shown that dolphins can discriminate between metal cylinders that differ by just 1/3000th of an inch in diameter. Researchers now know that dolphins generate the high-frequency clicks used in echolocation in their nasal sacks, which are located behind an oil-filled organ in the head called the melon. The melon acts like a lens, focusing the sound into a narrow beam that is projected in front of the animal.

Dolphin biosonar is remarkably sensitive. After the sound strikes an object, the echo is picked up by a specialized bone in the jaw, which transmits the signal to the brain. Not surprisingly, dolphin brains feature a huge sound-processing center.

“In an ocean full of dullards, what good is such a brain?” Norris once joked. “Certainly complicated nervous machinery is not needed for concourse with jellyfish, sea cucumbers, and sponges.” The answer, he noted, was that dolphins use sound to sense the world around them.

But not all dolphins produce the same kind of biosonar pulses. Some use so-called “broadband” pulses, which spread the energy out over a wide range of frequencies. This is like broadcasting on many radio stations at the same time. Others, in contrast, produce very narrow, focused pulses.

These differences have prompted some debate about exactly how dolphins use their biosonar. Some scientists believe it is primarily a tool for finding food. Others believe the animals use it to navigate. Whatever the answer, researchers agree that dolphin biosonar is a remarkably sensitive sensory system that people may never truly understand.

Norris had a similar view. “It is very hard for us to imagine sensory systems and processes we do not have,” he said in a 1980 interview. “It’s a bit like a man from outer space tapping into the [phone system] and trying to make sense of all the beeps and sounds.”