According to the pilot in the public use helicopter, he was conducting a wildlife anti-depredation flight, and maneuvering at a low altitude with two net gunners located on the left and right in the aft cabin. The pilot reported that he entered a left bank in pursuit of a deer. The deer was located, "between our 8 and 9 o'clock position, and the net gunner made the net shot at the deer." The pilot observed the net miss the deer and simultaneously heard an unusual "whooshing noise" coming from the helicopter. He made a precautionary landing, and during the shutdown, he noticed what he described as an unusual discoloration of the main rotor disc. An examination of the main rotor blades showed that one of the blades had a large hole in it, about 5ft from the blade tip. The net that was fired at the deer was recovered and was found to be missing one of the weights that are normally attached to the corners of the net. The helicopter sustained substantial damage to a main rotor blade. The pilot reported that there were no preaccident mechanical malfunctions or failures with the helicopter that would have precluded normal operation.