Whale Entanglements in Fishing Gear Surge Off U.S. West Coast During Marine Heatwaves

Each spring, humpback whales start to feed off the coast of California and Oregon on dense schools of anchovies, sardines and krill—prey sustained by cool, nutrient-rich water that seasonal winds draw up from the deep ocean. 

That process, known as coastal upwelling, turns the California Current into one of the most productive marine ecosystems in the world, giving whales a chance to rebuild the energy reserves they depleted during months of fasting in their winter breeding grounds in Mexico.

But according to a new study published on Wednesday in the scientific journal, PLOS Climate, rising ocean temperatures are shrinking and redefining this critical foraging habitat, putting the humpbacks at greater risk of entanglement in fishing gear. 

Marine heatwaves weaken upwelling, reducing the amount of cold, nutrient-rich water reaching the surface. That, in turn, reduces offshore krill blooms. Humpbacks then begin to move inshore, where other prey, like anchovies and sardines tend to swarm. There, they are more likely to overlap with dangerous fishing activity and fixed gear, like Dungeness crab traps.

Scientists refer to this process as “habitat compression,” when the whales’ normal feeding grounds are squeezed into a much narrower strip of cooler water along the coast. 

NOAA authors of the study were not available for comment. But the study states: “Reported entanglements within the California Current Large Marine Ecosystem (CCLME) have increased markedly over past decades with an unprecedented record number during a persistent multi-year marine heatwave,” the study says. “Habitat compression is an underlying driver of these entanglements.”

The highest number of entanglements across the U.S. West Coast occurred in years when warmer temperatures significantly reduced their foraging grounds. Before 2014, for instance, fewer than 10 humpback entanglements were reported annually. That pattern changed dramatically in 2015 and 2016 during a prolonged marine heatwave known as “the Blob.”

The unusually large and long-lasting mass of warm water was first detected in 2013. By 2014, it had extended from the Gulf of Alaska to Baja California. It caused sea surface temperatures to rise more than 7 degrees Fahrenheit above average. 

“We saw a spike in entanglements,” said Ryan Bartling, senior environmental scientist at the California Department of Fish and Wildlife. At least 40 entangled humpbacks were reported during that period, according to the study. 

With less cold, nutrient-rich water reaching the surface, the marine food web was completely disrupted, causing massive die-offs of seabirds and marine mammals like sea lions, which were unable to find enough food. 

In 2024, there was another surge in whale entanglements coinciding with unusually warm ocean conditions linked to El Niño—a naturally occurring climate pattern that develops every few years when surface waters in the central and eastern tropical Pacific become warmer than average. 

While 31 whales were reported entangled that year, the study noted that it was impossible to determine the exact number of animals affected. Many entanglements likely go undetected. Those that are spotted are often recognized by fishers and whale watchers. According to NOAA, most of these incidents are not immediately life-threatening if they are reported in time for one of its specially trained teams to respond and cut the animal free. 

The authors, including Jarrod Santora—a research fish biologist at the institution’s Southwest Fisheries Science Center—analyzed 25 years of data to better understand how and why humpbacks are particularly impacted. 

In the past, some scientists have suggested that growing humpback populations could explain the increase in entanglements. The whales have rebounded significantly since commercial whaling was banned in the 1980s, but NOAA authors of the study rule this out as a primary driver. 

“We suggest that the elevated entanglement reports observed during and after the heatwave were not merely a function of there being more whales, but that habitat compression led to both the higher overlap of whales with fishing gear and more whales concentrated closer to shore,” the report read.

Along the West Coast, the study identifies fixed gear used in the Dungeness crab fishery as the primary source of entanglement risk for humpbacks. The fishery relies on heavy pots set on the seafloor that are connected to surface buoys by vertical lines. At least 100,000 of these lines can riddle coastal waters when the season opens, usually in late fall, Bartling said. 

Whales can become entangled in various ways. 

At times, they may encounter lines while feeding or swimming. Humpbacks can be curious and are known to rub on kelp and fishing lines that could become wrapped around their bodies, said Kathi George, director of cetacean conservation biology at the Marine Mammal Center in Sausalito, California. As the whales lunge for prey, they can also snag a line in their mouths or baleen plates. 

“If it gets in the mouth, they can’t push it out with the tongue because it’s caught in the baleen. And then they go through that kind of throwing themselves, breaching, trying to get rid of the entanglement,” George said. That movement can tighten the line and worsen the situation, she said.

In other cases, loose or floating equipment, known as “ghost gear,” can wrap around fins or flukes as whales pass by. According to the study, the Dungeness crab fishery experiences a high rate of gear loss—upwards of 10 percent each season. 

When whales become entangled, Bartling said, they often are unable to feed. It also impedes their ability to move normally and reproduce. In some cases, the gear cuts deep wounds into the animals’ flesh. Some entanglements prove fatal, he said. 

When a live whale is reported entangled, responders mobilize through a coordinated federal network. Anyone who spots an entangled whale can call a 24/7 NOAA Fisheries hotline to immediately report entangled, injured, or distressed marine life on the West Coast. They can also alert the U.S. Coast Guard, said George, who is authorized by NOAA to respond and attempt to free entangled whales. 

Specially trained rescue crews approach the affected whale in small boats. Using drones and cameras mounted on long poles, they assess how the whale is wrapped before cutting anything.

“We review the footage and talk about a plan and how we can safely approach the whale and we can cut the entanglement off of it,” George said. 

Once a plan is devised, responders attach a buoyed “working line” to help track and slow the whale and then carefully remove the gear using specialized cutting tools designed to minimize additional injury. 

Retrieved gear is analyzed to determine which fishery it came from and whether it was active or lost equipment—information that helps regulators focus on prevention, which George said is the ultimate goal. 

A key tool used in the latest research is NOAA’s Habitat Compression Index (HCI), developed under Santora’s leadership. The index measures how much cool, nutrient-rich upwelled water is available along the coast and tracks when that productive habitat becomes compressed toward shore. By quantifying shifts in the extent of cold-water feeding grounds, HCI identifies periods when whales, their prey, and possibly other marine life, may converge in nearshore waters where fishing gear is deployed. 

The study found that the index not only explains past spikes in entanglements but can forecast ocean conditions up to a year in advance. Researchers say integrating HCI into fisheries management could provide an early warning system, allowing regulators to adjust season timing, trap limits or other mitigation measures before entanglement risk peaks.

The California Department of Fish and Wildlife is already working to reduce the risk of whale entanglements through its Whale Safe Fisheries initiative. Under the program, regulators can require fleet-wide trap reductions during high-risk periods. For example, a vessel permitted to fish 500 Dungeness crab traps may be limited to deploying only half that number, said Bartling, who supervises the program. When applied across the fleet, he said, such reductions can cut the number of vertical lines in the water from roughly 100,000 at a traditional season opener by half, to about 50,000. 

The program is also pushing for increased use of alternative fishing gear that eliminates the vertical lines connecting traps to surface buoys, mitigating the threat of entanglement to whales and other marine life. “It’s a way to really reduce risk to theoretically almost zero,” Bartling said. This spring, he said, between 20 and 50 crab fishers will be “ready to go ropeless.”