A female Hawaiian monk seal died Tuesday on a rocky shelf at Hanauma Bay while researchers were trying to tag it, marking the fourth time one of these endangered marine mammals died during a capture attempt in three decades.

Five researchers from the National Oceanic and Atmospheric Administration Fisheries were trying to place a net over the monk seal's head when it reared back and struck its head on a rock. It suffered severe fractures, which resulted in its death, said George "Bud" Antonelis, chief of the protected species division of the National Marine Fisheries Service's Pacific Island Fisheries Science Center.

There were no visible wounds; the injuries were internal.

"We are deeply saddened by (this) death," Antonelis said at a press conference yesterday. "We will review our recovery protocols to ensure such an accident never happens again."

In the past 30 years, more than 4,000 Hawaiian monk seals have been captured.

This incident marked the first time that a monk seal has died from injuries sustained from striking its head on a rock, Antonelis said.

The last time a monk seal died during a capture attempt was about 10 years ago.

"This is extremely unusual," Antonelis said.

The female monk seal was about 2 or 3 years old and weighed between 170 and 200 pounds, said Charles Littnan of NOAA's Pacific Islands Fisheries Science Center, who was at the capture attempt on Tuesday.

Reproductive age for these marine mammals is between 5 and 12 years old. The monk seal was not pregnant when it died.

Researchers were trying to capture the seal to place a satellite tracking device on its back. The device would allow scientists to monitor its behavior and demography, Antonelis said, and thereby help them better understand the factors affecting the seals' survival.

Currently, the Hawaiian monk seal population is about 1,200 worldwide, the lowest ever recorded.

"We're trying to learn as much as we can about these monk seals," Antonelis said.

Researchers conducted a four-hour necropsy of the monk seal on Tuesday to determine the cause of death and to learn more about the endangered animal.

They looked at its stomach contents, checked its blood values and screened for various diseases.

Aside from the head injuries, the monk seal was healthy, Littnan said.

"Even in its death it will provide us a lot of good information," Antonelis said.

Reach Catherine E. Toth at ctoth@honoluluadvertiser.com.