As more companies offer travelers the chance to swim with wild dolphins and other marine mammals, scientists are increasingly critical of the practice. Debra A. Klein dives into the murky waters of the aqua-tourism industry

"They're right below you," the skipper shouts from the catamaran that motored me and 11 others off Makaha Beach for an early-morning swim with wild spinner dolphins. "Look down!" A rush of warm-blooded torpedoes shoots beneath my dangling, finned feet.

Back on the boat a half-hour later, members of our group recall the encounter as "awesome" and "a once-in-a-lifetime experience."

But a growing number of researchers say that swimming with wild dolphins can be harmful to the animals, and Congress is considering legislation that would make the popular practice illegal in the United States.

"Swimming with dolphins in the open sea is a thrill for people, but it often disturbs the animals," says marine biologist Randall Wells, of the Mote Marine Laboratory, in Sarasota, Florida. "The encounters increase the dolphins" stress levels and cause them to alter their normal behaviors, which could place them at risk."

Although no long-term studies on the effects of dolphin-encounter programs in the wild have been carried out, short-term impacts have been closely observed by researchers, who say that man's forays into dolphin habitats are having a detrimental effect on the animals.

Ania Driscoll-Lind, a marine biologist at the Big Island–based Kula Nai'a Foundation, has noted a recent decline in the number of dolphins that use Hawaii's Kealakekua Bay as a place to rest. On weekends, dozens of kayakers and swimmers can be found in the bay, hoping to spot and even swim with a wild dolphin. Driscoll-Lind says that as a result of these intrusions, pods of as many as 300 dolphins have dispersed into smaller groups along the coast. For social creatures like dolphins, she says, this could signal a harmful change in their normal behavior patterns.

Similarly, other researchers have reported that in areas frequented by boats and swimmers, dolphins tend to alter their daily cycles of hunting and resting. Nocturnal spinner dolphins in Hawaii, for example, use shallow bays for protection and to recharge their batteries during the day. Scientists say that kayakers, motorboats, and swimmers in these areas disrupt important dolphin biological cycles. "These are critical habitats," says Naomi Rose of the Humane Society of the United States. "Swimmers are basically bugging them while they're in their beds."

According to Rochelle Constantine, a behavioral ecologist at the University of Auckland, "a decrease in resting behavior may affect foraging efficiency and vigilance over caring for their young or their ability to escape predators." As a result of her research in New Zealand's Bay of Islands, dolphin swims there are now permitted only at certain times of day, allowing the mammals an opportunity to rest. Constantine's research also led tour operators to adopt new regulations, which include dropping swimmers to the side of dolphin pods rather than in the midst of them, thus giving the animals a choice of whether or not to approach humans.