TROUT STOCKS IN RIVERS

Otago Daily Times, Issue 25800, 22 March 1945, Page 8

TROUT STOCKS IN RIVERS

(By J. T. L.) It is realised that there can be no trout in rivers which do not contain ample food supplies, and the question is often raised as to why our acclimatisation societies do not make a move to establish suitable food stocks. The bully is usually suggested as the most profitable type for the experiment. The answer is, of course, that the conditions are most probably unsuitable to the bully’s existence or else it would be found in the particular place where 'it is suggested putting it. It. did not take very long for the trout, once it had become established in a few New Zealand rivers, to work its way throughout the length and breadth of the accessible water of this country. Travellers from inaccessible places like South Westland often report well-stocked streams into which no trout have ever been liberated. Surely, then, a native like the bully, with thousands of years to accomplish this, will have established itself in every stream and river that is capable of affording it an existence. The number of trout that any given river can support is governed by several factors. First, there must be an adequate supply of food all the year round. It is not sufficient that there should be an abundance at any one time, if there is another time during which there is a famine, for under threat of starvation the trout will move off elsewhere. Secondly, there must be sufficient water throughout the year. It cannot be hoped to see heavily stocked a river that dries up along miles of its length eacli summer, as do several Canterbury streams. Next there must be ample spawning grounds—smooth stretches of clear, shingly bottom, free from silt, which may choke the eggs. There have frequently been reported from overstocked streams cases of a second female ploughing up redds laid by an earlier fish because she was unable to find space on which to deposit her own. It sometimes happens that a body of water has no suitable spawning ground at all, as is the case with the Tomahawk Lagoons, which must of necessity be restocked continuously with young fish. But perhaps the most important requirement of a river that is to carry a large number of trout is that it must be free from floods. Nothing is more detrimental than a flood. It destroys the eggs, it smothers the insect life on the bed, and it tears the vegetation from the banks, destroying the small living things that form such an important part of the diet of the young trout. It literally drives the fish out of the rivers. Thus it is seen that the food problem is not thp only factor that has to be contended with, and the introduction of other forms of fish, were it feasible (which in most cases it is not)'may not be the solution to the question of building up trout stocks.