Do fish Dream?

Juvenile snapper less than 4cm long photographed during the early evening at Goat Island.

Some fish are nocturnal like this Toadstool Grouper that rests during the day and hunts at night. I had my modelling lights on and I suspect it was frozen in place like a ‘possom in the headlamps’. Photographed in Blue Maomao Arch, Poor Knights Islands


Bailing Out

Sleeping seals at Red Rocks near Island Bay, Wellington.

A wrasse photographed at night while it was asleep against a rocky crevice at the Poor Knights Islands. The body pattern of the wrasse provides good camouflage.

By Iain Anderson

The sea is a dangerous place at night for reef fish. Wrasses, demoiselles, snapper, and other species that use their eyesight to hunt during the day must seek nighttime shelter when predators such as morays and sharks are on the hunt. For them it is sensible to stay out of the way by huddling against and between rocks or burying themselves in the sand. Some wrasses even secrete a mucous net around their body to keep their scent to themselves and to avoid detection by a predator.

When you approach a resting fish on a night dive, with torch or camera modelling lights ablaze, it will usually stay put. Is this a nervous reaction, like the possum that stands frozen in the light beam of an approaching car; or are they asleep? You can’t really tell whether they are sleeping as they lack eyelids.

I discussed these thoughts with Professor John Montgomery, Director of New Zealand’s Leigh Marine Lab. The ‘frozen possum’ analogy might be valid in certain cases. A fish can’t see the diver because of the glare of the torch and there must be some confusion about what to do next when under a spotlight. Fish also undergo some body pattern changes at night and there is good evidence that they become docile. Some can even be picked up when they are in a fishy sleeping state, but is it sleep as we experience it?

Sleep is a biological mystery and an area of scientific research. It is clear that birds, mammals and some reptiles sleep. If you walk up to a seal colony, like the one at Red Rocks near Island Bay Wellington (New Zealand), you will need to be alert to avoid stepping on a sleeping seal. Like us, seals can breathe without needing to think about it. This is not true for dolphins that think about each breath and must remain conscious in order to avoid drowning. They sleep without drowning by always keeping one hemisphere of the brain awake.

Besides being good for energy conservation, sleep improves the body’s healing ability. At least for humans, the lack of it compromises our immune system, ability to perform complex tasks (like diving and driving), and our longevity. Many a good creative idea or problem solution has been born in the early hours of the morning.

Scientists study sleep by using electrodes planted on the surface of the scalp to monitor the electrical impulses from brain nerve cells. Stages of sleep are associated with profound changes to specific electrical brain wave patterns. It is during one of the distinct stages of sleep that we actually dream. This is associated with rapid movement of our eyes beneath the eyelids.

Returning to fish, if we define sleep as a state of resting with an altered state of consciousness then it is pretty clear that at least some fish sleep. But do they dream? My dreams involve memories of past events and experiences with friends so memory must be important. It is clear that for fish some memory ability exists. Fish on reefs that are heavily hunted can be quite timid, especially when compared with fish in marine reserves that follow you around. Perhaps the marine reserve fish have fewer bad memories of human encounters. Some fish memory ability is outstanding! John told me of the homing capability of salmon who remember the smell of a particular branch of a stream from whence they came. Fish are territorial and I wonder if some reef fish can remember the location of their favorite rocky bedroom where they can bunk down at night. But whether or not they dream is still an open question and beyond the scope of this article!

References and further reading: Sleep on Wikipedia

scroll to top