Notes on the Biology of Lake Taupo.

By J. S. Armstrong, M.B.

The lake may be described as a deep cistern lake with a relatively flat bottom at an average depth of 350ft; the rise and fall in the lake level is very slight, the maximum variation having been only four feet in the last six years. As all the supply-rivers are short, the height of the lake is governed by the local rainfall; this rainfall may take place at any time in the year, with the result that there is no certain seasonal rise or fall, but on the average the lake is slightly higher in the spring than in the autumn.

About half the lake-edge is composed of precipitous cliffs of rock or pumice, while the remainder is mostly bare and fairly steep sand-shore, although in a few places, such as round the delta of the Tongariro, there are swamps. The sand-shores and cliffs show very few signs of erosion, although there is usually some wind and swell on the lake. The slight rise and fall in the level of the lake does not materially extend its area, thus bringing large alluvial areas under wave-action, so that we may assume that the main supply of dissolved salts found in the lake came from the rivers or from the decomposition of the organic matter taking place on the lake bottom.

The amount of organic and inorganic salts present in the water is small, the analysis of Phillipps and Grigg giving 10.98 parts of solids per 100,000 of water, and 9.0 milligrams of dissolved oxygen per litre. Their distribution through the water is very even, brought about by the free circulation of the lake water in the winter.

The Temperature.

The temperature of the lake varies little from year to year; the earliest records of Dieffenbach and Hochstetter are similar to readings obtained at the present time.

During the past six years the coldest lake-temperature that I have recorded was 49° Fahrenheit, in the shallow part of Taupo Bay, on June 30, 1931. On January 23, 1931, the minimum temperature in the water of the lake down to 420ft. was 50°, the maximum temperature at the same time being 62° on the surface of the lake. In the upper 60ft. of the lake the average fall in temperature was one degree for every 12ft.; if this average continued, then the thermocline in mid-summer would be at about 150ft.; this means that all the water below 150ft. will be at the minimum temperature and maximum density, and so will not circulate till the winter has equalised the temperature and density of all the lake-water.

The surface-temperature of the lake falls to 50°, and remains at this for three months every year, on the average; as breezes are nearly always present over the lake, this period will be ample to allow thorough circulation and aeration of the depths and the diffusion of any salts that have collected at the bottom during the period of stagnation in the summer.

The highest surface temperature that I have recorded over deep water was 68°, on January 31, 1932; on the same day the water at the edge of the lake was 72°. In some years the temperature probably rises a few degrees above this, as in 1926 and 1927, when the native fish died in thousands, and were thrown up on all the shores of the lake. High lake-temperatures appear to cause a heavy

mortality in the smaller fish of a lake all the world over, but in Lake Taupo such a condition is only produced when the maximum sun's rays fall on a glassy lake unstirred by a breath of air; then the cooling effect of evaporation is at a minimum and the aeration of the water will be low; under such conditions the infestation with parasites, such as Ichthyopthirius multifilus (Fouquet), is much more severe; these various factors will no doubt all play a part in the fish mortality.

The higher temperature is not harmful to all the inhabitants of the lake, for the molluscs, such as Isidora antipodea (Sowerby) and Myxas ampulla (Hutton), take advantage of the smooth lake surface and the stimulation of the high temperature to become air-breathers instead of water-breathers; this would appear to stimulate their metabolism to a very great extent, so that they grow and reproduce more rapidly, until, after a succession of two hot seasons, they almost completely cover the lake-bottom in shallow water; the first then, for a time, becomes the main food supply of the trout, while the other is that of the bulley (Gobiomorphus).

Light.

The penetration of light into the lake is governed in the first place by the altitude of the sun; if the sun's rays strike the water at an angle of less than 20° they are all reflected, so that lakes such as Windermere, in England, have no light penetration in the depth of winter. At Taupo the sun rises above 20° for some four hours at mid-winter and twelve hours in summer, so that we have the stimulating influence of light all the year round.

The penetration of light can roughly be estimated by the visibility of an object in the lake; with this view tests were made with a Secchi's 20 cm. disc at noon on several days. The day chosen for the test was sunny, with as few clouds as possible, and a smooth lake so that refraction of light was not caused by the waves. The sun was always shining brightly when the test was made. On 11th January the disc could be seen to a depth of 220ft., on 31st March to 210ft., and on 17th June to 60ft. When sufficient light can be reflected from an object to render it visible at a depth of 220ft., it is almost certain that some degree of light will reach all parts of the lake bottom, which is only 520ft. at its deepest spot.

The Lake-bottom.

Dredging shows that most of the lake-bottom below 300ft. is smooth, being covered with a brown sediment composed of the organic and inorganic remains of the plankton of the lake. In and on this there live numerous molluscs, turbellarians, nemerteans, Oligochaeta such as Taupodrilus simplex (Benham), and the larval stages of a large Chironomus;^* the last mentioned when they reach the pupal stage float up to the surface of the lake to hatch into the mature insect; the others live at the bottom of the lake throughout their lives. Two of the molluscs, Potamopyrgus antipodum zelandiae (Gray) and Potamopyrgus corolla (Fischer), are of interest because even when dredged from over 400ft., they do not appear to notice

the change in pressure that occurs when they come to the surface, but continue to live just as well under a few inches of water. This faculty allows the millions of individuals that live at the lake-bottom to act as a reserve that constantly restocks the lake-shores. Thus Potamopyrgus and Isidora become the chief food of the trout when the supply of small fish fails, due to the mortality associated with a high lake-temperature.

In places where the bottom is not covered with organic debris we find beds of sponge associated with chara and the liverworts; these beds appear from samples brought up by the dredge to extend to a depth of at least 300ft. in some places, and have the same fauna living in the sponge as is found over the sedimentary bottom of the lake.

The Plankton.

The plankton of Lake Taupo is remarkable for its uniformity; at any season of the year the same species will be found in all the areas of the lake, and in very much the same amount. During the last year the prevailing species has been Nostoc commune (Vaucher) or some similar species. This was noticed to be appearing freely in the lake in December, and remained the dominant species right through till the end of June. In March it was very freely devoured by a protozoan, so much so that for a time there was an appreciable diminution of of the Nostoc in the lake; but some factor, probably the cooling of the water, killed out the protozoan before the middle of June and the Nostoc became as thick as ever, but was associated with other species of algae and also large numbers of microscopic crustaceans of the orders Cladocera and Copepoda.

The growth of algae through a period of so many months is no doubt due to constancy of the sunshine over the lake, and to the fact that the temperature never falls low enough to inhibit growth.

The total duration of sunshine on the lake will average about 2300 hours a year, of which nearly two-thirds will fall during summer and one-third in winter; so that there is sufficient difference in the amount of sunshine to cause distinct seasons.

The Native Fish.

The native fish of the lake belong to two families, and have different habits. First we will consider Galaxias brevipinnis (Gunther), the kokopu of the Maori. This fish in its adult stage lies at the bottom of a stream or near the lake-shore, hiding under stones or any other cover that is available; its younger stages are spent as a free-swimming fish, when it may be seen going up a river or along the edge of the lake in large shoals; all the fish in the shoal are of approximately the same size and presumably of the same age.

The food of the kokopu varies considerably, but preference is shown for insects either in their larval or adult stage; also they seem to have a distinct liking for ova or spawn, as is common with many of the salmonoid fishes.

The second fish is the Gobiomorphus gobioides (Cuv. and Val.) or toitoi of the Maori. This is a typical ground-fish, for all its stages are spent on the bottom of the lake or stream; its colour varies

with its surroundings—on the black rocks it is almost black, and when hiding in holes it assumes a blue-black sheen which is most effective against the glistening rock, while on the light sand it becomes just as pale as the sand, with a few black specks; this change of colour can be accomplished in a few moments; and on it the fish relies for protection from its enemies. The adult Gobiomorphus likes to burrow under the cover of any stones just as the adult Galaxias, but the young of the Gobiomorphus, driven out from this cover by their larger brethren, lie on the open sand areas or on the surface of the stones and rely entirely on their colour for protection from their enemies. When they swim, they do so with a swift darting motion, to settle again on or under some object that suits their natural colouration; otherwise their colour quickly changes to suit the new surroundings. The food of Gobiomorphus consists of the small molluscs and insect-larvae found on the bottom; they never rise to the surface as does the Galaxias, not even when they are feeding side by side in the same stream.

In the early days Galaxias was the dominant fish of the lake; it was so numerous that it used to form one of the main items in the food-supply of the Maoris of this district, and the early writers describe it as having been washed up in cart-loads on the shores of the lake at times. That Gobiomorphus was also present there can be little doubt, for it is mentioned by Hector that “The Hawai of the Taupo Natives is probably the Eleotris or Bullhead”; and Sherrin states that “The Natives say that the Toitoi is found in Taupo in large quantities. It is caught with nets. Though dried, as the Kokopu, it is not considered so good a fish.” Toitoi is the name still used by the local Maori for Gobiomorphus.

The Introduction of Trout.

The Brown Trout (Salmo fario) was first introduced into the rivers that flow into Lake Taupo in 1894. They grew so rapidly that in 1906 fish of twenty pounds and over were taken.

In 1903 the Rainbow Trout was first introduced, and further supplies continued to be brought till in 1905 every stream, large and small, was stocked. This fish rapidly replaced the Brown Trout, so that now only a few remain in such places as the pools at the estuary of the Tongariro. This has probably been brought about because of the destruction of the spawn of the Brown Trout by the later-spawning Rainbow.

But more remarkable still has been the almost complete destruction of Galaxias in the lake; this fish with its stripes and mottling of dark brown on a light green or yellowish-brown ground is admirably coloured for protection in a river where the banks are overhung by the branches of trees and lower-growing vegetation; the colouration probably also serves quite well for concealment against the dark stones and vegetation on the lake-bottom from the attacks of the small shags which have always been its inveterate enemy on Lake Taupo. On the introduction of the trout, its dark bands, in the absence of silvery scales on the under surface, rendered it a conspicuous object in the clear waters of the lake to its new enemy.