KENNEDY ET AL.: TEMPERATURE-TIME RELATIONSHIPS 



reports elsewhere (Nelson, 1928; Belding, 1931; 

 Loosanoff, 1937; Porter, 1967). No temperatures 

 have been published for spawning in Maryland or 

 Virginia waters. We will assume conservatively 

 that the range of 23° to 29°C would apply in 

 these waters and that hard clam embryos and 

 larvae would be present in the plankton under 

 these conditions. Coutant (1970) estimated that 

 the average temperatui'e increase expected in 

 cooling water carrying entrained organisms 

 through a nuclear power plant would be 10.8°C. 



Therefore, entrained embryos and larvae of hard 

 clams could be subjected to temperatures of 

 33.8° to 39.8°C while passing through such a 

 facility. In spring (Figure 4), 9(K^ of the cleavage 

 stages could be eliminated if entrained for about 

 30 min. Fifty percent could be killed in about 

 13 min and 10% in about 6 min. For trochophores, 

 10% could be killed in about 25 min, perhaps 

 longer. Straight-hinge larvae would appear to be 



unaffected by the temperature increase in spring. 

 In late summer, 90% of the cleavage stages 

 might be killed in 1 min or less, with 90% of the 

 trochophores dying in less than 5 min. Over 180 

 min of exposure would be needed to kill 90% of 

 the straight-hinge larvae. 



The equations we have developed should allow 

 predictive evaluations to be made concerning the 

 effects of entrainment of hard clam embryos and 

 larvae in Maryland and Virginia waters and 

 elsewhere. Discharge canals of steam-electric 

 power plants are usually located to avoid directing 

 heated water over beds of commercial bivalves. 

 It appears that it is also important to avoid 

 taking in water that might come from the area 

 of a bed of hard clams during spawning season. 

 Should such water contain embryos and larvae of 

 hard clams, long exposures in the cooling system 

 of the plant (whether within the facility or in a 

 discharge canal) could be lethal to the entrained 



50 



40 



30 



Mercenaria mercenaria 





g 



straight - hinge larvae 



-1 1 — I — I— _i i_j_ 



^ 4 



o 



a> 

 a 



I 30 



i 



trochophore larvae 



-1 1 I L_ 



30- 



cleavage stages 



5 6 7 8 9 10 



20 30 40 50 



Minutes 



100 



200 300 400 500 



1000 



Figure 4. — Estimates of three percentage mortality levels for different exposure times. Percentage mortality for trochophore 

 larvae and straight-hinge larvae was estimated to be less than lO^f at 1 min and up to 30 min, respectively. For each stage, 

 the dashed lines represent an increment of 10.8°C over estimated spawning temperature in spring (23°C - lower dashed 

 Une) and late summer (29°C - upper dashed line) in Maryland and Virginia waters. 



1165 



