Fly Fishing New Zealand

Water Temperature and Trout Behavior

Water Temperature and Trout Behaviour by David Moate

Trout behaviour is affected by all-natural elements or events surrounding each fish. These events change all the time, requiring suitable responses from trout in order to survive. Water is a very sensitive medium, relaying scent, temperature, barometer pressure, vibrations, and sound (three times faster) more quickly and effectively than air. Trout respond to these messages quickly, too. Here is a look at some of the secrets of water temperature and trout behaviour. 

Water temperature and barometer pressure have profound effects on trout, changing day-to-day living patterns, yearly living cycles, and species distribution.

The optimum water temperature for trout is 14 degrees Celsius with a comfort range of 10 degrees to 18 degrees. Below 10 degrees, trout metabolism slows down, making the fish sluggish or torpid. Above 20 degrees, lower dissolved oxygen levels slow trout activity. Over 23 degrees trout become stressed, while at temperatures around 25 degrees, they can die.

Trout, therefore, are constantly moving locations to find the most preferred temperatures, and/or better feeding situations influenced by temperature.

Brown trout generally can tolerate wider ranges in temperature than Rainbows. Winter water temperatures influence trout distribution more than summer temperatures, with Browns spawning in water under 10 degrees and Rainbows in slightly warmer water.

Water temperature affects the whole ecosystem, influencing plant growth, insect behaviour and distribution, as well as chemical and oxygen levels in the water. For example, cold water limits plant growth and chemical absorption but can contain more dissolved oxygen. Warmer water encourages plant growth and carries less dissolved oxygen, but more nutrients and suspended solids.

Some years ago, I started keeping accurate records of water and air temperatures, trout locations and behaviour. Most of these 900 days of fly fishing have been recorded in the Nelson, Marlborough and West Coast areas, but all other regions of the South Island have been included.

Water temperature and trout behavior

I am certain the information collected will be similar throughout New Zealand. Here is a brief summary:

Four To Seven Degrees

Trout hold in deep, slower water. Feeding is lethargic and hooked fish fight poorly. There is little insect activity. Rainbows are more active than Browns. Warm weather increases trout activity. Lake trout are deep.

Seven To Ten Degrees

Trout hold more in the eye of pools, drop-offs, mid-stream lies and near banks. Feeding and insect behaviour increase. Fish put up a better fight and start to use the shallows.

Ten To Fifteen Degrees

Trout feed well in the shallows runs and on the surface. Insects are active with mayfly and caddis hatching. Fish fight aggressively.

Fifteen To Eighteen Degrees

Trout hold in faster runs and mid-stream lies. Feeding and insect activity occurs more at night, mornings, or on cool lays. Rainbows are deeper with Browns being more active. Once a fish is hooked, the fight is over quickly.

Many of the trout spotted in shallows during the day are inactive. There is less surface feeding.

Eighteen To Twenty Four Degrees

Trout hold in deep and heavy water. Feeding and insect activity slows down during the day. Prime fish eat small fish at night. Old or small fish are in runs or along the edges. Lake trout are deep or cruising over dropoffs.

Algae grow in the shallows. Fish migrate to cooler locations such as headwaters, river mouths, and springs, and downstream to deep areas of lakes. Browns are more active than Rainbows. Catching and releasing fish quickly is critical to their survival.

On average, water temperatures are higher from January to early March, but weather conditions can alter things quickly. Examples of this include:

Early in the season, warm weather can increase water temperatures in rain-fed streams, but it can also melt snow to cool big rivers and lakes. Sudden rain after a long, hot period can flush rivers with rain, and dirty water, pushing trout deep.

Steady rain can lower water temperatures by cooling the land and air, also filling underground water reservoirs and forcing out cold water. Rivers draining lakes warm very quickly in hot weather. Lake water temperatures are lower during strong winds, creating wind lanes and upwelling.

Clear, cold nights quickly cool water. Water temperature is undoubtedly a major factor in trout behaviour. Understanding this will help the angler to locate fish and select the most productive angling method to catch them.

Gathering a basic knowledge of water temperature changes in your local waters is easy and can lead to the discovery of some interesting trout secrets. Naturally, to do this, it becomes necessary to understand how to use your thermometer.

ln a river, always take the temperature reading from knee-deep, moderately moving water. I tie my thermometer to the tip of my rod, boots or landing net. Give it several minutes before recording. In lakes, use your rod, holding the thermometer in waist-deep water. This method will give you good average recordings in water where fly fishing is mainly practised.

Water temperature and trout behaviour – Barometer Pressure

I used to believe barometer pressure effects on trout behaviour were old fishermen’s tales. But my own experiences and records tell me differently.

I’m not certain whether barometer pressure affects trout directly or other aspects of the food chain environment first. But the most commonly observed trout reactions to a falling barometer include reduced feeding activity, and more easily disturbed and more difficult-to-approach fish.

Sometimes, trout appear to be sleeping, only moving when jabbed by a rod tip.

When the pressure is low for some time trout activity remains slow, but occasionally trout will feed actively, although briefly. ‘Severe’ marine heatwave to hit South Island waters.

Many unproductive days fishing have been saved by a sudden five-minute feeding spree triggered by what, I don’t know, but these occasions are most welcome.

Have you ever seen a trout suddenly leap out of the water for no apparent reason? Old-timers have told me it’s a sign of rain coming. It’s hard to believe when the sky is cloudless, but it’s true. I’ve witnessed this many times and usually, the pressure is falling and a front arrives shortly after.

Brown trout are most affected by falling barometer pressure, often totally disappearing at the same time, though Rainbows will feed confidently. Other times, you will only catch undersized trout. At spawning time you will only catch undersized trout.

Low pressure encourages trout to move upstream or to gather at river mouths – sea or lake – awaiting a fresh.

A rising barometer pressure heralds clearing weather and rivers, resulting in active, confident trout. They remain active until pressure drops or other influences such as temperature change trout behaviour. 

Sportinglife Turangi, Umpqua Stream Thermometer.

This post was last modified on 15/03/2024 12:23 am

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