492 



BULLETIN OF THE BUREAU OF FISHERIES 



point 1 foot above the level of low-water mark to 1 foot below it. Here the concentra- 

 tion of spat averaged 25 per square inch of surface. Above this strip, setting 

 gradually decreased in intensity untU at a level 2 feet above low-water mark scarcely 

 one spat was found per square inch. From a point 1 foot below low-water mark to 

 the bottom of the channel setting was comparatively light, averaging about one 

 spat per square inch. Directly on the bottom setting was slightly heavier, and an 



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BEAUFORT 

 SOUTH CAROLIHA 



GREAT SOUTH BAY 

 LONG ISLAND 



FiGUEE 32.— Vertical distribation and intensity of setting of oyster spat in relation to range 

 of tide. H. W.=bigh-water mark. S.=height of tide wlien slack water occurs 



average concentration of 3 spat per square inch was found. The variations in intensity 

 of setting at different levels can be correlated closely with the distribution and oc- 

 currence of the larvos and the velocity of the tidal currents. Dming low slack water 

 the zone in which the larvae are most abundant and where the heaviest setting of 

 spat takes place is that in which the velocity of the current is least, or practically 

 zero. The reason that setting is not of equal intensity from low-water mark to the 

 bottom of the channel can be accounted for by the fact that at low slack water, when 

 setting occurs, the zone of least velocity is at the surface only and does not extend to 



