Water temperature and fish activity: what it changes

Metabolism and oxygen

As water warms within the range a species tolerates, its body chemistry speeds up. It digests faster, needs more food and can swim harder. As water cools, everything slows: the fish eats less, eats less often and is less inclined to chase a fast-moving meal.

Warmer is not always better. Above its preferred range a fish's energy needs keep climbing while its ability to meet them falls, so feeding drops and stress rises. Preferred ranges differ widely between species, which is why fish are often loosely grouped as cold-water, cool-water or warm-water. For the species you fish for, a local fisheries agency or an experienced local angler is a better guide than a general figure.

Fish breathe oxygen dissolved in the water, and the amount water can hold falls as it warms. That creates a squeeze in hot weather: fish need more oxygen at the very time less is available.

Shallow, still, weedy water is usually affected most. Plants and algae produce oxygen in daylight but consume it at night, so levels in such water tend to be lowest around dawn. Moving water, wind-stirred water and cooler depths are generally better supplied, and fish often shift toward them during hot spells.

For anyone releasing fish, this is the time for extra care. A fish played hard in very warm water is under more strain, so a short fight and minimal time out of the water help.

Seasonal patterns

In temperate regions the year follows a broad pattern. Warming water in spring raises activity, and the shallows warm first. In summer the surface can exceed what some species like, pushing them deeper, into shade, toward cooler inflows or into feeding at night and at dawn. Cooling water in autumn brings temperatures back into a comfortable range for many species. In winter metabolism is slow; fish tend to hold in deeper, more stable water and feed less, although they do still feed.

The emphasis depends on the fish. For cold-water species, summer is the difficult season and the cooler months are the active ones. In the tropics the seasonal swing is small and other factors usually matter more.

Thermoclines in lakes

In summer many lakes settle into layers. The sun warms the surface, warm water is lighter than cold water, and wind mixes only the upper part. The result is a warm upper layer, a cold deep layer, and between them a fairly thin band where temperature falls quickly with depth. That band is the thermocline.

The deep layer is cut off from the air. In fertile lakes, decaying material can use up much of its oxygen as the summer goes on, leaving deep water that is cool but short of oxygen. Fish are then often found near the thermocline or just above it, where temperature and oxygen are both acceptable.

In autumn the surface cools, sinks and the lake mixes from top to bottom. Fresh water is densest at about 4 degrees Celsius (39 degrees Fahrenheit), so under winter ice the slightly warmer water lies at the bottom. Shallow or windswept lakes may never form stable layers at all.

What changes temperature locally

Wind. On a lake, steady wind pushes warm surface water toward the downwind shore, and cooler water can rise along the upwind shore to replace it. On some coasts, wind that moves surface water away from the land draws up colder water from below, a process called upwelling, and the temperature can fall noticeably in a day or two.

Tide. A flooding tide carries sea water into bays and estuaries, where it may be cooler than sun-warmed shallows in summer or warmer than them in winter. An ebb draining off sunlit flats can carry warm water with it.

Rain and river inflow. Cold rain, snowmelt or warm runoff change the temperature near river mouths and creek mouths, often forming a distinct edge. Inflows also bring colour and, in estuaries, fresher water.

Sun and depth. Shallow water over a dark bottom warms fastest by day and cools fastest at night.

What a sea-surface temperature figure measures

Sea-surface temperature from satellites or forecast models is a surface figure and an area average. Satellite sensors see only the very top of the water. The readings are then combined with other observations and averaged over grid squares that may be a kilometre or many kilometres across, with gaps under cloud filled in by estimates.

Such a figure can differ from the water near the bottom, in a shallow bay, beside a river mouth or under a thermocline. It is good at showing trends and broad patterns, such as whether the water is warming week by week and where warm and cool water meet. For the spot you are fishing, a thermometer lowered on a line or the temperature sensor on a fish finder gives the local answer.

The practical takeaway

Use temperature to decide where and how to fish as much as when. CatchWindow location pages show water temperature where it is available; treat it as background and watch the trend. In heat, look for depth, shade, flow and early mornings. In cold, expect slower fish, fish slower, and favour the mildest and most stable part of the day. Then measure the water in front of you whenever you can.