Parasites are not numerous in oysters and are rarely danger ous. In 0. edulis the spirochaete, Cristispira always occurs in the crystalline style. Occasionally the Coccidian, Chytridiopsis is found in the eggs, the gregarine (Nematopsis) in kidney and gills, and the bucephalus larva of the trematode (Gasterostomum gra cilescens) in the body tissues generally. The metacercariae of tre matodes cause serious damage to the Japanese gigantic oyster.
Weakness and heavy mortality may occur from functional derangement due to bad treatment by man or un favourable general conditions of life. A normal mortality of io% per annum due to all causes has been estimated to occur in Eng land, but figures of 20 to 25% are estimated for Dutch beds. In oysters carried from one bed to another a mortality of 5% or more may occur, with possibly higher rates in the second season after relaying. Excess of fresh water, especially from melting snow, may produce great weakness with subsequent high mor tality. Prolonged immersion in freshlets of water produces swelling. Sudden cold periods following warm weather in the summer are believed to be harmful. Gales sometimes result in the smothering of immense numbers of oysters on sandy beds, and a great loss may be experienced on muddy beds by silting up. 0. edulis will live well in sea-water under thick ice, and in water warmed to a temperature of 9o° F, and_several species can with stand deprivation of oxygen for a week or more in winter, but not in summer. Stunted shell growth occurs in 0. edulis on many beds, due apparently to a functional disturbance of the growth mechanism; such individuals are usually fat and esteemed
eating. Myolysis, or degeneration of muscular tissue, was asso ciated strongly with the mortality in the Thames estuary in 192o, and has been observed elsewhere, but its cause (or causes) is at present unknown.
The oyster industry constitutes an im portant source of income, especially in the United States, Holland and France, and in the former country is one of the most im portant fishery industries on the Atlantic seaboard. The statistics for the world's production—excluding Italy for which no figures are available—shown in Table III., give some indication of the fluctuations in production over a short period, and show mainly a marked decline from about 1920, especially in the United States, British isles, Holland, France and Japan. The chief causes of this decline are (I) over-fishing; (2) the failure of the spat fall in recent years to meet the demand; (3) possible occurrence of increased pollution, which reduces the spatfall, and (4) in Europe, the unusually heavy mortality in 1919-21.
Oyster culture is carried out in four main operations which may be enumerated as ( ) care of the beds; (2) production; (3) rearing, and (4) fattening. (I) On all grounds it is necessary to supervise and work on the beds to remove enemies, pests, sea-weeds and dangerous accumulations of all kinds, and otherwise keep the beds clean. (2) A breeding stock is maintained to supply a sufficiency of larvae. When the larvae are ready to settle various materials are placed in the water at suitable times and places to provide settling areas for the young oyster. This material is called cultch and varies in different countries. In England, America and Canada the commonest practice is to use clean shell, especially that of the oyster itself. In Holland and France earthenware tiles coated with lime are laid out in selected parts of the beds. In Italy, Australia, Norway, China and Japan branches of trees are thrown into or fixed in the water. Stones are used largely in Australia, New Zealand and China. It is especially important that the material used for pro viding settlement area should be clean at the time the larvae are ready to settle. If material be placed in water too soon it becomes slimy, overgrown with marine organisms, and, technically, dirty.
In Italy bundles of twigs are placed in the water as early as April; in France and Holland tiles are put out in June; in England shell cultch is usually put out in June, or, in a late season, in July. Bamboos are put out in Japan about April. The best place in a locality to put out the cultch can only be determined by local ex perience. Good spatfalls may be obtained in either a strong cur rent or on banks covered by calm water. Certain oyster beds are only suitable for producing young oysters, which are sold at ages of one, two or three years for transplanting to or relaying on beds suitable for fattening. (3) Different methods are used in different countries for rearing the young. When shell-cultch is used below low-water mark, rearing may consist simply of leaving the spat on the ground until they are large enough (II. to 2in.) to be separated from the cultch (culling from the cultch) and then either removed to special beds suitable for growth, or returned to the same bed. Spat on sticks or stones above low-water mark is left in position during the summer or longer and then shaken off and relaid on growing beds. (4) Fattening oysters for the market is the final stage in oyster-culture. Owners of beds suit able for fattening buy from producing grounds young oysters gen erally three to four years old. These are merely relaid in the sea on special beds and fattening occurs automatically according to the season without further aid. Fattening grounds are only found by experience and may fail to produce fattened oysters in certain years. Artificial manuring of the waters over oyster-beds with phosphates and nitrates has been suggested to ensure fatten ing. Oysters placed in fresh water absorb water and become falsely fat. In France oysters are fattened artificially and ren dered green by being kept in ponds in which an exuberant growth of the diatom Navicula ostrearia is cultivated. The greening is due to accumulation of pigments in the gill and palps from the ingested diatoms. Similar natural greening of oysters occurs in certain creeks in Essex and elsewhere. A green colour due to an accumulation of copper shows mostly in the body, rarely in the gills. The copper is present mainly in the blood cells. Metallifer ous oysters occur in many parts of the world, viz., Falmouth (Eng land), the Tagus, and certain American beds.
Artificial production of spat has been successfully performed in pits in France, in ponds in Norway, and recently in tanks by the British Government fishery research staff.
It is known that oysters are liable to filter and retain alive from water contaminated with sewage certain bacteria which when swallowed by man may pro duce enteric fever. All oysters sent to the London market are scrutinized or sampled by the Fishmongers Company and, if necessary, tested for purity by bacteriological analysis. Oysters from beds contaminated by sewage may be purified in tanks of sterilized water, or by transplanting them for two or three weeks to beds remote from sources of pollution, but in the latter case it is desirable to prove the purity by bacteriological analysis before such oysters are pronounced fit to eat and finally placed on the market.
As oysters are often in poor condition in summer during the breeding period, and as dangerous bacilli in contaminated oysters would rapidly reproduce in warm weather, a close season is im posed in most countries at some period. Such regulations are beneficial also for spawning and spatting. In England the close season for native oysters varies in different localities but generally extends from March-April to August-September. Foreign oys ters of any species may be sold in England at any time providing they show a clean health bill. A clean fat oyster may be eaten with impunity at any time of the year.