Temperature can make the difference with trout

by Tony Ritchie

Brown Trout find very comfortable temperatures between 12 and 20 Degrees Celsius and often feed best at the top end of this range for brook and rainbow trout is several degrees less. Trout can live for only limited periods in water temperature of 28 degrees.

They can tolerate without problems temperatures just above freezing to lengthy periods up to 24 degrees, so long as temperature changes slowly.  

Trout, as are most fish are cold blooded and have a body temperature the same as the water which surrounds them. As water temperature changes, so does the initial physiology of the fish. After such a change, the temperature needs to stabilise for several days so that trout acclimatises.

For trout, too much heat is usually the problem and at the peak of summer trout are often hard to find, especially in the top layer of warm water. However, the reverse applies early in the season. Many Australian lakes are classified as warm monomictic - meaning they form into different layers or strata in warmer weather and have complete circulation of water through them in winter. Early in the season layers of water different temperatures may be factor in some lakes with warmer water located well down near the bottom. This is because water is heaviest at 4 Degrees Celsius; it becomes lighter as it grows colder and moves towards the surface, where of course it may freeze. Because at 4 Degrees trout also start to show an interest in food, if a thermometer confirms that the water is warmer deeper, the lures should obviously be fished down there.

However, most Tasmanian lakes are shallow and if they do stratify for more than a day, can be classified as polymictic - stratifying for only some days throughout the year. Local lakes ice over only in parts and for only a few days at a time. Of more interest is the surface water that warms as spring progresses, the wind blows it to windward shores and it is often here that fish tend to congregate.

In summer, larger, deeper lakes may stratify. The epilimnion, the upper of the three layers, is less dense and is mixed by the wind to be uniformly warmer than the bottom layer, which is the hypolimnion of colder, more straight water, and is little affected, by wind. The section in between is called the metalimnion or thermocline, where temperatures drop rapidly as the depth increases; by definition falling by at least 1 Degrees Celsius per metre.

With trout usually preferring the epilimnion, sometimes the metalimnion and rarely the hypolimnion; knowing where these are in various seasons is useful.

In mid to late autumn, the ratio of epilimnion and hypolimnion by depth is 4:1:1 - that is, the top layer comprises two thirds of the lake's depth. In winter there are no layers and in early to mid spring the ratio is 1:1:4. However, it should be borne in mind that trout will tolerate deeper, colder water for short periods if that is where they need to feed.

Many of the main still waters, however, average only a few metres in depth and for the trout the discomfort of warm summer water is compounded by a double - barrelled problem. While warmer water holds less dissolved oxygen than colder water, it is at the same time raises the metabolism of the fish so they need more oxygen and water has to be pumped through the grills faster.

Active fish like trout, which normally live in cool and well oxygenated water, are particularly sensitive to reduced amounts of dissolved oxygen and this, combined with increased  metabolism and their dislike for warmer water, ensures that the trout will actively seek places where it is cooler.

Occasionally these are in the shade of vegetation and higher ground and in streams where springs add cooler water. Particular places where they well up from the bottom are known as spring holes and will nearly always be crowded by trout enjoying the coolness and increased supply of oxygen, which is also available to them in the faster runs where the rushing water enfolds in from the air. Feeder streams may also be cooler and trout numbers greater where they enter rivers or lakes. Obviously, methodical use of a thermometer to find cooler water will result in better sport, which will usually remain better for season after season.

Deeper water is also cooler in summer. After fishing the summer dun hatches on Little Pine Lagoon intensively over many seasons, I find when wading that the weedy shallows about a metre deep are best fished during the first half of summer, but later in the season I favour shores where I can cast over deeper basins and channels and especially over the course of the old river, in whose considerable depths trout seem to lie with gratifying consistency and in pleasing numbers.

Shallow still waters may not only be too warm for fish but if they grow much vegetation may also contain too little dissolved oxygen because of respiration of these aquatic plants. During the day, through the process if photosynthesis which involves sunlight, these plants produce useful oxygen but at night respire only and take it up. Dissolved oxygen levels in the shallows may be at their lowest early in the morning after a warm day of heavy overcast, which by reducing sunlight in turn limits photosynthesis and therefore the production of oxygen. If fish cannot move elsewhere, they may die. Late in summer and on into autumn the problem may be aggravated by plants beginning to die back and decompose. There may not be sufficient oxygen produced during the day to make up for the oxygen used at night through respiration and decomposition.

Pollution levels also play a key ole in the comfort of a trout, through its respiration rather than diet. The gill structure consists of a very fine, basket - like sieve through which water is pumped by the muscular action of the mouth and pharynx. This is done very efficiently by trout, which can extract u to 80 percent of the oxygen dissolved in the water, but this very efficiency also makes it vulnerable to poisons in the water.  The acidity or alkalinity of water, as measured on the pH scale with "soft"acid water reading below the neutral 7.0 and "hard"alkaline above, has an influence on trout, whose own blood is almost neutral at 7.1. They have problems with water registering below 5.0 or over 9.0 on the pH scale and seem to prefer a range of 6.8 to 7.8.

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