Non-Virtual Reality: Do fish sleep, or just hang in a stupor?

Last week’s cold snap caught Cocoa by surprise.

As is widely known, disgusting brown puddle water in the alley is so much more delectable than the fresh, clean water by the bowl of kibble in the kitchen. But wait … this one’s solid and cold, and so’s this one, and over here too. …

His confusion brought to mind another brown dog figuring things out, late one July evening some 20 years ago. …

The beams of light that had been flickering this way and that from the depths of the quarry for the past 20 minutes rose slowly to the surface and switched off. Closing in on midnight, all was quiet with hardly a ripple on the water.  On shore, the Weimaraner mix picked up his ears at the soft hiss of the two divers inflating their buoyancy compensators, or BCs.

“The best part was when we came up on that group of four or five white suckers hanging, like in a stupor, over the little rise on the bottom.”

“No kidding! For one thing, they were the largest whites I’ve seen — all of them over two feet, easy. But yeah, the way they were suspended there motionless, facing different directions, one atop the other like a mobile made of fish — so cool.” 

Their 10-minute break on the surface was interrupted by the sound of thrashing water.

“Yikes!  Hershey, NO! Go back, NOW!”

He didn’t need to be told twice. It was a couple hundred yards out to the middle of the quarry, a long way to dog paddle. He’d been with us on many a daytime dive here, nosing along the shoreline until we got back.  It never occurred to me he’d swim out tonight to join us.

Once it was clear the pup had made it safely back to land, we turned on our lamps, released the air from our BC’s and, facing one another, descended 25 feet to the bottom, with the goal of finding more sleeping fish.

Do fish really sleep? The answer depends on how you define the term. Sleep researchers working with mammals or birds typically recognize sleep when an animal’s eyes close and certain electrical patterns are detected with an electroencephalograph (EEG machine). But fish have no eyelids and they lack the parts of the brain that generate the EEG signals seen in the land vertebrates.

But those white suckers sure looked somnolescent. In fact their appearance nicely fit the four criteria for “behavioral sleep” used by fish biologists: prolonged inactivity, a typical resting posture, a high arousal threshold, and alternation of rest and activity within a 24-hour cycle. 

By these criteria, most fish that have been studied do experience sleep, or at least a sleep-like pattern, each day. But there are some interesting exceptions. Pelagic (open ocean) fish like tuna, mackerel and bonitos actively swim 24/7, and while many night-feeding coral reef fish seek out refugia in which to sleep during the day, others form largely stationary schools that are nonetheless alert and responsive to predatory threats.

A lot of fish — bass, yellow perch, bullheads and many minnow species for example — appear to forgo sleep when spawning and/or guarding a nest with eggs or young. Then too, different species vary in how long they’ll sleep and in their ability to respond to disturbance. Even within a given species, you’ll generally find a few relatively active individuals while everyone else is zoned out. 

… Settling on the bottom of the quarry, we took a compass bearing and looked around. Even our powerful dive lights only provided a limited field of view in the quarry’s murky water. At a signal from my dive buddy we switched them off. The depth of darkness encasing us for that minute or two was a lesson in perspective. Sighted humans hardly know how to think about such blackness; it is a daily reality for aquatic organisms.

Swimming in closer to shore, we came upon a mixed species group of about 10 fish suspended in 3 to 15 feet of water. A couple of decent-sized large-mouth bass slowly finning in place were first to catch our attention, but the prize was the several seemingly comatose rock bass.

A large-mouth eased a few feet to one side as we approached. Slowly extending an arm beneath one of the rock bass, cupping the hand beneath its body, the foot-long fish tipped slightly on being lifted, but did not stir otherwise. The animal was out.

But was it really asleep? Researcher Steven Reebe of the the Université de Moncton in New Brunswick says, “Although signs of sleep are present in fishes, we should not equate it with the kind of sleep that we, humans, are used to experience.”

You’d have to be a fish to know what kind of sleep it was.

Ken Baker is a retired professor of biology and environmental studies. If you have a natural history topic you would like Dr. Baker to consider for an upcoming column, please email your idea to fre-newsdesk@gannett.com.