It’s been a grim new year for whales off the coast of Virginia, with a flurry of casualties that has experts dumbfounded.

In the month of February alone, four humpback whales turned up dead. Three washed ashore — the first near Hampton Roads, the second on the Eastern Shore and the third on Cape Henry — all apparently after being struck by large ships, and two with apparent propeller wounds. A fourth badly decomposed carcass washed up near Chincoteague. And these weren’t even the year’s first fatalities; a badly decomposed humpback had washed up in the Cherrystone area of the Eastern Shore in January.

Baffled scientists are continuing to investigate the cause of this unusually rapid succession of deaths. But it has brought a sense of disquiet to those studying and working to preserve Megaptera novaeangliae, which many regard as the world’s most charismatic whale species. Their long pelvic fins and unusual comfort interacting with humans make them a favorite of whale watchers.

The number of victims found this winter is “quite unusual,” said Alexander Costidis, stranding response program coordinator with the Virginia Aquarium & Marine Science Center in Virginia Beach.

“Mortalities are typically spread out more across our humpback whale season,” Costidis, explained, “which typically runs from about November through April. What makes it even more unusual, is that three of those four are believed to have been killed by vessel collisions.”

Before last year, Virginia averaged 2.3 large whale deaths annually for a decade, according to a report by the aquarium’s marine mammal stranding program. In 2016, Costidis said, there were four fatal strandings: a minke whale on the Eastern Shore, two unrecovered floating humpback whales and a dead humpback first seen in Virginia waters that eventually washed up in the Outer Banks. The cause of death remains unknown in the first three; the fourth was determined to be caused by entanglement in fishing gear.

Experts continue to investigate the circumstances around the recent whale deaths. While ship strikes apparently figured in most, there’s no easy solution to preventing more. Costidis said that, “Since we do not anticipate shipping traffic to decrease any time soon and the whales typically stay in this area well into April or longer, there is a distinct possibility we may see additional mortalities.”

Humpback whales spend the winter in tropical waters, then migrate northward as the weather warms. But along the way, they’re drawn close to Virginia’s coast to feed. Unlike other baleen whales, which consume krill or tiny fish, humpbacks can eat larger fare such as menhaden. Their feeding forays take them into waters as shallow as 18 feet deep, with channels about 60 feet in depth. Those near-shore waters happen to be marine super-highways for shipping.

While the number of dead humpbacks this year has been unusual, it’s likely, Engelhaupt said, that those washing ashore may not reflect the full extent of the casualties, as mortally wounded animals could swim out to sea before dying.

Though some humpback whale populations elsewhere are considered threatened or endangered, the Atlantic population is not, and is thus not protected under the Endangered Species Act. Even so, it is still covered by the federal Marine Mammal Protection Act, which generally prohibits killing, harming, capturing or harassing them in U.S. waters.

Dan Engelhaupt, with the engineering firm HDR, Inc., has been contracted by the U.S. Navy to help it understand the whale mortality. Engelhaupt, who serves as program manager for marine species management and monitoring, said that although Norfolk is home to the world’s largest Navy base, military traffic constitutes only 10–20 percent of the coastal shipping traffic in the area. The rest are tankers, cargo and container ships, many of them hundreds of feet in length.

Preventing ship-whale collisions is complicated, according to Engelhaupt. Although whales have acute underwater hearing, he suggested that they might not be able to pick up the sounds of a rapidly approaching cargo ship soon enough to move out of the way. The underwater background noise of Norfolk and other busy ports is such that the whales may grow inured to it and be unable to distinguish the threat of an oncoming ship, he explained.

Navy ships are required by military directives to take precautions to avoid whale collisions. Observers are supposed to be stationed on deck to keep a lookout, and captains have orders to slow, divert or even stop if necessary to avoid hitting one. The aquarium’s Costidis said he doesn’t believe commercial vessels are required to have observers looking out for marine mammals, so it’s not clear if they take such precautions.

Even if they do, the circumstances are ready-made for trouble, according to Regina Asmutis-Silvia, executive director of the nonprofit group Whale and Dolphin Conservation.

“Large ships cannot slow or alter course easily, and whales cannot necessarily hear oncoming ships,” she said. The front of the ship can block the engine noise coming from the propeller in back of the ship, she suggested, a phenomenon known as acoustic masking or Lloyd’s Mirror Effect. For that reason, she said, “the best solution found to date is simply to have areas where speeds are limited so that voyages can be preplanned.”

Under a federal rule first imposed in 2008 to protect endangered right whales, all ships 65 feet or longer are limited to speeds of 10 knots or slower within 20 miles of ports. The speed limit had originally extended out 30 miles, but the area was reduced to ease the burden on shipping.

There doesn’t appear to be any quick technological fixes to this problem.

“There have not yet been any technology solutions to significantly reduce risk,” Asmutis-Sylvia said. “Experiments with alarms, forward-facing sonar, thermal detection, etc., have not proven useful.”

In fact, acoustical techniques to scare whales away from oncoming ships might be counterproductive, Asmutis-Silvia said.

“Alarm research on North Atlantic right whales showed that the whales who reacted to the alarm did so by surfacing, thereby putting themselves at a greater risk,” she said. Even if they got out of the way of ships, she noted, they might instead wind up getting entangled in fishing gear. Disrupting the big marine mammals by itself “can reduce fitness and increase stress as well,” she said.

Another option, Asmutis-Silvia suggested, would be to have ships avoid whales’ seasonal feeding grounds altogether.

Asmutis-Silvia said that as far as she knows, no drift analysis has been done on the whales killed off the Chesapeake to determine where they were struck. That could be key, she suggested, to learning how to prevent future collisions.

“If they were not struck within the current speed restriction zone, it may…call for a need to increase the zone. If they were struck within the zone, then it is important to review data to determine if vessels are complying with the speed rule. If they are (complying),” she concluded, “then it may be time to consider mitigation specific to feeding humpback whales.”