Warming Ocean Waters Increase Humpback Whale Entanglements Along the US West Coast

Warming ocean waters are altering the areas where humpback whales feed along the US West Coast. As marine conditions change, these enormous animals are increasingly at risk from fishing gear set in coastal waters.

Scientists from the National Oceanic and Atmospheric Administration (NOAA), alongside several research organisations, examined the issue in depth.

Their findings indicate that the loss of cool-water habitat is a major factor behind whale entanglements.

Increase in whale entanglements

Humpback whale entanglements have risen markedly since 2014. Prior to that year, fewer than ten incidents were generally reported annually.

During the intense marine heatwave in 2015 and 2016, more than 40 confirmed entanglements were documented.

Figures have remained above earlier levels even after new fishing regulations were brought in. In 2024, 31 entanglements were reported.

The California Current Large Marine Ecosystem runs along the West Coast and sustains major fisheries, including the Dungeness crab fishery. It is also an important feeding area for humpback whales.

The likelihood of entanglement increases when whales and fishing equipment occupy the same waters.

Shrinking feeding habitat for humpback whales

Humpback whales feed chiefly on krill and small fish, including northern anchovy.

These species rely on cool, nutrient-rich coastal waters produced by upwelling. This process draws cold deep-ocean water to the surface, creating productive feeding areas for marine life.

As sea-surface temperatures increase, the extent of cool water contracts. Scientists describe this as habitat compression.

Researchers measure this effect using the Habitat Compression Index. Its annual form, known as the cumulative Habitat Compression Index, records how much cool habitat accumulates between January and December.

When cool habitat forms as usual, whales can disperse across broad feeding grounds. If it develops slowly or remains restricted, whales congregate in smaller coastal areas, where fishing traps and lines are often present.

Marine heatwave changed whale feeding patterns

The marine heatwave of 2015 to 2016 demonstrated this pattern clearly. Areas of cooler habitat suitable for whales were substantially reduced. Krill abundance fell, whereas anchovy numbers started to increase.

From 2017 onwards, anchovy biomass rose to more than two million metric tonnes by 2021 and remained elevated, although it fluctuated between years.

Large shoals of anchovy assembled close to the coast in central and southern California. Whales followed this food source nearer to land, while fishing gear is likewise concentrated along the continental shelf. This overlap raised the risk of entanglement.

Whales that feed nearer shore are also more visible. Whale-watching vessels and coastal observers encounter more animals when they gather near ports. This could partly account for the greater number of entanglements reported in years of compressed habitat.

Is population growth the principal cause?

Humpback whale populations have rebounded considerably since commercial whaling ended. A continuing increase in whale numbers does raise the underlying likelihood of interactions with fishing gear.

Nevertheless, the study indicates that population growth by itself cannot account for abrupt surges in entanglements. Even when whale population size was taken into account, years with limited cool-water habitat continued to show exceptionally high entanglement reports.

“Habitat compression is a primary driver of entanglements. We found strong negative correlations between cool-thermal habitat availability and entanglement reports,” the authors explain.

“Years with low cumulative thermal habitat (high compression) consistently resulted in the highest number of entanglements across all US West Coast regions, making cHCI a critical predictor of both risk and detection.”

Ocean changes increase entanglements

“Environmental shifts influence risk more than population growth. While population recovery contributes to baseline interactions, it does not explain extreme spikes (e.g., 2015–2016 and 2024),” the authors add.

“Analysis of “excess entanglements” (residuals controlled for population size) confirmed that environmental conditions exacerbate risk significantly beyond density-dependent factors.”

Put simply, unusually high entanglement reports still occur when habitat is compressed, even after allowing for the number of whales at sea.

Heatwaves concentrate whales near shore

“Additionally, our “compression-assisted resolution” hypothesis suggests that extreme compression concentrates whales nearshore, artificially inflating recapture rates and explaining biologically implausible population growth estimates (>30%) during heatwaves,” the authors add.

This indicates that scientists may see and record more individual whales than normal when the animals cluster close to shore.

Consequently, population estimates during heatwaves may appear far greater than would be possible through natural growth alone.

Forecasts signal elevated risk

“Forecasting provides an early warning system. The cHCI effectively predicts ocean conditions 6–12 months in advance. For example, January 2024 forecasts correctly predicted the year’s low thermal habitat, which coincided with a rise in entanglements to 31 reports,” added researchers.

This forecasting capacity provides managers with useful time to prepare for hazardous conditions before the fishing season reaches its busiest point.

“Operationalizing a ‘rate tracker’ aids management. We recommend integrating cHCI into risk assessments as a ‘rate tracker’. Monitoring thermal habitat accumulation during winter and spring enables managers to make strategic decisions regarding seasonal fishing activity and mitigation efforts,” noted researchers.

In practical terms, following the annual build-up of cool habitat can support more informed decisions and reduce harm to whales.

Fishing changes and continuing challenges

Following the 2015 to 2016 crisis, new rules altered the California Dungeness crab fishery.

Managers postponed season openings, ended seasons sooner, capped trap numbers and introduced depth limits. Oregon and Washington also implemented measures aimed at reducing risk.

Despite these actions, entanglements remain more frequent than they were before the heatwave years. Lost or discarded fishing equipment, commonly known as ghost gear, creates an additional hazard. In certain seasons, up to ten percent of crab gear may be lost.

When salmon fisheries are closed, fewer boats are available to retrieve lost gear, potentially increasing the risk still further.

A tool for the future

A key outcome of this research is its predictive value. The Habitat Compression Index can project ocean conditions as much as a year ahead.

Forecasts issued early in 2024 had already indicated that cool habitat would stay limited. Entanglement reports increased again later that year.

This ability to forecast gives fishery managers time to intervene. Changing fishing seasons, cutting trap numbers or expanding monitoring during periods of elevated risk could prevent harm before it occurs.

Warming seas raise the threat

Climate change is transforming ocean ecosystems. Warmer water changes where prey congregate and where whales feed.

Monitoring habitat compression gives scientists and managers a clearer understanding of when whales are at greatest risk.

The study delivers a clear message: as cool feeding habitat contracts, entanglement risk increases. Recognising this relationship can help safeguard humpback whales while supporting fishing communities along the US West Coast.