Clusters of adhesive demersal eggs, 0.74 to 0.88 

 mm, are laid on sandy bottom (Bigelow and 

 Schroeder 1953). Upon hatching, the planktonic lar- 

 vae are 3 to 3.5 mm long. Metamorphosis occurs 

 when the larvae reach lengths of 8 to 9 mm. The lar- 

 vae remain free-swimming until prior to meta- 

 morphosis, when they become bottom oriented 

 (Bigelow and Schroeder 1953). 



In the planktonic state the larval stages are suscep- 

 tible to the hazards of entrainment by steam electric 

 generating stations, which usually use once-through 

 cooling because it is the most economical method for 

 condensing exhaust steam from turbines. However, 

 cooling requires the passage of large quantities of 

 water through the steam condensers. During its 

 passage the temperature of the water is elevated by 

 5.5° to 23.3°C, while residence time may be as much 

 as half an hour or longer, depending on the geometry 

 and operational methods of the cooling system (Com- 

 mittee on Entrainment 1978). 



The present study determines the temperature- 

 time combinations which produce a thermal shock 

 leading to significant mortalities in 5-d-old winter 

 flounder larvae. 



Materials and Methods 



Winter flounder, collected from Narragansett Bay, 

 were induced to spawn artifically by injecting freeze- 

 dried carp pituitary hormone in saline into the back 

 muscle below the dorsal fin (Smigielski 1975). Hor- 

 monal treatments were repeated daily until the 

 mature ova were released. The fish were then 

 stripped by hand. Fertilized eggs were secured from 

 two females. An even layer of eggs was deposited on 

 the bottom of a plastic bowl and then milt from 

 several males was added. After fertilization, a dense 

 suspension of diatomaceous earth (50 g/1) was mixed 

 with the eggs to retard clumping (Smigielski and 

 Arnold 1972). After the fertilized eggs remained in 

 the slurry for 10 min, they were washed on a 550 jum 

 mesh screen. The eggs were then transferred to a 

 hatching jar where they remained until being 

 transported from the Environmental Research 

 Laboratory of the U.S Environmental Protection 

 Agency, Narragansett, R.I., to Flax Pond Laboratory, 

 Old Field, N. Y. To ensure maintenance of the spawn- 

 ing (acclimation) temperature during transport, the 

 eggs were placed in plastic bags and maintained in an 

 insulated container holding seawater. Aeration was 

 continuous, using a battery-operated air pump. 



At Flax Pond Laboratory, the eggs were placed in 

 incubation baskets at the 5° C spawning temperature 

 and acclimated to reduced salinity. The spawning 



salinity at Narragansett Bay was 3 l%o, while at Flax 

 Pond the salinity was 27%o, a typical salinity for Long 

 Island Sound in this region. The fertilized eggs 

 remained in the incubation baskets until after hatch- 

 ing. Five-day-old larvae-were then transported about 

 8 km to the Marine Sciences Research Center. 



Larvae were pipetted in to 27 two- compartment 

 hatching boxes, consisting of a polyvinyl chloride 

 frame (7.5 X 6.0 X 16.0 cm) covered with monofila- 

 ment bolting cloth, 243 jum mesh opening. In each 

 compartment of a hatching box, 15 to 69 larvae were 

 placed, providing 27 samples and their replicates. 

 The hatching boxes were then placed in one of four 

 partially filled 114 1 aquaria, having a salinity of 

 27%o, and immersed in a constant temperature 

 water bath system set at an acclimation temperature 

 of 5°C. The constant temperature water bath system 

 is a rectangular plywood box, measuring 2.4 m X 1.2 

 m X 0.45 m, filled with fresh water. Water tempera- 

 ture was controlled by thermostatically regulated refri- 

 geration units and monitored with a continuous 

 recording thermometer. The water was circulated by 

 two submersible pumps. 



For each test exposure, a hatching box with its lar- 

 vae was placed in a polyethylene foam container 

 holding 12 to 15 1 of seawater for 4, 8, 16, 32, or 64 

 min. The excess temperature, AT, over the acclima- 

 tion temperature of 5°C, in the initial trial was 22°C. 

 For each subsequent trial, the AT was increased by 

 an increment of 2°C until a final AT of 30° C was 

 attained. Temperature in each container was mon- 

 itored and maintained by the addition of hot or cold 

 water of 27%o salinity. Following each exposure 

 period, the hatching boxes were returned to the 

 acclimation aquaria (5°C) where they were held for 

 24 h after which survival/mortality counts were 

 made. A larva was counted as living if it showed 

 transparency or heart beat; otherwise, it was con- 

 sidered to be dead (Table 1). Of the 27 hatching box- 

 es, two were chosen as controls. These controls 

 received no thermal shock, but were handled in a man- 

 ner similar to the experimental boxes. 



Results 



The larval survival/mortality counts (Table 1) for 

 samples and their replicates were converted to 



square root transformations, VN + 0.5, and com- 

 pared using the Chi-square test (Sokal and Rohlf 

 1969). Samples did not differ materially (x : = 3.841, 

 df = 1,P< 0.05). It was therefore feasible to combine 

 the samples to enlarge the sample size. 



The data for each hatching box sample (Table 1) 

 were converted to percentages of corrected mortality 



914 



