OCR Text |
Show 21 MR. MORTON ALLPORT ON THE [Jan. 13, The proportion of living ova deposited was estimated at above 45 per cent, of all sent out. Since the deposition of the ova in April, 1864, several great improvements had been effected by the Commissioners in the arrangements at the Plenty, the chief of which was the alteration of the gravel in the breeding-boxes. To explain the change and the advantages of the present plan, it must be remembered that in its own rivers the Salmon chooses for its spawning-beds shallow rapids running over a bottom of coarse river-gravel, consisting of pebbles weighing from half a pound to three or four pounds, the spaces between which are of course large enough to permit the ova to roll down to depths varying from a few inches to a foot and a half. This is no doubt a wise provision of nature for the protection of the ova and the helpless young fry from their innumerable natural enemies, but has serious objections in artificial rearing. To begin with, it is absolutely impossible, in the first instance, to separate the dead from the living ova; all must be rapidly transferred to the water together, and the dead ova gradually picked out afterwards. In 1864 numbers of dead and living ova together got out of sight between the interstices of the gravel purposely made to resemble as nearly as possible the natural spawning-beds, and much of the living ova was assuredly destro}*ed by contact with that which was decomposing, to say nothing of the ill effects which the decaying ova would have upon the water generally. Again, it is now an ascertained fact that a considerable admixture of atmospheric air is indispensable in hatching the ova of most of the Salmonidae, and that, consequently, the further the ova are from the surface of the water, the more tumble and splash you must have in the water to drive bubbles of air through and amongst the gravel. It follows that if in artificial rearing the ova are allowed to get some 3 or 4 inches down into the gravel, a sharp stream of water must be directed over the artificial beds to supply them with the air necessary-but that if it is desired to keep the ova in sight, they must be placed on fine gravel, and an even, gentle stream of water about an inch or an inch and a half in depth must flow through the beds. As in the artificial process the boxes are thoroughly guarded from all possible enemies, the advantages are so manifestly in favour of keeping the ova in sight, that the Commissioners replaced the coarse gravel formerly used by an even bed of very fine pebbles on which the ova rested about an inch from the surface of the stream, which was made to flow gently and evenly through the boxes. The result was, that the moment an egg became opaque, or, in other words, died, it was removed, and all danger to the neighbouring ova was avoided. Besides this alteration, a long series of gravelly rapids, with a few deep places interspersed, was added to the lower end of the Salmon-pond, and a new circular pond with a gravelly rill attached was constructed for the Salmon-trout. On the 8th of May, 1866, the first Salmon-ovum per 'Lincolnshire ' hatched, and on the 12th of the same month the first Sea-trout ovum. By the time all were hatched the Superintendent reported that he had counted up to 4490 Salmon-fry and 496 Sea-trout fry |