Can dolphins survive the winter in US rivers?

A group of bottlenose dolphins have been confounding humans since they took up residence in two rivers near the Jersey shore six months ago. Now that it’s winter, some people are worried they’ll never make it out.
Three dolphins have died out of the original group of about 15 that spent the summer and fall in the Shrewsbury and Navesink rivers, waterways just north of Asbury Park.
Federal wildlife experts say the remaining dolphins are healthy, and should be able to make it through the winter if they choose to stay. They cite the cases of dolphins that successfully spent winters in Massachusetts, Virginia and even northern Scotland.
But some animal advocates worry the dolphins will meet the same fate as four that drowned in the Shrewsbury River in 1993 when ice closed in on them, or the 26 dolphins killed by a sudden freeze in 1990 in Texas’ Matagorda Bay.
“It would seem to me that the natural habitat for dolphins in the winter when it gets cold is much farther south in warmer waters,” said US Republican Frank Pallone Jr. “Isn’t it stressful for them to be in this colder environment? Since they are mammals, what happens if the ice freezes over and they can’t breathe?”
The National Oceanic and Atmospheric Administration, which has jurisdiction over the dolphins, doesn’t plan to intervene unless the dolphins appear to be in imminent danger.

“This is a normal sort of way of life for these critters,” said David Gouveia, marine mammal programme coordinator for the agency’s fisheries service. “We’re optimistic that things are going to work out just fine.”
Others are not as confident.
“We have seen the disastrous consequences when dolphins remain in the area during the winter months,” said state Senator Sean Kean, R-Wall. “Now it’s up to NOAA to determine a course of action to try to get the dolphins back to the ocean before it’s too late.” Looming over this debate is the dismal history of dolphins that have wound up in the Shrewsbury in previous years. In at least two instances, dolphins lingered too long in the riv
er and died when rescue attempts went awry. Still, dolphins can handle cold weather, said Randall Wells, dolphin research programme manager for the Chicago Zoological Society.
“There are examples of the Navy using dolphins in very cold situations with ice around, and a naturally occurring population of dolphins off the coast of northern Scotland, where ice reaches into the water and snow is in the mountains nearby, and these animals get by fine,” he said. “Blubber is a pretty amazing substance in these animals; it’s able to maintain body temperatures quite well.”
Virginia’s coastal water is usually home to bottlenose dolphins from April through November, when they
leave for warmer climes south of Cape Hatteras, N.C. But during the winter of 1996-97, one bottlenose dolphin stayed in Broad Bay off Virginia Beach, making it through 43-degree water in February. It was seen the following June with a healthy calf.
Five others joined the pair that October, and stayed through the winter of 1997-98. In March 1998, they were all examined and found to be healthy.
There are other success stories, but cold is not the only problem winter brings to dolphins. There is also the matter of finding food.
“The food may be plentiful now, but it won’t be plentiful in February,” said Andrew Mencinsky, executive director of the Surfers’ Environmental Alliance who lives in Sea Bright and watches the dolphins almost every day.
Scientists had hoped that when the animals’ summer food source, menhaden, left the area in late October or early November, the dolphins would follow them out to sea.
Instead, the dolphins have switched to other, smaller fish such as alewife, a species of herring that they continue to consume. In four of the last five years, Mencinsky said, the Shrewsbury River has frozen solid. Teri Rowles, lead veterinarian for the Fisheries Service and leader of the National Marine Mammal Health and Stranding Response Programme, predicted the Shrewsbury dolphins will be able to keep part of the river open by their constant surfacing to breathe