A group of about 15 bottlenose dolphins that have been swimming in New Jersey's Shrewsbury River since mid-June now appears to be moving farther inland. Local marine officials had originally planned to coax them out of the river, but they have shelved those plans for now. How would you coax a dolphin back into the ocean, anyway?
With nets or noise—but both methods can be dangerous. Nets have been used to save dolphins before, usually by deploying divers to ensnare the dolphin, removing the animal from the water with a special stretcher, and quickly transporting it to safety. But capturing the dolphin is not always an easy feat, and there's also the possibility the animal will get entangled in the webbing and drown.
An alternative is to create a barrier upstream using boats and then bang pipes or use high-frequency pingers to scare the dolphins in the opposite direction. But this method has downsides, too. Because dolphins are so sensitive to sound, the loud noises can create a highly stressful situation. In extreme cases, the stress can even send a dolphin into shock or cause it to beach itself. Last year, an effort to rescue a group of common dolphins in Long Island's Northwest Creek using this method had mixed results: Eight dolphins were safely corralled into the Atlantic, but 11 could not be saved.
Despite these risks, there are times when rescuing a dolphin is worth the danger. Keep a bottlenose dolphin too long in fresh water, and the process of osmosis can start causing serious health problems. (River dolphins do exist, but they are classified in a different family from ocean dolphins and aren't found in the United States.) After about three days in freshwater, a bottlenose dolphin's skin would begin to swell, and its corneas would become cloudy. Soon afterward, the animal might develop skin lesions, eventually leading to infections that can spread throughout its body. (To see a case study of one bottlenose dolphin rescued from a Florida river, click here.)
Fortunately for the dolphins in New Jersey, they have been swimming in the Shrewsbury and the neighboring Navesink—two bodies of water that are better described as estuaries rather than rivers. Estuaries contain a brackish mix of freshwater and seawater that is probably salty enough to keep the dolphins in good health: Veterinarians say dolphins can get in trouble when salinity levels drop below 15 parts per thousand; measurements near the bridge the dolphins recently passed under were more than 25 parts per thousand on Thursday. These particular dolphins may also be well-suited to water with a slightly lower salinity content, since they appear to be part of a stock that sticks closer to the land even when they aren't lost. (Marine biologists believe the dolphins may have been attracted inland by schools of menhaden, a fish the animals prey on.) But no matter the salt content, these estuaries could be dangerous come winter: Back in 1993, a group of four dolphins that stayed in the frozen Shrewsbury River until December died after rescuers tried unsuccessfully to free them from the ice.
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Explainer thanks Gregory Bossart of the Harbor Branch Oceanographic Institution; Teri Frady, Mendy Garron, and Larry Hansen of the National Oceanic and Atmospheric Association; and Daniel Odell of the Hubbs-SeaWorld Research Institute.