Page:EB1911 - Volume 21.djvu/679

 Fish-rearing.—Of the first type of pisciculture there are few examples of commercial importance. The pond-culture of carp is an important industry in China and Germany, and has been introduced with some success in the United States, but in England it has long fallen out of use, and is not likely to be revived so long as fresh fish can be obtained and distributed so rcadxly as is now the case Other examples are to be found in the cultivation of the lagoons of the Adriatic, and of the salt marshes of various parts of France. Here, as in ancient Greece and Rome, it is the practice to admit young fish from the sea by sluices, into artificial enclosures or “viviers,” and to keep them there until they are large enough to be used. An interesting modification of this method of cultivation has been introduced into Denmark. The entrances to the inner lagoons of the Limfjord are naturally blocked against the immigration of flatfish by dense growths of sea-grass (Zostera), although the outer lagoons are annually invaded by large numbers of small plaice from the North Sea. The fishermen of the district consequently combined to defray the expenses of transplanting large numbers of small plaice from the outer waters to the inner lagoons, where they were found to thrive far better than in their natural habitat. The explanation has been shown by Dr Petersen to be due to the abundance of food, coupled with the lack of overcrowding of the small fish. This transplantation of plaice in Denmark has been annually repeated for several years with the most successful results, and a suitable subvention to the cost is now an annual charge upon the government funds.

As a result of the international North Sea fishery investigations, it has been proposed to extend the same principle for the development of the deep sea fishery in the neighbourhood of the Dogger Bank Experiments with labelled plaice, carried out in 1904 by the Marine Biological Association, showed that small plaice transplanted to the Dogger Bank in spring grew three tunes as rapidly as those on the inshore grounds, and the same result, with insignificant variations, has been obtained by similar experiments in each succeeding year. In this case the deep water round the Dogger Bank acts as a barrier to the emigration of the small plaice from the shores. It has consequently been proposed that the small plaice should be transplanted in millions to the Bank by well vessels every spring. It is claimed, as a further result of the experiments, that from May to October the young fish would be practically free on the shallow part of the Bank from the risk of premature capture by trawlers, and that the increased value of the fish, consequent upon their phenomenal growth-rate, would greatly exceed the cost of transplantation.

The methods of oyster- and mussel-culture are similar in principle to those just described. A breeding stock is maintained to supply the ground, or the “collectors,” with spat, and the latter, when sufficiently grown, is then transplanted to the most favourable feeding-grounds, care being taken to avoid the local over-crowding which is so commonly observed among shell-fish under natural conditions.

Fish-hatching.—The second, and more familiar, type of pisciculture is that known as fish-hatching, with which must be associated the various methods of artificial propagation. The fertilization of the spawn is very easily effected. The eggs are collected either by “stripping” them from the mature adult immediately after capture, or by keeping the adults alive until they are ready to spawn, and then stripping them or by keeping them in reservoirs of sea-water and allowing them to spawn of their own accord In the two former cases a little milt is allowed to fall from a male fish into a vessel containing a small quantity of water-fresh or salt as required and the eggs are pressed from the female fish into the same vessel In fresh-water culture the eggs thus fertilized may be at once distributed to the waters to be stocked, or they may be kept in special receptacles provided with a suitable stream of water until the fry are hatched, and then distributed, or again they may be reared in the hatchery for several months until the fry are active and hardy.

The hatching of eggs, whether of fresh-water or salt-water fishes, presents no serious difficulties, if suitable apparatus is employed, but the rearing of fry to an advanced stage, without serious losses, is less easy, and in the case of sea-fishes with pelagic eggs, the larvae of which are exceedingly small and tender, is still an unsolved problem, although recent work, carried out at the Plymouth laboratory of the Marine Biological Association, is at least promising It has been found possible to grow pure cultures of various diatoms, and by feeding these to delicate larvae kept in sterilized sea-water, great successes have been attained. In fresh-Water culture little advantage, if any, has been found to result from artificial hatching, unless this is followed by a successful period of rearing. Thus the Howietown Fishery Company recommend their customers to stock their streams either with unhatched ova or with three month-old fry. Their experience is “that there is no half-way house between ova sown in redds and three-month-old fry Younger fry may do, but only where ova would do as well, and at half the cost " In marine hatcheries, on the other hand, it is the invariable practice to hatch the eggs, although the fry have to be put into the sea at the most critical period of their lives. If it is a risky matter to plant out the robust young fry of trout under an age of three months, it would seem to be an infinitely more speculative proceeding to plant out the delicate week-old larvae of sea-fishes in an environment which teems with predaceous enemies.

Objects and Utility of Fish-hatcheries.—The earlier advocates of artificial propagation and fish-hatching seem to have been under the impression that the thousands of fry resulting from a single act of artificial propagation meant a corresponding increase in the numbers of edible fish when once they had been deposited in suitable waters; and also that artificial fertilization ensured a greater proportion of fertilized eggs than the natural process. For the second of these propositions there is no evidence, while the first proposition is now everywhere discredited. It is recognized that the great fertility of fishes is nature's provision to meet a high mortality—greater in sea-fishes with minute pelagic eggs than in fresh-water fishes with larger-yolked eggs, partly because of the greater risks of marine pelagic life, and partly because of the greater delicacy of marine larvae at the time of hatching. Artificially propagated eggs and fry after planting must submit to the same mortality as the other eggs and fry around them. Consequently it is useless to plant out eggs or fry unless in numbers sufficiently great to appreciably increase the stock of eggs and fry already existing. It is this, combined always with the suitability of the external conditions, which accounts for the success of the best known experiments of American pisciculturists. The artificially propagated eggs of the shad from the eastern rivers of the United States were planted in those of California and the Mississippi, where the species did not naturally occur. The conditions were suitable, and the species became at once acclimatized. Similarly reservoirs and streams can be stocked with various kinds of fish not previously present. But in the case of indigenous species the breeding stock must be very seriously reduced before the addition of the eggs or fry of a few score or hundreds of fish can appreciably increase the local stock.

In the case of sea-fishes it is becoming increasingly recognized that the millions of cod fry which are annually turned out of the American, Newfoundland and Norwegian hatcheries are but an insignificant fraction of the billions of fry which are naturally produced. A single female cod liberates, according to its size, from one to five million eggs in a single season. Yet the annual output of fry from each of these hatcheries rarely exceeds 200 millions, i.e. the natural product of a few hundred cod at most. In Britain marine hatcheries have been established by the Fishery Board for Scotland in the bay of Nigg, near Aberdeen, by the Lancashire Sea Fisheries Committee at Peel, and by the government of the Isle of Man at Port Erin. These establishments have been principally devoted to the hatching of the eggs of plaice But again the maximum output of fry from any one of these establishments has not exceeded 40 millions in any single year As a single female plaice produces about 200,000 eggs per annum, this output does not exceed the natural produce of a few hundred fish Under these circumstances the probable utility of the oneratinm mnld be