The Nature Column.

Digger (Invercargill RSA), Issue 33, 29 October 1920, Page 11

The Nature Column.

(By "Student.")

("Student" will be pleased to receive notes on any brancn of Natural History. Observations on birds, insects, plants, etc. , will be equally welcome. lf using a pen-name, will correspondents please enclose real' name and address. ) Dr. Tillyard has prepared a very interesting report on the Neuropteroid insects of the Hot Springs . region of N.Z., in relation to the problem of trout food. This report ifi of interest to nature students because it exemplies once again the folly of interfering with the balance oi nature, without due regard to the consequenoes. Examination of trout stomachs showed that a great diversity* of food was eaten. The green manuka beetle, Pyronota festiva, was the most abundant iood. Next to this was the larvae of the Caddis-fly family Leptoceridae, which form their cases on a green weed (Nitella). Another important food was a sxna.ll Molusc Potamopyrgus sp. Other foods were larvae of Dragonflies, Mayflies, Stoneflies and and other Caddis flies. B.esides these, occasional remains of crayfish, small fish, and other insects were found. Some stomachs were quite empty and others filled with stones. A considerable number of the fish were slabs, and this appeared to be due to semi-starvation and indigestion.^ In some cases the slabby condition appeared to be caused by th6 indigestibility of the food. The claws of large crayfish were found obstructi'ng the diges. tive passage, and Dr Tillyard does not think that the introduction of a larger species of crayfish shoxild be encouraged. The Neuropteroid insects inhabiting the Thermal Region belong to six orders and of these the Stoneflies, Mayflies, Dragonflies, Alderflies, and Caddisflies, are those important from a food standpoint. The Alder fly larva, known as the black creeper or toe biter, is large and fat. Though generally abundant in N.Z. it has been exterminated in this region. The Stoneflies, which are a valuable trout food, have been reduced by some 80 y:er cent. The Mayflies inhabit both streams and lakes and are a most important article of trout diet, being soft and easily digested. In the Northern Hemisphere they have evolved alongside the trout and have resorted to various devices fln order to escape attack. In this country they were not subjected to the same rigorous weeding out, and have not evolved the protective expedients of similar European species. Some of the finest Mayflies in the world are found in New Zealand, and some species of these have, also been wiped out. A moderate estimate of the diminution of the Mayflies in the hot springs region is over 50 per cent; The Dragonflies are important trout food and they have Deen much reduced in numbers. lue Caddisflies . constitute. another first-class food for trout and probably not 10 per cent remain. It will thus be seen that . in this place where the rapacious Rainbow trout is abundant, the native fauna suitable for trout has been almost extinguished. Briefly it may be stated that before the ti'out were introduced the rivers of New Zealand swarmed with an aquatic insect' fauna which had not formed protective hablts. The introduced trout, especially the Rainbow, gorged themselves on the rich food, and soon record size fish made the district famous. No proper attempt was made to readjust the food supply, and it began to fail. Slabbiness and disease appeared. Netting produced some improvement. There is not enough food for the trout now., and some of the streams are eaten out. Dr. Tillyard somewhat caustically s1 ras up the position, by comparing it with that . of a grazier who puts 10,000 animals on a 1000 acres of rich land, and wlfen the food begins to fail, prepares to r,emedy it, by putting in a fresh supply of calvcs every year. The remedy proposed by Dr Tillyard for the present state of affairs is one which our Acclimatisation Societies might well take note of. He does not pxopose to irapbrt foreign insects to further upset the balance, but suggests that certa.n streams should be blocked off and the trout cleaned out. Restocking of the d.enuded waters would then take place from points in the streams which the trout have not yet reached, and the iiaci .-e- ;iisect fauna would reassert itself. One stream after another would thns be treated until a normal balance betw.een trout and food fs established. In additicn a lake and its streams should fce set aside as a sanctuary for insects. At the same time the lak,es should be netted to remove all large fish as they " ara ft serious detriment to the feed. and to the naor,e normal sized fish. He says "with the maximum poesible native food supply, a New Zealand fishery ougfit not to be expected to produce anything beyond a steady and assured supply of reasonar>te

eized fish." From the foregoing ir, wiJi be seen that large fish and good Ssh-Ing cannot both be had. The members of the local Acclimatviation Society would do well to get Dr. Tillyard'g paper and read it with diligenee. It is puiblish.ed by the Linnean Society of New South Wales.