490 BULLETIN OF THE BUREAU OF FISHERIES 



number of spat collected per bushel was only 2,000 and 2,500, respectively. Such 

 annual variations in the production of seed on both natural and cultivated oyster 

 beds have long been observed and are largely the result of the annual and monthly 

 fluctuations in the physical conditions that have been discussed previously. 



DISTRIBUTION OF SET 



On a given area of bottom the setting was found to be distributed unevenly and 

 varied in intensity according to the distance from the spawning bed and the depth 

 of water. In the harbor the set was found to occur on such areas as are covered with 

 water when the tide is 2 feet above mean low water mark, with the exception of a 

 small portion above Station 1, where setting occurs rarely because of the discharge of 

 fresh water. In 1925 a set of commercial value was found principally within a radius 

 of 300 yards from the spawning bed, the number of spat ranging from 5 or 6 spat per 

 shell on the outside edge to 200 and 300 per shell in the central portion. The spat 

 were most abundant on shells planted over the spawning bed and within approxi- 

 mately 100 yards of its center. The concentration of spat at the 100-yard circle 

 averaged 50 per shell. Though setting occurred at virtually the same distance up- 

 stream, or above the bed, as it did below, it was found to be of slightly greater in- 

 tensity in the areas Ijdng below or toward Long Island Sound. The horizontal 

 distribution of the set in relation to the spawning bed clearly shows how close the 

 oyster larvse remain and attach to the place where they were produced. No notice- 

 able difference in the distribution of set from the spawning bed could be found in the 

 last three yeare. These facts make possible the development of special methods of 

 oyster-seed collection on the same areas where the heaviest setting is found to occur. 



The variations in intensity of setting according to depth, or, in other words, 

 the vertical distribution of spat, has been found to be quite peculiar in Connecticut 

 waters. Setting occurs in a zone extending from the bottom of the channel to a 

 point 2 feet above mean low-water mark, while from above this level to high-water 

 mark, a distance of about 5 feet, no setting takes place. The vertical range of set- 

 ting found in other bodies of water is quite different, as shown in Figure 32. In 

 Great South Bay, Long Island, setting occurs from the bottom to nearly high-water 

 mark, while in South Carolina and Georgia the set is found chiefly between low and 

 high water marks and not below low-water mark. Since the velocity of the tidal 

 current was found to be an important controlling factor in the distribution and 

 occurrence of the oyster larvae, it appeared likely that it might also exert considerable 

 influence on the distribution of setting. In the studies of the relationship between 

 current velocity and setting it was found that in MUford Harbor the larvse began to 

 attach at low slack water and continued to do so during the first two hours' run of 

 flood tide until the current had developed a velocity of one-third foot per second, or 

 20 feet per minute. This was determined not only by plankton collections of f uUy de- 

 veloped larvae but especially by observations as to the position of the larvse on the col- 

 lectors in respect to the stage of tide and the velocity and direction of currents. The 

 portion of the collectors covered at low water, when there was no current, was found to 

 be covered equally with spat on all sides, while above this level spat occurred chiefly 

 on the lee side or that which was protected from the force of the flood current. Shells 



