FISHERY BULLETIN: VOL, 72, NO, 4 



organisms. Whether such mortality would affect 

 the continued existence of the local hard clam 

 resource depends upon local circumstances. 

 Minimum destruction of the resource during use 

 of cooling water from the vicinity of hard clam 

 habitat requires that entrainment of embryos and 

 larvae be as short as possible. The thermal 

 discharge should be mixed with receiving water 

 as quickly as possible to provide a rapid return to 

 ambient temperature. In making evaluations it 

 should be remembered that organisms passing 

 through cooling systems are also subject to 

 various stresses due to pressure changes, mechani- 

 cal effects, and chlorination, in addition to 

 temperature. 



ACKNOWLEDGMENTS 



H. Hidu was closely involved in early experi- 

 mentation using the thermal gradient apparatus. 

 We thank L. Douglass for his helpful advice on 

 the computer program and associated statistical 

 matters. K. Drobeck kindly supplied the appara- 

 tus for simultaneous injections of larvae into the 

 test tubes. Other associates lent essential equip- 

 ment as needed. H. Zion and R. Karney helped 

 us greatly in the course of the experiments. 

 N. Lewis provided us with the embryos and 

 larvae. F. Younger drew the figures. Financial 

 support, in part, came from U.S. Department of 

 the Interior Water Resources Research Center, 

 University of Maryland, Project A-OllMD, and 

 from Maryland Department of Natural Resources. 



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