"At any temperature colder than freezing, a fish's gills are very susceptible to freezing when exposed to air," says Sue Marcquenski, DNR fish health specialist. "It is best to keep the fish in the water as much as possible while removing the hook."
When temperatures start running below 10 degrees F, a fish's eyes are also at risk of being damaged.
"When it's really cold, you might notice the fish's eyes become opaque when they're out of the water," says Marcquenski. "It's possible that the cornea has frozen and although the eyes clear up when the fish is put back in the water, the cornea or lens might have been damaged. Fungus or bacteria in the water can also infect the damaged cornea, perhaps leading to blindness.