Shrinking Cold-Water Zones Drive Humpback Whale Entanglements on the West Coast

The number of humpback whales confirmed entangled in fishing gear off the U.S. West Coast stayed below 10 per year for most years before 2014. By 2015 and 2016, during a sustained marine heat event known as "the blob," confirmed entanglements exceeded 40 in a single year. In 2024, reported entanglements reached 31. The question researchers have struggled to answer is whether this rise reflects population growth, worse luck, or something more predictable - and therefore preventable.

A 25-year analysis published in PLOS Climate by Jarrod Santora of NOAA and colleagues argues it is largely the latter. The study finds that entanglement numbers track a measurable environmental variable - the total area of cool-water habitat available off the West Coast - with enough consistency that a metric called the cumulative Habitat Compression Index (cHCI) can forecast entanglement risk six to twelve months in advance.

The Compression Mechanism

Humpback whales on the West Coast feed primarily in cold, productive waters upwelled from depth along the continental shelf. When sea surface temperatures rise during El Nino events or marine heat waves, that cold-water zone shrinks and compresses toward the coast - toward shallower, nearshore areas where trap-based fishing for Dungeness crab is concentrated.

The collision of hungry whales and dense fishing gear is not random. It is geometrically determined by how much cold water is available and where it sits relative to the fishing grounds. When the cold zone contracts, whales and gear occupy the same compressed space at higher density than either would normally.

The 2015-2016 heatwave made this mechanism visible at scale. Statistical analysis confirmed that habitat compression explained the spike beyond what population density alone would predict. The humpback population had been recovering steadily from historical whaling, but that growth rate could not account for a near-fivefold increase in entanglements in a single season.

Forecasting Entanglement Risk

Santora's team found that the cHCI can predict ocean conditions well enough to serve as an early warning system. January 2024 forecasts made using winter cHCI values correctly predicted the year's low thermal habitat - and that year's entanglement reports climbed to 31, consistent with the forecast signal.

This predictive capacity has direct management relevance. The timing and intensity of the Dungeness crab fishery - one of the most economically valuable fisheries on the West Coast - is already subject to regulatory adjustments. If managers can anticipate high-compression years six to twelve months out, they can consider preemptive changes to fishing season timing, trap limits, or gear configurations before whales and gear come into contact.

What the Data Cannot Fully Resolve

Entanglement records are almost certainly incomplete. Many whales that become entangled are never observed by humans, and those observed may carry gear acquired days or weeks earlier at unknown locations. The study addresses this by analyzing broad associations between habitat conditions and reported entanglements rather than treating the count data as an accurate census of all entanglement events.

The analysis covers only humpback whales and cannot be directly applied to other species. The relationship between population size and entanglement risk is real but nonlinear; the researchers recommend treating population growth as a background factor and habitat compression as the primary predictor of year-to-year variation.

"Habitat compression is a primary driver of entanglements," the authors write. "Years with low cumulative thermal habitat consistently resulted in the highest number of entanglements across all U.S. West Coast regions, making cHCI a critical predictor of both risk and detection."