102 COMMISSION OF CONSERVATION 



can close their shells and live until the return of the sea-water, but eggs, 

 embryos, and larvse left stranded on the surface, that might live in cloudy 

 weather until the next tide, may be crushed by the falling drops, beaten 

 into the mud, smothered with debris, or drowned in pools of the fresh 

 water. 



Tides, tidal currents, current-movements produced by the wind, 

 continuations of river-currents, waves and storms, may carry floating or 

 suspended stages out to sea or throw them upon the beach, or heap sedi- 

 ment, silt, sand, mud, gravel, stones, shells, weeds and drift on to fixed 

 spat and adult stages, crushing or smothering them. 



Eggs may go unfertilized, or settle into masses and pollute one another, 

 or sink into mud, or be eaten by animals. Larvffi may swim or be floated 

 on to unsuitable bottom or eaten up. Spat and adults may be covered 

 with drift, eaten, or otherwise injured or destroyed. All stages may 

 suffer from lack of food, the larvse from lack of cultch. All are preyed 

 upon by larger or smaller carnivorous or omnivorous fishes and crabs. 

 The oyster crab does not occur in our regions. The star-fish and the drill 

 are not plentiful. The boring sponge attacks some of the shells, softening 

 and weakening them as organs of defence. There are parasitic protozoa 

 and worms and associations with other animals as sponges, anemones, 

 hydroids, bryozoa, tube-worms, tunicates, etc., that result in little or no 

 recognizable injury. More formidable troubles arise from old age and 

 disease. 



A very great disadvantage in some respects arises from the gregari- 

 ousness of oysters, which brings them into competition with one another 

 for place and food and causes crowding and interference with normal 

 growth as well as with the processes of feeding and breathing. 



To the preceding may be added those changes which take place on 

 a large scale and which have to do with the elevation or subsidence of 

 continents or portions of them to such an extent that the sea falls off from 

 or flows up farther on to the land. Luckily for the oyster such changes 

 are generally of a slow and gradual nature so that, although a fixed animal, 

 its successors are able to move a little in one direction or the other as the 

 case requires. Much more formidable effects are brought about by 

 changes in the circulation of the ocean water, such as the bringing of 

 frigid currents closer to a region of comparatively mild temperature. 

 It has already been pointed out that this is the case with the oyster areas 

 of Canada. They must be regarded as lying near the northern limit of 

 possible occupancy by the oyster. While the lowering of the temperature 

 of the surrounding sea-water by the Arctic or Greenland current is so 

 slight as to cause no immediate concern, yet, when any area from other 

 causes becomes depleted, its chances for restocking in a natural way are 

 poor. The adult oyster could doubtless become accommodated to a 

 considerable lowering of the mean annual temperature, but what tells 



