Short‐Term Thermal Resistance of Hexagrammid Eggs and Planktonic Larvae from Puget Sound

Eggs of the kelp greenling, Hexagrammos decagrammus; painted greenling, Oxylebius pictus; whitespotted greenling, H. stelleri; and larvae of these species and of lingcod, Ophiodon elongatus, were subjected to tests that simulated thermal stress associated with steam‐powered electric stations using seawater for cooling in a once‐through system. Ambient temperatures for holding the eggs and larvae were 8 C for the lingcod, 9 C for the kelp and whitespotted greenlings, and 13 C for the painted greenling. Embryos subjected to daily 4‐hour exposures where temperature changes were gradual, as could occur with thermal plumes passing over egg nests, survived a maximum temperature of 22 C but not 24 C. Unfed larvae were subjected to conditions simulating passage through heat exchangers (held at elevated test temperatures for 10 or 20 minutes with an abrupt increase and decrease from ambient) or mixing with thermal plumes (held at test temperature 1–8 hours with temperatures gradually rising and decreasing from ambient). Lingcod, the most sensitive species, had a 48‐hour, 50% survival at 24 C for the 10‐minute test and at 22 C for the 20‐minute test. However, lingcod and kelp greenlings became torpid at 18 C for the 10‐ and 20‐minute tests. The 48‐hour 50% survival of the greenlings subjected to the thermal plume test ranged from 26 C for the 1‐hour test to 16 C for the 8‐hour test. The maximum termperature increase that would not cause death or torpor to the least resistant species is 8 C.