What a whale does not see

This story was originally published by bioGraphic and appears here as part of the Climate Desk collaboration

Humpback whales make some of the longest migrations on Earth, swimming roughly 8,000 kilometers (4,970 miles) from their high-latitude feeding grounds toward the equator to breed and calve. Along the way, they often “lunge feed” near coastlines, opening their maws to engulf mouthfuls of fish and crustaceans. But they don’t always get the perfect gulp—sometimes fishing gear gets in the way.

Off the west coast of Canada, for example, almost 50 percent of humpbacks spotted by people bear scars from encounters with ropes and nets. During spring and summer, crews rescue and release whales on a near-weekly basis. The process can be expensive and time-consuming: In fall 2024, a crew from Fisheries and Oceans Canada took four days to extricate a humpback from fishing gear entangling the whale’s head, body, and mouth.

While humpback populations are stable, fishing gear still poses a threat to conservation, says Jacob Bolin, a PhD student in biology at the University of Tulsa in Oklahoma. Whales, like many animals, learn from experience, so why they keep swimming into nets and ropes is a mystery. Furthermore, animals with large eyes tend to have better vision, Bolin says. If whales are intelligent creatures with big eyes that can presumably see well, why don’t they just avoid the hazards?

To understand why humpbacks frequently become enmeshed in fishing gear, Bolin researched the problem from the whale’s point of view. Literally. In 2023, when he was a student at the University of North Carolina Wilmington, Bolin sliced into a preserved, grapefruit-sized left eyeball taken from a whale stranded in Thorofare Bay, North Carolina. Bolin’s study was the second time researchers cut into the eye of a humpback for the sake of studying vision. The first whale was from a juvenile that drowned in Australia in 2005 and may not have had fully formed vision.

To understand what Bolin found, it helps to first understand some basics about how vision works. Visual acuity, or sharpness of vision, is dependent on two factors: focal length and the number of retinal ganglion cells. Focal length is measured from the lens, near the center of the eye, to the retina, near the back. A large eye has a longer focal length, which generally means sharper vision. Retinal ganglion cells are located in the back of the eye and transmit visual information to the brain. Like pixels on a camera, the more retinal ganglion cells, the higher the resolution.

When Bolin dissected the whale’s eye, he found it had a large schlera, or white of the eye. The schlera was thick near the back of the eye, Bolin says, which would have shortened the whale’s focal length. The humpback also had a low density of retinal ganglion cells—a maximum of 180 cells per square millimeter, or an area about the size of the head of a typical straight pin. Humans, in comparison, have between 12,000 and 38,000.

After dissecting the eye, Bolin and his research team entered the data into a computer program that projected what a bait ball—a dense school of small prey fish—and a gill net would look like in the water to a humpback from different distances. The projections show that a bait ball is quite visible, whereas the fine lines of netting become harder to discern with distance. It’s possible that humpbacks can’t see nets and can’t change course before it’s too late, Bolin says.

The data revealed that humpbacks can see at 3.95 cycles per degree (CPD), the unit for visual acuity, compared to humans at 60 to 100 CPD. According to health guidelines in the United States, the threshold for human blindness is 6 CPD. “[A low CPD] is bad for a human, but not bad for a whale at all,” says Thomas Cronin, a visual ecologist at the University of Maryland, Baltimore County, who was not involved in the study. Other whales are known to have comparable eyesight, he says. Having low visual acuity is fine for whales, since they don’t usually need sharp vision to catch prey.

Cronin also notes that nets may be even harder for whales to discern because northern coastal waters where migratory whales feed tend to be dark and murky. He says the study is a good baseline for understanding humpback vision, and he’d like to see further research that delves into whales’ sensory perceptions, including smell.

By giving people a glimpse into a whale’s perspective, the latest study is a step toward helping humpbacks see danger more clearly.