DOMINION TROUT STOCKS

Press, Volume LXXVI, Issue 23130, 20 September 1940, Page 10

DOMINION TROUT STOCKS

System of Liberating

Fry

WISDOM QUESTIONED

The utility of liberating trout fry. as is widely* practised by acclimatisation societies throughout New Zealand, to maintain the stocks of fish in the streams is questioned by Mr Derisley F. Hobbs, fresh-water biologist at the Fisheries Laboratory, Wellington, in a bulletin issued by the Fisheries Branch Of the Marine Department.

The bulletin is a study of the natural reproduction of trout, and its relation to the density of trout populations, in New Zealand waters. In it Mr Hobbs says that trout populations are limited by natural causes and that it is vain to endeavour to increase them above those limits by liberations. As a result of extensive researches in streams of Otago and Auckland provinces, Mr Hobbs ascertained that trout populations appeared to be generally limited by the extent of the spawning grounds available.

The natural reproduction of introduced rainbow and brown trout was found to be a highly efficient process under favourable conditions. Such conditions were usual in New Zealand trout streams. Where satisfactory facilities for natural reproduction existed. the development of stocks of trout had been correspondingly satisfactory. It appeared that, whether or not liberations continued to be made regularly, the introduction of trout was successful largely according to the extent ■ that facilities for reproduction existed. Spawning Areas Trout required very special conditions for spawning. They were very particular as to the size of stones, degree of consolidation, and gradient of their spawning-redds. The extent of suitable areas was therefqre very limited, even in streams with beds predominantly of gravel from the hills down to the sea. Rather than spawn in ■ unsatisfactory materials, late spavvners would occupy sites used by previous comers, and this superimposi. tion caused fairly extensive losses of eggs through displacement and damage. New Zealand stocks had developed to the point where this condition occurred in a considerable proportion of spawning grounds. Discussing other causes that could limit the stocks of trout, Mr Hobbs points out that lack of food would result in cannibalism, reduced growth of individual trout, and lowered fecundity. Cannibalism appeared exceptional among New Zealand brown trout. Study of rates of growth and intensity of spawning suggested that trout populations developed till checked by lack of further spawning grounds, not lack of food. Natural enemies—shags, eels, and bullies—had not restricted the development of stocks.

When the stock of trout in a stream was reduced to the level where its natural reproduction was most efficient, it was capable of producing more mature fish than were necessary to maintain the stock at that level—thereby providing scope for exploitation in the way of angling. To keep the stock below this level, and make regular liberations, was uneconomic, as it necessitated expenditure of money to effect what the stock itself would do if intelligently exploited. If the stock was maintained at that level, on the other hand, no advantage accrued from liberation of fish to provide more than would be taken by angling. Liberations of fish even just sufficient to balance the numbers extracted by anglers might not serve a useful purpose if they safeguarded from exploitation the quantity of fish which was produced naturally. "The wisdom may well be doubted of attempting artificially to aid the natural increase of populations (be they of deer, oysters, or trout) by releases into areas where a natural ability to ihcrease is already being frustrated by environmental limitation," Mr Hobbs concludes.