FISHERY BULLETIN: VOL. 75, NO. 3 



Zoeae used as controls had survival rates 

 ranging from 53 to 100% (Table 2). Guidelines set 

 in the American Public Health Association (1971) 

 state that losses of greater than 10% of control 

 subjects invalidate an experiment. Control zoeae 

 of the Bering hermit crab, Pagurus beringanus, 

 with a survival of 53%, L. bellus with a survival of 

 89%, and Petrolisthes eriomerus, with a survival 

 of 83% fall below this standard. Although the 

 TL 50 's are invalid for these species, the point of 

 torpor is valid as it demonstrates an immediate 

 condition the zoeae lapse into with a given 

 temperature stress. 



Activity and survival of a species of zoeae de- 

 creased with increasing temperature and dura- 

 tion at an elevated test temperature (Table 2). 

 In heat exchanger tests, zoeae experienced a rapid 

 temperature change and were initially hyper- 

 active, probably as a result of thermal shock 

 (Kinne 1964). With time, zoeae at a temperature 

 4°C below the TL 50 appeared normal. Those at 

 2°C below TL 50 had reduced activity and had 

 difficulty maintaining themselves off the bottom. 

 Subjects placed in water at the TL 50 temperature 

 and above were initially hyperactive, but in 2 to 

 7 min became torpid and sank to the bottom. Heat 

 exchanger test temperatures producing torpor 

 were 26°C for Pagurus beringanus and Petro- 

 listhes eriomerus and 30°C for most other test 

 species; the maximum was 32°C for L. bellus, 

 Pugettia producta, and H. oregonensis. After the 

 zoeae were returned to ambient conditions, those 

 tested at the TL 50 temperature had not become 

 active after 20 min. 



Zoeae subjected to the heat exchanger tests 

 generally had high survival to the point of the 

 TL 50 (Table 2). Thereafter, mortalities were 

 complete at 2° to 4°C higher except in the case of 

 the hairy hermit crab, Pagurus hirsutiusculus, 

 where all died at 6°C above the TL 50 . The minimal 

 TL 50 was at 28° and 30°C for most other crabs; it 

 was at 32°C for Pugettia producta and L. bellus 

 (Table 2). The most tolerant species was H. 

 oregonensis with a TL 50 at 34°C. 



Zoeae subjected to the thermal plume tests had 

 lower TL 50 's than those given the heat exchanger 

 tests (Table 2). The TL 50 of zoeae given the 60-min 

 test was similar to or 2°C lower than those given 

 the 20-min heat exchanger test; TL 50 's were at 

 progressively lower temperatures for the 120- and 

 240-min tests. Mortalities were complete at 2° 

 to 4°C above the TL 50 . The least tolerant species 

 were the Cancer crabs (Table 2) with TL 50 's at 



28°C for the 60- and 240-min tests. TL 50 's were 

 generally at 30°C for the other crabs for the three 

 time durations they were tested. The species with 

 the highest tolerance was H. oregonensis with a 

 TL 50 at 34°C for the 60-min test and at 32°C for the 

 120- and 240-min tests. 



DISCUSSION 



The situation postulated to be most critical to 

 the survival of the planktonic zoeae is their 

 passage through heat exchangers; zoeae will be 

 entrained into heat exchanger systems but those 

 encountering thermal plumes will probably only 

 be exposed to lowering temperatures (Coutant 

 1970) at the periphery where turbulence occurs. 



The maximum temperature limit that should 

 occur in heat exchangers is best described as the 

 one causing no adverse effects to the least 

 resistant species — to be consistent with the 

 protection of all species tested. Conditions that 

 could be overtly recognized as affecting the 

 survival of the zoeae were the degree of stress 

 causing torpor and the TL 50 . While the TL 50 

 directly relates to death, torpor indicates a 

 condition that could indirectly cause death. 

 Torpid zoeae would have their feeding interrupted 

 and they would not be able to evade predators 

 until they recovered. Selective predation on zoeae 

 subjected to a stress below that causing torpor 

 could also be a factor of survival at sublethal 

 temperature-time combinations. In fish, for ex- 

 ample, Coutant (1973) experimentally observed 

 that rainbow trout, Salmo gairdneri, predators 

 selectively preyed on juvenile rainbow trout and 

 chinook salmon, Oncorhynchus tshawytscha , that 

 had been exposed to shock temperature treat- 

 ments of durations below that required for the 

 prey to lose equilibrium. 



The maximum temperature that had no observ- 

 able effect on the species studied was 24°C, as this 

 was the greatest stress that did not cause Pagurus 

 beringanus and Petrolisthes eriomerus to become 

 torpid. The maximum for other species should be 

 no greater than 28°C for Cancer and up to 30° to 

 32°C for the most resistant species. 



A properly sited steam electric station should 

 not discharge hot waste water in quantities or at 

 locations where thermal plumes would retain 

 their integrity over periods of 1 to 4 h. This could 

 be a problem if Puget Sound waters were 

 intensively used for cooling. TL 50 's for the zoea 

 subjected to the 1- to 4-h thermal plume test 



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