Art. XXVII.—On the Cause of the Disappearance of Young Trout from our Streams.
[Read before the Wellington Philosophical Society, 9th January, 1889.]
During the past fifteen years trout-fry have frequently been put into the streams in this district (Palmerston North), but, so far as I am aware, they have never been seen afterwards. Some thought they had been devoured by eels, and others that they had been washed out of the streams by freshes. As it is a subject in which I take considerable interest, and one which has cost many a pound to no purpose, I set myself some twelve months ago to discover, if possible, what was really the matter. It occurred to me that there was little use in examining the streams, or in speculating on the cause of the mischief, and that something more practical would require to
be done. I had a tank erected, 12ft. long by 30in. wide and 24in. deep, divided into three compartments with movable partitions. The front of this tank or aquarium is of glass, so that I can observe all the movements of the fish; and, although I have not a constant flow of water, there is sufficient to answer my purpose. I observed that in all the streams in this district there are large numbers of minnows (Galaxias attenuatus), which are pretty to look at, and apparently innocent and timid; but, as they take the worm freely, I thought they might just as readily take the young trout. I placed seven of them in the tank, and gave them worms nearly as long as themselves. These they ate greedily. I then gave them dead whitebait, which they ate as freely. My next experiment was with twenty-five live trout six weeks old; and within twelve hours they had all disappeared except one, which may have been hidden among the plants, and so escaped their observation. By this time I was satisfied that they were at least one cause of the mischief, but to make certain I tried once more. I placed two minnows in the tank with fifteen trout nine weeks old; and within half an hour three of the latter had disappeared. I then dissected one of the minnows, and took from it one of the trout, which had been swallowed whole. I may add that the minnows had been well fed before they were placed with the trout; and if they will devour them under these circumstances, we may well understand how readily they will attack them in the streams. There may be other reasons for our want of success in stocking the streams; but, apart from all others, this of itself is sufficient to account for it.
Experience has proved that it will not do to place the young fry in the streams and leave them to take care of themselves, and an important question would be, at what age will they be old enough and strong enough. This I cannot answer definitely. I have had the trout with the minnows when they (the trout) were seven months old, and they seemed to live in harmony. Whether they would do so at an earlier stage I cannot say.
I think if the following plan were adopted we might reasonably look for success: Let the fry be brought from the breeding-ponds, as formerly, when they are, say, four or five weeks old, before they have lost the umbilical sac, for reasons which will be apparent to any one at all acquainted with the subject. Place them in a pond through which there is a constant flow of water, and where other fish cannot get at them. Let them be kept there until they are, say, twelve months old, after which it will be quite safe to turn them into the stream. The pond could be cut out close to the stream from which the water-supply would be taken, and the overflow could go back
to its former channel. I would have a number of large stones in the bottom of the pond to provide hiding-places, as the trout of all ages seem inclined to lie quiet during the day, and come out at dusk to rove about. While kept in the pond they should be fed regularly.
I have pleasure in adding that I have been greatly indebted to Mr. Ayson, of the Wellington Acclimatization Society, at Masterton, from whom I have received much valuable information, and small parcels of trout for experimental purposes.