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PISCICULTURE (from Lat. piscis, fish)
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See also:PISCICULTURE (from See also:Lat. piscis, See also:fish) . The See also:species of fish which can be kept successfully in captivity throughout their lives from See also:egg to adult is exceedingly limited. in number. The various breeds of See also:goldfish are See also:familiar examples, but the See also:carp is almost the only See also:food-fish capable of similar domestication. Various other food-fishes, both marine and fresh-See also:water, can be kept in ponds for longer or shorter periods, but refuse to breed, while in other cases the See also:fry obtained from See also:captive breeders will not develop. Consequently there are two See also:main types of pisciculture to be distinguished: (I) the rearing in confinement of See also:young fishes to an edible See also:stage, and (2) the See also:stocking of natural See also:waters with eggs or fry from captured breeders. Fish-rearing.—Of the first type of pisciculture there are few examples of commercial importance. The See also:pond-culture of carp is an important See also:industry in See also:China and See also:Germany, and has been introduced with some success in the See also:United States, but in See also:England it has See also:long fallen out of use, and is not likely to be revived so long as fresh fish can be obtained and distributed so readily as is now the See also:case. Other examples are to be found in the cultivation of the lagoons of the Adriatic, and of the See also:salt-marshes of various parts of See also:France. Here, as in See also:ancient See also:Greece and See also:Rome, it is the practice to admit young fish from the See also: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 See also:Denmark. The entrances to the inner lagoons of the Limfjord are naturally blocked against the See also:immigration of flatfish by dense growths of sea-grass (Zostera), although the See also:outer lagoons are annually invaded by large See also:numbers of small See also:plaice from the See also:North Sea. The fishermen of the See also:district consequently combined to defray the expenses of trans-planting 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 See also: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 trans-See also:plantation 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 See also:annual See also:charge upon the See also:government funds. As a result of the See also:international North Sea See also: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 See also:Bank. Experiments with labelled plaice, carried out in 1904 by the Marine Biological Association, showed that small plaice trans-planted to the Dogger Bank in See also:spring See also:grew three times as rapidly as those on the inshore grounds, and the same result, with in-significant See also:variations, has been obtained by similar experiments in each succeeding See also:year. In this case the deep water See also:round the Dogger Bank acts as a barrier to the See also: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 See also:October the young fish would be practically See also:free on the shallow See also:part of the Bank from the See also:risk of premature See also:capture by trawlers, and that the increased value of the fish, consequent upon their phenomenal growth-See also:rate, would greatly exceed the cost of transplantation. The methods of See also:oyster- and See also:mussel-culture are similar in principle to those just described. A breeding stock is maintained to See also: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 See also:local over-crowding which is so commonly observed among See also: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 See also: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 See also:accord. In the two former cases a little milt is allowed to fall from a male fish into a See also:vessel containing a small quantity of water—fresh or salt as required—and the eggs are pressed from the See also: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 See also: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 See also: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 andtender, is still an unsolved problem, although See also:recent See also:work, carried out at the See also: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, See also:great successes have been attained. In fresh-water culture little See also:advantage, if any, has been found to result from artificial hatching, unless this is followed by a successful See also:period of rearing. Thus the Howietown Fishery See also:Company recommend their customers to stock their streams either with unhatched ova or with threemonth-old fry. Their experience is " that there is no See also:half-way See also:house between ova sown in redds and three-See also: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 See also:hand, it is the invariable practice to See also:hatch the eggs, although the fry have to be put into the sea at the most See also:critical period of their lives. If it is a risky See also:matter to plant out the robust young fry of See also:trout under an See also:age of three months, it would seem to be an infinitely more speculative proceeding to plant out the delicate See also:week-old larvae of sea-fishes in an environment which teems with predaceous enemies. See also:Objects and Utility of Fish-hatcheries.—The earlier See also:advocates of artificial propagation and fish-hatching seem to have been under the impression that the thousands of fry resulting from a single See also: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 See also:process. For the second of these propositions there is no See also:evidence, while the first proposition is now everywhere discredited. It is recognized that the great fertility of fishes is nature's See also:provision to meet a high mortality—greater in sea-fishes with See also:minute pelagic eggs than in fresh-water fishes with larger-yolked eggs, partly because of the greater risks of marine pelagic See also:life, and partly because of the greater delicacy of marine larvae at the See also: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 See also:external conditions, which accounts for the success of the best known experiments of See also:American pisciculturists. The artificially propagated eggs of the See also:shad from the eastern See also:rivers of the United States were planted in those of See also:California and the See also: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 See also: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 See also: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 See also:cod fry which are annually turned out of the American, See also:Newfoundland and See also:Norwegian hatcheries are but an insignificant fraction of the billions of fry which are naturally produced. A single female cod liberates, according to its See also:size, from one to five million eggs in a single See also:season. Yet the annual output of fry from each of these hatcheries rarely exceeds 200 millions, i.e. the natural product of a few See also:hundred cod at most. In See also:Britain marine hatcheries have been established by the Fishery See also:Board for See also:Scotland in the See also:bay of Nigg, near See also:Aberdeen, by the See also:Lancashire Sea See also:Fisheries See also:Committee at See also:Peel, and by the government of the Isle of See also:Man at See also:Port See also:Erin. These establishments have been principally devoted to the hatching of the eggs of plaice. But again the maximum out-put 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 operations could be admitted only if the fry were sedentary and could be planted in suitable localities where young fish were naturally scarce. But the fry See also:drift with the currents as helplessly as the eggs, and the a priori objections to the utility of the operations have in no case been met by evidence of tangible results. The plaice fry hatched in the Scottish See also:establishment have been distributed for many years in the waters of See also:Loch See also:Fyne. Yet in this See also:area, according to the investigations of Mr See also:Williamson (See also:Report of the Scottish Fishery Board for 1898), nearly 500 millions of plaice eggs are naturally produced in one spawning season. Evidence is still lacking as to whether the 20 to 30 million fry annually added from the hatchery have appreciably increased the quantities of young plaice on the surrounding shores. Supposing this could be established, the question would still remain whether the same result could not be obtained at far less expense by dispensing with the hatching operations and distributing the eggs directly after fertilization. In the United States the utility of the cod-hatching operations has been constantly asserted by representatives of the See also:Bureau of Fisheries, but practically the only evidence adduced is the occasional See also:appearance of unusual numbers of cod in the neighbourhood. It has not been established that the fluctuations in the local cod fisheries See also:bear any fixed relation to the ex-See also:tent of the hatching operations, while the earlier reports of the Commissioners of Fisheries contain evidence that similar fluctuations occurred before the hatching of ." fish See also:commission cod " had begun. The situation may be summed up in the words of Mr Fryer, H.M. Superintending Inspector of Fisheries, who critically examined the evidence bearing upon the operations of the Newfoundland Hatchery at Dildo (Reports x.–xii. of the Inspectors of Sea Fisheries, E. & W.): " Where the establishment of a hatchery, even on the smallest See also:scale, is followed by an increased take of fish, there is a tendency to connect the two as cause and effect on insufficient evidence, and without any regard to the many conditions which have always led to fluctuations in the case of any particular See also:kind of fish." The most exact investigations bearing upon this problem are those which have been recently undertaken in See also:Norway in connexion with the cod-hatching operations at See also:Arendal under See also:Captain Dannevig. Four fjords were selected in the See also:south See also:coast of Norway in proximity to the hatchery, and the usual number of fry (10--3o millions) were planted in the spring in alternate fjords, leaving the intermediate fjords unsupplied. The relative number of young cod in the various fjords was then carefully investigated throughout the succeeding summer and autumn months. It was found that there was no relation between the abundance of young fish and the presence or See also:absence of " artificial " fry. In 1904, 33 million fry were planted in Sondelefjord and young fish were exceptionally abundant in the following autumn (three times as abundant as in 1903 when no fry were planted). But their abundance was equally striking in other fjords in which no fry had been planted, while in 1905 all the fjords were deficient in young cod whether they had been planted with fry from the hatchery or not. For a See also:summary of these investigations see papers on " Artificial Fish-hatching in Norway," by Captain Dannevig and Mr See also:Dahl, in the Report of the Lancashire Sea Fisheries Laboratory for 1906 (See also:Liverpool, 1907). It would thus seem clear that the attempts hitherto made to increase the supply of sea-fish by artificial hatching have been unsuccessful. The experience gained has doubtless not been wasted, but the direction to be taken by future work is See also:plain. The See also:energy and See also:money devoted to hatching operations should be diverted to the serious See also:attempt to discover a means of rearing on a large scale the just-hatched fry of the more sedentary species to a sturdy See also:adolescence. When that has been done (it has been achieved by the present writer in the case of the sea fish Coitus with demersal eggs,) it would be possible to See also:deposit the young fish in suitable localities on a large scale, with a reasonable prospect of influencing the local abundance of the species of fish in question. For further details, see J. T. See also:Cunningham, Natural See also:History of the Marketable Marine Fishes of the See also:British Islands (See also:London, 1896) ; A See also:Manual of Fish-Culture (See also:Washington, 1897) ; See also:Roche, La Culture See also:des mers (See also:Paris, 1898) ; W. Garstang, Experiments on the Trans-plantation of Marked Plaice (First Report of the North Sea Fisheries Investigation Committee, 1905). (W. Additional information and CommentsThere are no comments yet for this article.
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