By Zachary Fagenson

MIAMI (Reuters) - A study by University of Miami scientists says mahi-mahi, a popular fish among restaurants and anglers and exposed as infants to oil from the 2010 BP Deepwater Horizon spill, swim nearly half as fast as their unaffected counterparts.

"The worry is that if you have reduced swimming performance you’re going to be less effective at capturing prey, and less effective in avoiding (predators)," said Martin Grosell, a professor at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

Researchers treated mahi-mahi embryos and young fish with oil collected from near the damaged wellhead and from the gulf’s surface. Individual fish were then transferred to clean water for at least 25 days before their swim speeds were tested in a kind of aquatic treadmill.

The speed of the young, inch-long fish, thought to be among the fastest on the planet at about five body lengths per second, "dropped by about 37 percent," Grosell said.

Assessing the extent of the damage on the population, however, is proving difficult as few statistics are kept on gulf mahi-mahi.

Scientists in 2000 failed in an attempt to count the number of brilliant, green-blue fish, also known as dolphinfish, living between Florida’s west coast and Texas, according to Steve Branstetter, a scientist with the National Oceanic and Atmospheric Administration (NOAA).

“No management actions have ever been needed for this species,” the agency’s website said.

Grosell and scientists from across the country partnered with NOAA following the disaster to study the effect of the 87-day-long spill that dumped an estimated 4.9 million barrels of oil into the Gulf of Mexico.

Researchers said they used oil concentrations similar to those measured in the gulf shortly after the spill, which oil company BP disputed on Friday.

“The study does not provide any evidence to show that an effect on that group of fish would have had a population-level impact,” BP spokesman Ryan Jason wrote in an e-mail.

In March, researchers from the University of Miami and Stanford University released test results showing oil caused heart deformations in young Bluefin tuna and a variety of other species, and may have led to low catch numbers in the years that followed the spill.

(Editing by David Adams; Editing by Grant McCool)