Figure 1. — Collecting stations in the Tred Avon River and Broad Creek, Md., 1961-62. 



At station 10 in the Tred Avon River and at 

 station 4 in Broad Creek three rows of 

 seasonal bags and a single row of weekly- 

 bags were attached to chains. There were 

 six bags in each row--one at every 0.3-m. 

 vertical interval of the water column (fig. 2). 

 Depth 1 extended from the bottom to 0.3 m. 

 off bottom; depth 2 was 0.6 m. off bottom, . . . 

 depth 6, 1.8 m. off bottom. 



Identical bags and similar placement 

 methods were used in 1962. Because of the 

 general lack of oyster setting in the Tred 

 Avon River in 1961, the number of stations 

 was reduced to five in 1962 (stations 2, 4, 

 5, 7, and 9 were abandoned). Setting in various 

 depths at station 10 was not investigated. 

 Station locations and off-bottom arrangements 

 in Broad Creek remained as in 1961. 



Oyster setting in weekly bags was estimated 

 by examining the inner faces of 20 average- 

 size cultch shells (76- 127 mm. long) taken 

 at random from each bag. Spat and spat 

 scars (the attached lower shells or fragments 

 of lower shells with their upper shells missing) 

 were counted under 20X magnification. 

 It was assumed that: (1) the weekly counts 

 would reveal the time of setting, and (2) the 

 total of such counts at each station for the 

 season would approach the maximum pos- 

 sible set for the setting period. The total spat 



yield per seasonal bag was determined by 

 counting spat on both shell faces of all shells 

 in a bag, but only counts on the inner faces 

 were used in estimating survival rates, since 

 this was the only surface examined in the 

 weekly shell bags. 



Survival rates were estimated as follows: 

 The total number of spat on the inner faces 

 of all shells in each seasonal bag was 

 reduced to an average per 20 inner shell 

 faces. Next, an average of the duplicate or 

 triplicate seasonal bags per 20 inner shell 

 faces at each station and depth was derived. 

 Percentage of survival was then calculated 

 by dividing the average count per 20 shell 

 faces on the seasonal bags X 100 by the 

 accumulated counts of spat per 20 shell 

 faces on the weekly bags. For example: In 

 1961, Broad Creek, station 1, depth 0.3 m. 

 off bottom, the average count of spat per 

 20 shell faces of the duplicate seasonal bags 

 was 20.9. At this same station the total 

 count of setting per 20 shell faces in all 

 weekly bags was 92 spat. Finally, 20.9 X 100 

 divided by 92 gives 23 percent. 



Oyster setting in bagged shells was com- 

 pared with setting on shell cultch broad- 

 cast loosely on the bottom; the latter pro- 

 cedure is commonly practiced in Maryland. 

 A bottom plot about 30.5 m. by 4.6 m. beside 



