24 July, but in 1962 setting was heavy each 

 week fronn 25 June to 13 August. The set 

 was light in both tributaries between 

 5 September and 18 September 1961, and 

 little or no setting was found after 13 August 

 1962. 



SPAT SURVIVAL RATE 



The calculated spat survival rates for 1961 

 and 1962 in Broad Creek and Tred Avon 

 River ranged from 1 to 75 percent (tables 3 

 and 4). Because counts of weekly setting and 

 end-of- season counts were extremely low 

 in the Tred Avon River during 1961, it is 

 doubtful that the survival rate for that year 

 in that area is of any significance. When these 

 data are excluded, the rates range from 1 

 to 27 percent. In 1962 a set occurred in the 

 Tred Avon River, but survival rates were 

 extremely low--ranging from. 1 to 2 percent. 

 Survival rates were higher in Broad Creek- - 

 ranging from 8 to 27 percent in 1961 and from 

 1 to 20 percent in 1962. 



Generally, when total set on weekly shells 

 was high, survival rates were low. For example, 

 total weekly counts of 1,392, 2,096, 1,262, and 

 1,025 spat in Broad Creek in 1962 had cor- 

 responding survival rates of 8, 1, 3, and 6 

 percent, whereas stations with lower total 



Table 3. — Setting and survival of oyster spat by station 

 and depth, Tred Avon River and Broad Creek, Md. , 1961 



Table A. — Setting and survival of oyster spat by station and 

 depth, Tred Avon River and Broad Creek, Md. , 1962 



Station 



Tred Avon River; 



1 



3 



6 



3 



10 



Broad Creek: 



1 



2 



3 



4 



4 



4 



4 



4 



4 



Distance 



off 



bottom 



Total 

 of all 

 weekly 

 counts^ 



Average 



count on 



shells in 



seasonal 



bags^ 



Percentage 

 survival 

 of spat 



0.3 

 .3 

 .3 

 .3 

 .3 



No. 



112 

 283 

 821 

 4^3 



106 



No. 



2.4 

 3.5 

 10.8 

 4.8 

 2.2 



No. 



2 



1 

 1 

 1 

 2 



18 

 8 



U 

 9 



20 



10 

 6 

 3 

 1 



Counts of spat on inner surface of 20 shells per bag per 

 week (weekly bags ) . 



See text for method of computing average. 



■■■ Counts of spat on inner surface of 20 shells per bag per 

 week (weekly bags ) . 



^ See text for method of computing average. 



weekly spat counts of 624, 771, 808, 503, and 

 690 had survival rates of 18, 14, 10, 20, and 

 9 percent, respectively (table 4). 



SETTING AND SURVIVAL OF SPAT AT 

 VARIOUS DEPTHS 



Setting in the Tred Avon River at station 

 10 was too light to yield results on either 

 setting intensity or survival. In Broad Creek, 

 however, setting intensity was sufficiently 

 heavy to permit the following evaluations. 

 On the basis of accumulated counts on weekly 

 bags, setting tended to be heavier in the upper 

 0,9 m. than in the lower 0.9 m. This dif- 

 ference was especially large in 1962, In 

 contrast, percentage survival in 1962 was much 

 higher in the lower depths (table 4). 



EFFECTS OF BAGGING SHELLS 



Catching spat in bags of shells was a 

 departure from the usual practice in Maryland, 

 where seed is collected on shell cultch broad- 

 cast loosely on the bottom; therefore, we 

 investigated in 1962 the relation between spat 

 yield on bagged shells and on loose bottonn 

 shells. At the end of the setting season the 

 number of spat found averaged 9 per bag 

 (i.e., the number of shells equal to one bag) 

 on broadcast shells, and 29 per bag on sus- 

 pended shells. 



FACTORS OF SETTING AND SURVIVAL 

 OF OYSTER SPAT 



Past surveys by Federal and State biologists 

 indicated that setting is poor in Tred Avon 



