Art. XXI.—On Diseased Trout in Lake Wakatipu.
[Read before the Otago Institute, 15th August, 1882.]
For several years back the large trout in Queenstown Bay, Lake Wakatipu, have been a subject of notoriety and interest to visitors and others. These trout, in weight from 2 lbs. up to 15 lbs. or more, and in shoals of several hundreds, frequent the east margin of the bay near the mouth of the Town Creek, the reef at the end of the Peninsula, and the shore near the Onemile Creek on the west side. They appear quiet and lazy in habit, except when, as often occurs, they throw themselves with great vigour into the air.
It is also to be remarked that they are to be seen at these places both in summer and winter, caudal and dorsal fins on the surface, and an occasional one may also be observed in the middle of the bay or at the steamboat jetty. The water is very deep throughout this bay, but has a shallow margin a few yards in width running round parallel to the beach. The great body of the lake itself is abysmal, the only sounding got as yet being at a depth of 1,300 feet. Along the east side of the Queenstown Bay there is a belt of weeds growing on the bottom on the outer edge of the aforesaid shoal-water, the bottom itself consisting of shingle, gravel, and sand. The water of the lake is remarkable for clearness and purity, and is snow-fed through the Dart, Rees, Greenstone, and such rivers, which drain the surrounding mountains of the Southern Alps. Dr. Black, of our University, remarks, on his analysis of this water, that he never examined any water so destitute of common salt—a fact of great importance as regards the health of trout. His analysis I may repeat is,—
|Organic Matter in Solution.||Table Salt.||Degree of Hardness.|
|Wakatipu—0.5 grains per gallon.||Scarcely a trace.||3.1 degrees, very soft.|
In summer storms are frequent on the Wakatipu, but in winter its surface is generally calm or nearly so. The trout S. fario, ansonii, Günther, were put into the feeders of the lake about the year 1874.
Besides the abnormal habit of great trout herding together in shoals, these Wakatipu fish almost without exception have refused the baits of anglers who have fished for them. The exception I refer to is that of a beautiful but small trout of 1½ lbs. weight, taken by Mr. J. P. Maitland with minnow, while fishing from the beach of Queenstown Bay in January, 1880. This fish was remarkable by its bright silvery scales, the absence of all spots excepting on one gill-cover, and the absence of teeth on head of vomer. The condition generally of these trout is suggestive of good living, as they are fat and sometimes very much so, and are said to possess excellent edible qualities. But for some years I have heard of the presence of
fungus among them, eventuating occasionally in the death of very large ones. It was only recently, however, that I was so fortunate as to see these trout and to observe this fungus on them. One morning in June of this year I took a walk round the shore, past the mouth of the small Town Creek already referred to. The water from this stream enters the bay at its north-east corner exactly, and in almost direct alignment with shoal water frequented by the trout, consequently the influence of the creek water is noticeable for thirty or forty yards along the shore. Here within a few yards of the mouth of the stream I saw several large trout quietly resting, but the great body of them was stretched along from this point for a distance of about a hundred and fifty yards, in three shoals or “schools,” containing more than a hundred fish each, and distant from the water's edge as far as the belt of weeds grew. In weight I estimated them from 3 lbs. up to near 20 lbs. They all lay with their heads towards the creek mouth, except when one or two took a leisurely turn round and resumed their former position again, or when others evidently excited by some influence threw themselves wildly into the air, falling heavily, or splashed along the surface. They did not seem to care much for the presence of a human spectator, and in this their habits differ from what obtains in rivers or streams usually. Presently I was surprised to observe a trout of about 6 lbs. in weight, swimming within a rod of me parallel to the shore. As it showed no alarm I moved along with it, and then discovered that it was all covered with fungus. Concluding that it would probably soon die from its unnatural movements, I endeavoured to secure it for examination, but although I passed the crook of my stick easily over its tail, yet it resented the effort I made to draw it ashore, and swam off into deep water. Abandoning all hope of seeing this fish again, I examined as carefully as I could the other fish in the shoals, when I perceived easily, as the water makes the white spots very plain to the eye, that at least 25 per cent. of them had marks of fungus on their bodies. On the larger trout a patch or two of dirty white was seen on the head generally, and a tuft hanging out of the side of the mouth; while in breathing they could not close their jaws, and showed very little motion in them at all. Some of the smaller ones were worse, their bodies and fins being covered with spots or patches. The most of these trout were dark in colour, while one or two I occasionally noticed were light-coloured, but whether these were diseased or not I could not make out. So clear was the water, and so tame and subdued were the trout, that standing as I did on the shingle, I could plainly distinguish the sexes from one another in the larger individuals. I then walked along the beach towards the reef at the point of the peninsula, but saw no more trout till I reached that place. There, however, I soon saw a number of them, from
10 lbs. to 20 lbs. in weight, springing out of the water. These seemed also to be dark in colour as they rose to view in the air, leaping to a height of four or five feet. I may mention here that the previous night on visiting the beach I heard many trout splashing about and out of the water, so nightfall with its keener air did not put a stop to their gambols, or irritation, whichever it might be. Returning towards the point where I had seen ths diseased six-pounder, I observed one with its tail-fin out of the water, belly up and head on the bottom, in shallow water, drifting ashore. Wading in I seized it by the tail and easily ran it out and laid it on the shingle. As it was nearly dead I did not kill it, and in a few minutes it succumbed. It was evidently the same trout that I had been watching half an hour before, as it had the identical fungus marks I had been observing, and it weighed 7½ lbs. The fish was a female trout, fat, but dark in colour, badly spotted on dorsal and all the other fins with fungus; the gills were full of it, and a tuft hung out of the right side of the mouth, while the back and sides had a number of distinct marks or patches, some appearing as if due to the mucous covering having been eaten away by the disease. The margin of right opercula and origin of right pectoral fin were also eaten away. Lying on the beach near the creek mouth I saw the skeletons of two other trout which had evidently come ashore after death.
Examination of the Diseased Trout.
The same evening, twelve hours after getting this trout, Dr. Douglas, of the Wakatipu Hospital, and I made an examination of it. Immediately after death it had visibly swelled, and continued to do so till the abdomen became very much distended—a thing which never occurs with healthy trout. On opening it we found it full of ova nearly ripe, the roe-lobes having a hard appearance; pyloric cæca fatty, but not healthy; stomach quite empty, and air-bladder very much swollen with gas. The other viscera seemed healthy. A number of the blood-vessels lining the abdominal wall were full of coagulated blood, but that is not unusual. The teeth on body of vomer were gone and the gills were of a dull purplish hue. In attempting to remove a patch of fungus from the gills it could not be separated, so firmly had the roots taken hold, and the tissues came away easily with it. The gills, in fact, were rotting.
A small portion of fungus placed under the microscope showed a form and structure remarkably like Saprolignia feraw (figs. 1 and 2, pl. XXIII.), and, so far as I can judge, apparently the same disease; but of that I cannot be positive. Plenty of long sacs full of spores were present, with innumerable free cells floating about them, and some in circular sacs resembling Ogonium (or spherical sac containing spores and supported by short stems), but destitute of stems. No sacs were seen in the protoplasm
stage or without cells, and the cells themselves were exceedingly minute, just visible clearly under a two-inch objective. A subsequent examination made since returning to town (the fungus meanwhile having been preserved in glycerine and boracic acid) gave similar appearances, but no Ogonium. It, however, revealed the fact of the spores being contained really in an inner sac or tube, the space between which and the outer covering of the main sac appeared to be full of a colourless fluid. From the appearance of this trout and that of others in the shoal from which it was taken, it is manifest that these fish are in a chronic state of disease, and that not confined to this the spawning season, for I have ascertained the presence of fungus at other times, as in the month of March of this year. And here it may be observed in passing that fungus has been found on trout at the Wallacetown ponds in 1876, and recently a Marlborough gentleman told me of his taking out of some still pools in a stream in that district fungussed trout years ago, while our native fish the Galaxias and silver fish are not always free from it. At the same time the identity of the fungus among all those fish has not been determined. A consideration of the above facts naturally suggests two questions, first, what is the cause of, and second, what the cure for, the fungoid disease in the Wakatipu trout.
The Cause of the Disease in the Wakatipu Trout.
In the first place, so many difficulties surround the investigation, that the cause or causes of the disease cannot well be presumed to be stated exhaustively. At the same time, so far as our knowledge of the habits of trout and of the conditions necessary to their healthy life enable us to judge, we are warranted at least in advancing an opinion. I assume then that the trout in Queenstown Bay were spawned in the Town Creek, a stream far too small for the subsequent accommodation of the size of fish to which these attain. Growing too large for this stream they have naturally dropped down to the lake during floods, and when there have so increased in size in the course of a few years as to become physically incapable of again ascending the stream at their regular spawning season. No stream large enough seems to be sufficiently near, and the great depth of water along the shore to the west, without leading shoals, tends to confine the trout so to speak to one place, or at least to operate against their migration in that direction. In this respect the Wakatipu is totally different from the streams where the progenitors of our trout live in England, where the water does not probably have a greater average depth than four feet. With the true instincts of the Salmonidæ, however, the trout in Queenstown Bay linger near their parent stream, unable so to speak to convince themselves how it is they cannot be again admitted, and, diseased as they have become, presenting an appearance suggestive of the lame and sick folk of old who
waited for the “troubling of the water.” Being unable then to fulfil the functions of nature at the spawning season, is the first contributing cause to the outbreak of the fungus.
Again the chemical constituents of the water have an important bearing on the health of the trout. Trout under domestication when attacked by fungus have in almost all cases been cured by the addition of common salt to the water supplying the ponds or tanks containing the fish, provided the disease has not been permitted to go too far. As already mentioned, Dr. Black reports that the Wakatipu water has less salt in solution than any water ever examined by him. Now, as salt is an essential to health in trout, its entire absence in the water under consideration must act prejudicially on these fish. This is the second and only known cause tending to accelerate the outbreak of the disease. But there is yet another cause which I suspect, although not in a position to prove, namely,—the absence of a due proportion of oxygen among the gases held in solution by the water. To determine this, not only is a gaseous analysis required, but it is also necessary to find out what that quantity of oxygen is which trout require. Science has yet to discover this ratio so far as I know, and it is an important element in its bearings on this question. As already stated, the fact of the trout seeking those places, as the mouth of the creek and the reef, where oxygen is likely to be most abundant owing to the constant agitation of the water, shows that the instincts of these trout teach them to look for water where the best aeration is to be found.
These causes, then, seem to me sufficient to prove that the disease among the Wakatipu trout has been consequent on functional derangement, and that this has so lowered the vital force of the fish as to leave them powerless to resist the attacks of the fungus, a plant which the best authorities tell us is present in all fresh waters.
Can the Disease be cured in the Wakatipu Fish?
And here I confess that, considering the unfortunate situation of these trout in Queenstown Bay, no ordinary remedy could be applied efficiently. For although the submergence of rock salt at the places frequented by the fish, and the artificial increase of the water supply to the Town Creek, might probably lessen the extent of the evil, yet these applications could effect but a partial and temporary check on the disease. Moreover, there would be no finality to these operations, and their cost would exceed the means of the local Acclimatization Society I fear. No doubt it would assist if the trout were netted and all affected fish killed and burned; but in this there might be no finality either, still it ought to be done. While I am bound then to admit that I see no specific cure of an easy and cheap nature, there is yet hope, I think,
from a most unlooked-for quarter—that is in the disease itself. For you may remember that I have mentioned the circumstance of finding the remains of only two trout on the beach, and, from anything I could learn, these trout, frequenting the same localities, have not as yet died in large numbers at a time. It is possible, therefore, that this fungoid disease, loathsome in appearance and widespread as it certainly is among the shoals, may either die out, or so inoculate the healthy fish, as in process of time to render them proof against severe attacks. A correspondent of “Land and Water,” in the number of that journal for March 25th, 1882, records his having seen trout in the river Kent which had been affected by fungus and had recovered without any artificial treatment. Nature, then, may yet work out a recovery among the Wakatipu trout in her own time and way; but that must be assisted by our providing facilities for natural spawning in the Town Creek.