20 U. S. BUREAU OF FISHERIES. 



The virtual absence of free-swimming oyster larvae from the Mil- 

 ford region is believed to be due to the low temperature of the water, 

 which may have prevented either the fertilization of the eggs or the 

 development of those that were fertilized. The larvae found were 

 all of the smallest sizes. The bottom temperature, where the water 

 was 20 feet or more in depth, registered as high as 70° F. on only one 

 occasion. For the remainder of the time during the usual spawning 

 season of oysters, it ranged from 65 to 69.5° F., being usually about 

 68. Oysters in this region are found to begin spawning at a tem- 

 perature between 68 and 70°. It was observed that oysters in the 

 deeper water did not spawn until about the middle of August, the 

 process continuing slowly and in patches over the beds during the 

 rest of the month. Some of the oysters apparently gave out no 

 spawn at all. 



While the shallower water of the coves and harbors warmed up 

 earlier than that outside, and the oysters there spawned earlier, the 

 incoming tide was so cold this season as materially to affect the inside 

 waters, and oyster eggs or larvae carried from the shallow waters 

 doubtless perished in the cold water outside. In any event, and cor- 

 responding with the absence of free-swimming larvae, as noted, no 

 set of oysters was found or reported anywhere in Long Island Sound. 



As emphasized in the report covering the season of 1918, the best 

 evidence obtainable indicates that in former years the set was pro- 

 duced in favorable seasons by natural inshore beds of oysters in warm 

 waters where spawning occurred relatively early. All are agreed 

 that such beds are largely exhausted. The result of the Bureau's 

 investigations tends to substantiate the view that the securing of set 

 in commercial quantities on planted beds, even during favorable 

 seasons, is contingent upon the restoration of the natural beds in 

 inshore waters. Acting upon suggestions to this effect previously 

 offered by the Bureau, the State of Connecticut has appropriated 

 $10,000 for building up the spawning beds, and the results of this 

 practical experiment may be awaited with interest. 



WATER POLLUTION AS AFFECTING GROWTH AND BREEDING OF OYSTERS. 



It is the inshore waters which are most directly affected by pollu- 

 tion from sewage and industrial wastes, but indirectly the pollution 

 may have a still more significant economic effect on offshore beds 

 even though none of the polluting substances actually touch these 

 beds. It must be remembered that oyster larvae are free-swimming and 

 may be borne by currents for some distances. The beds in deeper 

 waters are not necessarily self-perpetuative, but may well have been 

 reseeded in former times by set arising from larvae borne in the waters 

 from breeding oysters in shallow waters. Our observations have in- 

 dicated, first, that successful propac^ation of oysters in the Long 

 Island region is dependent upon early spawning, which allows time 

 for development to an advanced and resistant stage before cool 

 weather; second, that this in normal years may occur only in the 

 warmer waters; and thirdly, that spawning beds in shallow waters 

 have virtually disappeared, partly in consequence of excessive fishery, 

 but principally because the excessive pollutions have led to the 

 general abandonment of such areas for oyster planting. 



