It’s peak season for kohola, the North Pacific humpback whales that migrate from Alaska to Hawaii each winter, and new guidelines ask that watercraft slow down to 15 knots during general travel in whale season and 6 knots when within 400 yards of the species, which is protected under the Marine Mammal Protection Act.

It is illegal under federal law to approach within 100 yards of humpback whales, but research shows the animals react when boats come within 400 yards at speeds higher than 6 knots (7 mph), Jens Currie, chief scientist with Pacific Whale Foundation on Maui, said Monday at a news conference organized by the state Department of Land and Natural Resources and National Oceanic and Atmospheric Administration.

“We’re seeing responses of increased swim speeds, decreased dive times and (changed) lines of travel of those whales when vessels are coming in to do a whale watch,” indicating the animals were not comfortable, Currie said of research which, since 2013, has observed and tracked more than 400 humpback whales off Maui.

These reactions, including changed respiration rates, waste precious energy the whales need in order to successfully mate and reproduce in Hawaii waters, where they fast, then survive the 3,000-mile swim back to their Alaska feeding grounds, Currie said.

Also Monday, NOAA’s Division of Fisheries announced that a humpback whale calf found dead near Oahu’s Wailupe peninsula Feb. 6 had probably been killed by a boat.

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 news release.

Vessel-whale collisions occur every year in Hawaii, especially from November through May, when an estimated 10,000 humpback whales migrate to the islands, said Jeannine Rossa, who works for DLNR’s Division of Aquatic Resources.

“(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 for the Hawaiian Islands Humpback Whale National Marine Sanctuary.

But in addition to averting strikes, it was important to avert harassing whales, Currie said, noting it is illegal under the Marine Mammal Protection Act to harass the animals, “(defined as) altering their natural behavior,” which is what researchers observed.

The new guidelines apply to any size of watercraft, he said, noting that studies had been done in various regions on bigger vessels such as containerships, and “we really wanted to focus on that smaller-size vessel and do it locally, here in Hawaii.”

With a majority of vessel types, slowing down to 6 knots or less produced a noise equivalent to ambient ocean noise, mitigating negative acoustic effects, Currie said.

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

According to the sanctuary website, research shows that collisions with whales by vessels traveling above 10 knots cause more whale deaths and serious injuries than collisions at slower speeds.

“When a whale strike does happen, we know we can do better,” Rossa said of the scientific and boating community.

An estimated 40 whale watch companies operate in Maui, according to the DLNR news release.

“Whales are more likely to stick around for people to see them if the vessels they’re on are moving at slow speeds,” said Blake Moore, chief operating officer for PacWhale Eco-­Adventures, which helped with the research.