River and good in Broad Creek (Beaven, 1954). 

 These findings were supported by the counts on 

 shells in weekly and seasonal bags. In the 2 

 years of monitoring, the only significant set 

 in the Tred Avon River came during the week 

 of 30 July to 6 August 1962. Despite this 

 set, the final counts on seasonal bags, on 

 the average, were less than 0.5 spat per 

 shell. In contrast, setting was of commercial 

 significance in Broad Creek in both years. 

 Average counts were 1.8 spat per shell in 

 1961 and 7.4 in 1962 on shells in the sea- 

 sonal bags. 2 



The reasons for these differences in setting 

 intensity between the two tributaries are 

 not known. These streams have similar salin- 

 ity and temperature ranges. Some factors 

 that could contribute to the differences are 

 flushing rates of the two systems, circulation 

 patterns, and abundance of brood stock. 



Rates of oyster spat survival or mortality 

 have been reported by a number of investi- 

 gators. Butler (1952), who kept three separate 

 groups of spat under observation for 1 3 months 

 near Pensacola, Fla., found mortalities of 

 67, 33, and 22 percent at the end of that time. 

 He reported that about 70 percent of the total 

 mortality occurred within 4 months after set- 

 ting. Loosanoff and Engle (1940) reported 

 spat survival of to 13.95 percent at various 

 stations in Long Island Sound during the sum- 

 mer of 1937. Manning (1952) calculated spat 

 survival of 37 to 62 percent after their setting 

 in the St. Mary's River, Md. Engle (1955) 

 estimated percentage survival of spat at Mill 

 Hill Bar in Eastern Bay, Md. Survival rates 

 were less than 10 percent between initial 

 set and seed- size oysters 3 to 4 months 

 later. Walne (1961) studied the controlled 

 collection of spat of the European oyster in 

 a tank. Spat transferred from the tank to 

 natural grounds 3 weeks after setting, as 

 well as spat kept in the tank, had a loss of 

 about 50 percent per month the first few 

 months after setting. At this rate the net 

 survival was about 12 percent in 3 months. 

 The spat survival calculated for the Tred 

 Avon River and Broad Creek, ranging from 

 1 to 27 percent, closely approaches that in 

 Long Island Sound (Loosanoff and Engle, 

 1940), in Eastern Bay, Chesapeake Bay (Engle, 

 1955), and in England (Walne, 1961). 



Several explanations can be offered for 

 difference in spat survival rates. When setting 

 intensity is great, crowding is almost imme- 

 diate, and the faster growing individuals 

 smother those which grow slowly. When setting 

 is light, the crowding is less severe and sur- 

 vival rates are higher. 



2 Averages derived from tables 3 and 4 with the assump- 

 tion that equal numbers of oysters set on the Inner and 

 outer faces of all shells in seasonal bags. 



Another possible explanation of different 

 rates of survival is the influence of predation. 

 Loosanoff and Engle (1940) calculated spat 

 survival rates in Connecticut; they used 

 methods essentially the same as those 

 described here. They believed differences in 

 survival rates between stations were due to 

 predation by starfish ( Asterias ) and oyster 

 drills ( Urosalpinx ). Neither of these predators 

 occurs in the Tred Avon River and Broad 

 Creek, but the predatory flatworm, Stylochus 

 ellipticus , does, and this forni has been found 

 to cause oyster mortality (Loosanoff, 1956; 

 Webster and Medford, 1961). 



To date no mortalities in Broad Creek or 

 Tred Avon River have been attributable to 

 oyster diseases. It is, therefore, unlikely 

 that difference in survival rates can be as- 

 cribed to this cause. 



LITERATURE CITED 



BEAVEN, G. FRANCIS. 



1954. Various aspects of oyster setting in 

 Maryland. Proc. Nat. Shellfish. Ass. 

 45: 29-37. 



BUTLER, PHILIP A. 



1952. Growth and mortality rates of sibling 



and unrelated oyster populations. Proc. 



Gulf Carib. Fish. Inst., 4th Annu. 



Sess.: 71 . 

 ENGLE, J. B, 



1955. Ten years of study on oyster setting 

 in a seed area in upper Chesapeake 

 Bay. Proc. Nat. Shellfish. Ass. 

 46: 88-99. 



LOOSANOFF, VICTOR L. 



1956. Two obscure oyster enemies in New 

 England waters. Science, 123: 1119- 

 1120. 



LOOSANOFF, VICTOR L., and JAMES B. 

 ENGLE. 

 1940. Spawning and setting of oysters in 

 Long Island Sound in 1937, and dis- 

 cussion of the nnethod for predicting 

 the intensity and time of oyster setting. 

 U.S. Fish. Wildl. Serv., Fish. Bull. 

 49: 217-255. 

 MANNING, JOSEPH H, 



1952. Setting of oyster larvae and sur- 

 vival of spat in the St. Mary's River, 

 Maryland, in relation to fouling of 

 cultch. Addresses Delivered at the 

 Conv. of the Nat. Shellfish. Ass. Aug. 

 1952: 74-78. 

 WALNE, P. R. 



1961. Observations on the mortality of 

 Ostrea edulis. J. Mar. Biol. Ass. 

 U. K. 41: 113-122. 

 WEBSTER, J. R,, and R. Z. MEDFORD. 



1961. Flatworm distribution and associated 

 oyster mortality in Chesapeake Bay. 

 Proc. Nat. Shellfish, Ass. 50: 121-132. 



MS. #1690 



GPO 9 39 - 177 



