When humpback whales began to appear in large numbers off the California coast in 2015 and 2016, people celebrated the comeback of the whales after a near-miss with extinction.

However, the excitement was quickly met with new worries – the whales increasingly got caught up in fishermen’s crab ropes. By 2016, there were more than 50 recorded entanglements that left whales injured or killed. Whales got ropes tangled around their mouths, making it difficult for them to eat. Crab lines cut through tissue and caused infections.

Although whales and fishing had coexisted for decades, this was a new problem. So what was driving it?

A new study published in the journal Nature Communications points at climate breakdown as a factor in the mass entanglements.

When the situation was unfolding in 2015 and 2016, it surprised most people, but not Jarrod Santora, an ecologist at the University of California, Santa Cruz and the lead author of the paper.

Santora was studying the ecosystem effects of the marine heat wave, known as “the blob”, that was happening off the coast of California at the time. Heat waves alter the ocean’s upwelling – the process in which deep, cold, nutrient-rich water rises to the surface. The upwelling in 2015 and 2016 shrunk to just a narrow band along the coast, causing organisms to cluster there. Due to a heatwave-related decline in krill, whales switched to feeding on anchovies in shallower and shallower waters. In addition, the crab fishing season – an $88m industry on the US west coast – had been delayed from November to April, and came to coincide with the whales’ presence.

“It’s like when only one restaurant is open in a town, and everyone ends up piling in around that restaurant,” Santora explains. “And that can cause conflict. There were thousands of whales and thousands of crab pots going out, so was a perfect set of events to cause entanglements.”

In September 2015, conservationists, wildlife managers and fishermen came together to form a working group to address the issue. Santora joined the group at a time when emotions were running high on both sides of the issue. “It was a great opportunity to bring in a scientific view of long cycles in the ocean,” he said. He began to work as an embedded scientific advisor, spending three years studying and listening to the concerns of the group’s members.

Entanglements are down now, and the scientists are helping to build a data-driven website that can predict when and where the whales are likely to be feeding off the west coast. At the same time, crab fishermen are experimenting with new ways to log their movements using solar-powered vessel tracking information systems that record boat positions every few seconds. And there’s the development of ropeless crab traps, which can be dropped and come up on their own. These technologies cost two to six times more than traditional crabbing gear, making them a hard sell in an industry with already razor-thin margins.

Crab lines are far from the only challenges humpback whales face in coastal waters. It’s clear that climate breakdown driven by human activity is causing our oceans to get warmer – marine heat waves have doubled in frequency since 1982, Santora says. “Increasing climate variability means increased uncertainty. This is the new normal: not having any vision of what is ahead. And that’s really scary.”

But for now, the whale populations off the west coast are thriving. “We got here because of successful management and acts put in place in the 1960s and 1970s like the Endangered Species Act and the Marine Mammal Protection Act,” says Santora. “The grey whales are the highest they’ve ever been, and humpback whales have remarkably rebounded.” Seals and sea lions are also close to their maximum numbers, and all those bodies need to eat.

Armed with better data, Santora is optimistic the problem of whale entanglements can be solved – and that managers can be more proactive in anticipating potential issues that pop up between humans and marine creatures.



