Room 3 at Roxburgh Area School asks :-
Do fish sleep?
Jean McKinnon, a marine zoologist at the University of Otago, responded.
Most fish spend time in an energy-saving state that can be called "rest" and we might even call their behaviour "sleep", though it is probably different than "sleep" in most land animals. There is no evidence to suggest that they go into the deep sleep that we do when we are dreaming, they just blank their minds and perhaps daydream.
While they are doing this they are alert to danger. Most fish can't close their eyes when they sleep; as only some sharks and some of their relatives have eyelids. What do Goldfish do? Goldfish sleep basically in one spot. They hover with periodic movements to keep stable and are often seen hiding behind aquarium decorations or close to the bottom of your aquarium. You will see Goldfish suddenly swim erratically when you turn the aquarium light on. They are much like us; they were startled and are seeking a place to hide.
Many marine fish rest on or under rocks at night, or they may float in place, wedge themselves into a spot in the mud or the coral they may even build themselves a nest using seaweed or pebbles. There are some animals that never stop swimming, like many species of shark, so they "sleep swim"!
The resting behavior of fish is very different from their behavior at other times of the day. Many small fish, such as mullet, which are very active in schools during the day, scatter and remain motionless in shallow water at night. Other fish "rest" or "sleep" during the day and are active at night instead. Some fish even change colour depending on the colour of their surroundings, the stage of their life cycle or even their state of excitement. For example, Trumpeter and Moki change their colour pattern between day and night almost as if they are putting on pyjamas! So as you can see fish do rest but exactly how they do it depends on what species they are, where they live, and what predators are likely to attack them.