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TROUT FISHING WITH MAURICE RODWAY - Weekly Column: January 11, 2002
 Southland, New Zealand

Lake Outlets Are Trout Heavens

The greatest densities of trout occur in lake outlet streams. The upper Clutha near Lake Wanaka has hundreds of trout per kilometer. The Waiau is similar. Even smaller lake outlet streams, such as the Buller at Lake Rotoiti, the Gowan at Lake Rotoroa,  in Nelson, the Arnlod at Lake Brunner on the 'Coast and the Hurunui at Lake Sumner in North Canterbury have trout packed into them, almost like proverbial sardines.

None of these streams are really small of course and that in itself is an important reason for their large trout populations. There are other reasons too. Nothing in nature is so simple. Lake outlets have relatively constant temperature ranges. In the winter they are not too cold so as to stunt the growth of trout and in the summer temperatures are never too high to cause stress. This is largely a function of the large body of water from which they come. Water has a relatively high thermal inertia so it resists large fluctuations in temperature. All the lakes mentioned are in the high country of the South island so air temperatures, which affect the water, are not extreme either.

The constant flow of water provides a stream of food that can sustain a large number of trout. Actually there is evidence that a more disturbed regime is more productive. For example many rainfed rivers with unstable gravel have more stream insects per square metre than lake outlets streams. The Mataura is an example. But the floods and the low flows in these streams, especially the latter, result in a lower population than in lake outlet streams.

The food in lake outlet streams is often based on filter feeding insects such as net building caddis flies. Sandflies are also found in abundance as their larvae are filter feeders too. The lake outlet water contains a large amount of plankton, which has developed in the lake. Filter feeders make the most of this and their numbers can be high as a result. They need large, stable boulders to live on and lake outlets usually have these – remnants of glacial moraines that were deposited during the ice ages of the last one hundred thousand years.

Lake outlets are often difficult places to fish. They rarely have exposed gravelly beaches from which to cast from and their large size makes their fish difficult to reach. For much of the year water flows amongst scrub making fishing almost impossible in large parts of them.

Trout in lake outlets therefore have an ideal environment. They are protected from over harvest, from extremes of temperature and flow fluctuations, and they have a steady supply of food drifting past. No wonder lake outlets are trout heavens.

Maurice Rodway
Southland, New Zealand                           E-mail: mrodway@southlandfishgame.co.nz

Article © 2002 Maurice Rodway, All Rights Reserved.

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