Cold Water Signals Along West Coast Could Help Assess Whale Entanglement Risk

NOAA Fisheries scientists have found telltale changes in cool, highly productive water upwelling along the West Coast. These changes can provide "an early warning system" signaling greater risk of humpback whales getting entangled in fishing gear.

The wind-driven upwelling of cold, deep-ocean water fuels the West Coast's rich coastal ecosystem. It attracts marine life to the cool corridor where the nutrient-rich water rises along the coast. The problem is that whales drawn to plentiful prey become entangled at high rates in gear from commercial fisheries attracted to the same productive waters.

Scientists found that entanglements increase especially when the cool water corridor shrinks in what they call "habitat compression." This draws the whales into even closer proximity to fishing gear with lines to the surface.

California and other West Coast states have restricted lucrative fisheries for Dungeness crab in recent years to avoid entangling protected whales. NOAA Fisheries and others are also exploring new fishing technologies that could reduce entanglement risk.

Earlier research in 2023 found that changes in the cool waters could forecast marine conditions that increase risk of whale entanglements up to 6-12 months in advance. NOAA Fisheries and NOAA's Physical Sciences Laboratory produced draft forecasts based on the findings. New research in PLOS Climate confirms the forecasts are accurate and could provide valuable intelligence on how to reduce entanglement risk so fisheries can continue their important contributions to the coastal economy.

Earlier research had suggested a connection between the ocean changes and entanglement risk; the new study confirmed the link and showed how to measure the changes. California fishermen welcomed the research insight, saying it could better help assess the risk of entanglements. They noted that fishing crews have local insight-such as details of currents-that can help interpret the research findings for certain areas of the coast.

"That is one piece of information, but a lot of other factors are also at play," said Dick Ogg, who has fished for crab and other species off Northern California for almost 30 years. "We need to make decisions based on the whole picture."

Forecasts Prove Accurate

Scientists recounted when advance notice may have been helpful. For instance, "forecasts available in January 2024 correctly predicted the low thermal habitat conditions observed later that year, which coincided with a rise in entanglements" to 31 humpback whales, they wrote. More than 30 humpbacks have been confirmed entangled along the West Coast in each of the past few years. Entanglements have been running high since a marine heatwave from about 2013 to 2016 warmed West Coast waters.

The scientists measured changes in the corridor of cold thermal waters along the coast with what they call the cumulative Habitat Compression Index. It reflects changes in the extent of the cool, nutrient-rich, deep-ocean water along the West Coast over the course of the year.

When the habitat narrows, for example, it can push whales and fishing gear even closer together. Scientists believe this also includes large numbers of derelict crab or other fish traps lost during past seasons. That can further elevate entanglement risk.

"This is a way to assess the compression of that habitat, which increases the risk of conflicts," says Jarrod Santora, a research scientist at NOAA Fisheries' Southwest Fisheries Science Center and lead author of the new research. "This is not a solution to entanglements, but can help us foresee when the risk will be greatest."

He first recognized the importance of the cool coastal habitat during earlier research in the aftermath of the 2013-2016 marine heatwave known as The Blob. His research found that higher temperatures from the heatwave compressed the cool coastal habitat. This pushed species together with fishing gear and drove a sudden spike of more than 60 whale entanglements in 2016. Entanglement numbers had previously hovered around 10 a year, but have remained elevated at much higher levels since the marine heatwave.

Heatwaves Compress Habitat

A continuing series of marine heatwaves since then has continued pressure on the cool coastal habitat, sometimes compressing it enough to concentrate entanglements at certain times of year. Santora recognized that there may be a relationship between the two.

Humpback whale numbers have also increased substantially off the West Coast as the species has recovered from near-extinction during the whaling era. Santora set out to test whether there are more entanglements simply because there are more whales, or whether changes in ocean temperatures or other factors may be compounding the risk.

The answer is that whale population numbers do contribute to an increased trend in entanglements over the long term. But they cannot explain sharp increases or decreases in entanglements such as the jump recorded during The Blob. Those more sudden changes matched up closely with changes in the availability of cool thermal habitat along the West Coast.

"This strongly suggests that habitat compression exacerbates reported entanglements significantly beyond what is expected from changes in population abundance alone," the scientists reported. They recommended the findings be incorporated into modeling that helps assess the risk of entanglements in the crab and other fisheries.

"The Habitat Compression Index helps show when ocean conditions are likely to push whales and fishing activity into the same places at the same time," said Elliott Hazen, a research ecologist at the Southwest Fisheries Science Center and coauthor of the new research. "Flagging higher risk periods before waiting for entanglements to occur, gives fishers and managers a chance to plan, adapt, and avoid problems before they happen-reducing entanglement risk and supporting more sustainable fishing going forward."

The rise in entanglements has also matched a sharp increase in anchovy along the California Coast, the largest increase in the region since the 1970s. Anchovies drawn to the cold coastal corridor have helped attract humpback whales. The whales then become entangled in fishing gear set for crab and other bottom species along the continental shelf. Whale watch tours along the West Coast and wider public awareness may also help spot entanglements that might have previously gone unreported, the scientists stated.

The scientists also expressed concern that derelict fishing gear lost at sea in past years adds to the risk. Many California vessels participate in both the crab and salmon fisheries. While fishing for salmon, vessels often help identify and remove derelict crab gear that snags their lines. That hasn't been happening recently, though, because salmon fishing has been closed in California for the last 3 years.

Ogg, the California fisherman, said solutions demand "ideas outside the box." However, fishing crews and fishing managers should weigh signals from habitat compression with other information. It may not carry the same weight when fewer whales are present, for example. "It is a piece of information, not a decision maker," he said.

Avoiding Increased Risk

The compounding factors make it more important to identify ways of reducing entanglement risk, researchers said. They recommended incorporating the Habitat Compression Index into risk assessments for fisheries to help avoid the riskiest conditions and times of year.

"The 31 reported entanglements in 2024 (relatively high compared to the record) and overall elevated entanglement risk, likely could have been forewarned by using these forecasts," the research found. "Furthermore, short-term forecasts of the cumulative Habitat Compression Index from December into spring may benefit strategic decision making about changes in habitat compression during a time period when humpback whales are migrating back to California waters and when the spring Dungeness crab fishery is operating."