A humpback whale calf found dead near Oahu’s Wailupe peninsula Feb. 6 was probably killed by a boat, the National Oceanic and Atmospheric Administration Division of Fisheries announced today.

The 12-foot-long, male calf, which was less than a week old with evidence of nursing, had suffered traumatic brain injury that “may have been caused by an impact of concussive force,” which could have been produced by another whale but was “more likely caused by a vessel strike,” NOAA said in a press release.

Vessel-whale collisions occur every year in Hawaii, where kohola, or humpback whale, season runs from November through May, when an estimated 10,000 of the cetaceans migrate from their Alaska feeding grounds to mate and calve in Hawaiian waters, Jeannine Rossa, an officer with the state Department of Land and Natural Resources Division of Aquatic Resources, said in a separate press conference, announcing new recommendations for boat speeds around whales, held by DLNR today.

“(Vessel strikes) tend to happen more to juveniles and calves, which spend more time at the surface,” added Rossa, who serves as state co-manager with NOAA officials for the Hawaiian Islands Humpback Whale National Marine Sanctuary.

Federal law prohibits approaching within 100 yards of humpback whales in Hawaiian waters, but the guidelines released today ask boaters to reduce speeds to 15 knots (17 mph) while in transit and 6 knots (7 pmh) within 400 yards of the whales.

That’s because years of research tracking a total of more than 400 humpback whales from Maui shoreline cliffs showed when vessels came within 400 yards of the whales, they reacted by swimming faster and more at the surface said Jens Currie, chief scientist with Pacific Whale Foundation.

“Harassment is (defined as) altering their natural behavior,” Currie said, adding that was what researchers observed was happening when boats came within 400 yards of humpback whales.

The recommendations were voluntary rather than mandatory, Rossa said, for two reasons: “It’s really difficult to enforce, and we have an awesome on-water boating community.”

“They all want to do the right thing in the boating community; we don’t need to go the route with policy,” Currie said.

The new recommendations were developed and released in collaboration withvoluntary guidelines forn the vicinity of humpback whales were released today by DLNR in collaboration with NOAA, Pacific Whale Foundation and the state.

The dead whale calf off Oahu was removed from the reef Feb. 6 by teams from NOAA, the University of Hawaiʻi Institute of Marine Biology, Hawaii Marine Animal Response, and native Hawaiian practitioners; the body was transported to the Health & Stranding Laboratory, where the postmorten exam was conducted.