Humpbacks' Eyes Might Explain Why They Get Caught in Nets

Researchers peered deep into the grapefruit-sized eye of a humpback whale and discovered one possible reason the animals keep getting tangled in fishing gear: Their eyesight sucks. It's worse than researchers expected and much, much worse than the eyesight of humans. Human vision is measured as 60 to 100 cycles per degree (CPD), meaning the eye can distinguish 60 to 100 pairs of black and white lines within a one-degree area of the visual field, per Smithsonian. Humpback whales have a CPD of just 3.95. That means "they can detect large, simple shapes from a distance, but need to be very close (within 3–4 body lengths) to see fine details," according to a release.

Researchers from the University of North Carolina Wilmington and Duke University dissected the preserved left eye of a juvenile humpback that stranded in North Carolina's Thorofare Bay more than a decade ago, finding the white of the eye was "particularly thick at the back," per Smithsonian. In other words, the distance between the lens and retina, known as focal length, was shorter than expected. Though one might presume big eyes mean good vision, a short focal length typically implies poor vision. The whale eye also showed a low density of cells that convert the image on the retina into brain signals. They numbered 180 per square millimeter. "Humans, in comparison, have between 12,000 and 38,000," per bioGraphic.

After learning all this, researchers plugged their findings into computer models to simulate how a humpback whale might see in the ocean. They are likely to detect simple shapes, like a school of fish, from afar, but details only within about 150 to 200 feet, according to the study published Wednesday in Proceedings of the Royal Society B: Biological Sciences. That's fine for a whale looking to catch prey, but bad for those that might swim in the way of boats and fishing nets. However, researchers say the findings could lead to fishing nets that are more visible to the whales, so they should be able to avoid them. (More whales stories.)