Do fish sleep

FISH DO NOT HAVE EYELIDS, DO NOT LIE DOWN, AND MOST NEVER STOP MOVING ENTIRELY. That makes the question look harder than it is, because the useful definition of sleep is behavioural rather than visual.

THE BEHAVIOURAL DEFINITION has four parts: a period of reduced activity, a species-typical posture or location, reduced responsiveness to stimulation, and — crucially — rapid reversibility, which distinguishes sleep from unconsciousness or torpor. A further test is HOMEOSTATIC REGULATION: an animal deprived of the state compensates afterwards.

BY THOSE CRITERIA MOST FISH SLEEP. Many species become quiescent at a predictable point in the daily cycle, adopt a resting position — on the bottom, wedged into structure, hanging in mid-water, or lying against something — and become measurably harder to disturb. Some reef fish secrete a mucus cocoon at night. Some bury themselves. Studies of sleep deprivation in fish have shown compensatory rest afterwards, which is the strongest evidence available that this is a regulated state rather than merely inactivity.

WHAT IS DIFFERENT FROM MAMMALIAN SLEEP. Fish do not show the same electrophysiological signatures — the brainwave patterns and the alternating stages of mammalian and avian sleep are not present in the same form in most fish. Whether that means fish sleep is a different phenomenon or the same phenomenon in a different brain is exactly the kind of question that has no agreed answer.

AND SOME FISH DO NOT STOP. Species that must swim continuously to ventilate their gills — many sharks and some fast pelagic fish — cannot become fully quiescent, and what rest looks like in them is an open question with several proposed answers.

WHAT FOLLOWS FOR FISHING, and it is more than a curiosity.

THE DAILY CYCLE IS REAL, and species differ in when they are active. Some are strongly diurnal, some nocturnal, some crepuscular. The species records carry it, and it is a better basis for deciding when to fish than any general rule.

A RESTING FISH IS A CATCHABLE FISH IN THE WRONG SENSE. Fish located while at rest — spotted at night with a light, found wedged in structure — are not feeding and are unusually vulnerable, and taking advantage of that is a choice worth making consciously.

AND DISTURBANCE HAS A COST. Repeated disruption of the rest period is a stressor like any other, which is one argument among several for restrictions on continuous night activity on pressured waters.