The number of endangered Hawaiian hoary bats killed by spinning wind turbines on an Oahu wind farm is on pace to far exceed U.S. Fish and Wildlife Service projections.

The limit for accidental bat deaths, known as incidental takes, over a span of two decades at Kawailoa Wind LLC was set at 72. But in less than four full years of turbine operation, the death count is already approaching the halfway mark.

The number of hoary bat deaths has been confirmed at 29 since operations got underway at Kawailoa in November 2012. The facility is on the North Shore on land owned by Kamehameha Schools.

D.E. Shaw Renewable Investments LLC, the owner of Kawailoa Wind and the company that contracts Sun Edison to operate the wind farm, declined comment.

Wildlife service spokesman Brent Lawrence said the owner plans to seek an increase in the agency’s acceptable limits for hoary bat deaths.

Hawaiian hoary bats, or Lasiurus cinereus semotus, are the only native Hawaiian land mammal. They roost in trees with dense foliage and eat night-flying insects, such as moths, beetles and termites, according to federal officials.

In an effort to reduce incidental takes, the wildlife service said a limit of 11 mph for turbine wind speed from March through November has been expanded to nearly year-round.

“This modification to operations has proven effective for reducing hoary bat fatalities on the mainland,” Lawrence said.

He said the U.S. Department of Energy is also researching other ways to reduce deaths.

“We are hopeful that bat fatalities will be dramatically reduced in the near future, once deterrents are proven effective,” Lawrence said.

Among bat species, Lawrence said, hoary bats seem to be most affected by wind turbines. He said the agency suspects that certain habitat characteristics in Kawailoa could leave the bats especially vulnerable, compared with other wind turbines in the state.

Wind turbine ventures have been successful in offsetting deaths of certain endangered species in Hawaii, such as the nene , by providing protected pens where their numbers have been allowed to increase.

But wildlife service officials acknowledge they don’t know much about the nocturnal native hoary bat, or opeapea, which was listed as an endangered species in 1971. It weighs about half an ounce and has a wingspan of between 10.5 and 13.5 inches, according to federal officials.

Lawrence estimates the bat’s population at hundreds to thousands, based on limited and incomplete information. The wildlife service does not know with certainty what the bat’s preferred habitat is for breeding, roosting or foraging.

“The Hawaiian hoary bat is an incredibly cryptic species for which little is known,” Lawrence said. “They are difficult to capture and impossible to mark and recapture.”