FISHERY BULLETIN: VOL. 73, NO. 2 



1975). These conditions (which remained constant 

 from fertilization to hatching, plus the period of 

 time between hatching and transfer to the test 

 conditions) constituted the conditions of acclima- 

 tion. Larvae were not fed during the experiments. 

 Test salinities were prepared by dilution with 

 deionized water from a stock solution of 60%, 

 which had been made by adding artificial sea salts 

 to seawater (May 1975). 



Upper Thermal Tolerance 



Larvae were acclimated to temperatures of 21°, 

 24°, 27°, and 30°C, and to salinities of from 15 

 to 45^/00 (Table 1), covering the ranges of these two 

 factors within which successful embryonic 

 development can take place (May 1975). Since 

 developmental rates were more rapid at the higher 

 temperatures (May 1975), the period of acclima- 

 tion (fertilization to transfer to test vials) was 

 shorter at the higher acclimation temperatures. 

 The median tolerance limit (TLm)^ of yolk-sac 

 larvae to high temperatures was determined by 

 the method of Doudoroff (1942). Larvae were 

 transferred directly from the acclimation condi- 

 tions to a series of 25-ml capped vials maintained 

 in the dark at a series of high temperatures in a 

 thermal gradient block (Thomas et al. 1963) within 

 5 to 10 h after hatching, at which time they were 

 1.8 to 2.0 mm in length. The highest test tempera- 

 ture was 36°C, and between five and eight test 

 temperatures, 1.5°C apart, were used depending 

 on the acclimation temperature; the salinities in 

 the test vials were the same as the acclimation 

 salinities. Approximately 10 larvae were placed in 

 each vial, and the test temperatures did not vary 

 by more than ±0.1°C. Antibiotics were added to 

 the water in the vials (May 1975), and the survival 

 of larvae under optimal conditions in these vials 



'The term "median tolerance limit" and the symbol TLm are 

 recommended in "Standard Methods" (American Public Health 

 Association 1971). 



was comparable to that in larger containers. The 

 number of larvae surviving at each test tempera- 

 ture was recorded at 0.5, 1, 3, 6, 12, 24, 48, and 72 h 

 after transfer, and for each time the TLm-the 

 temperature at which just 50% of the larvae sur- 

 vived the given time interval-was estimated by 

 graphical interpolation as described by Doudoroff 

 (1942). At each observation, larvae which showed 

 no movement were removed from the vials by 

 pipette and examined under a dissection micro- 

 scope. If the heart was not beating and the larva 

 was opaque, the larva was considered dead and 

 was discarded; live larvae were returned to the 

 vials. In one instance (see Results) moribund lar- 

 vae were found and counted as dead. 



Salinity Tolerance 



Larvae were acclimated to salinities of 15, 20, 25, 

 30, 35, and 40'*/oo, and their upper and lower 

 salinity TLm's were determined by transferring 

 larvae directly to a series of 25-ml vials containing 

 test salinities ranging from (deionized water) to 

 58°/oo. Between five and seven larvae were trans- 

 ferred to each vial within 8 h after hatching. 

 Because of limited material, only the upper TLm 

 was determined for larvae from an incubation 

 salinity of 40*'/oo, and only four larvae from this 

 salinity were available for each vial. The 

 temperature during fertilization, incubation, and 

 testing was maintained at 24 ± 0.2°C by thermo- 

 statically controlled water baths, and vials were 

 kept under continuous room light of low intensity 

 (May 1975). The number of larvae surviving at 

 each salinity was recorded 24, 48, and 72 h after 

 transfer, and the upper and lower TLm's were 

 estimated graphically for each time interval as in 

 the case of thermal tolerance. Larvae were con- 

 sidered dead on the basis of the same criteria used 

 in the study of thermal tolerance. 



RESULTS 



Table 1. -Acclimation conditions for larvae used in determina- 

 tion of heat tolerances. 



Upper Thermal Tolerance 



The upper TLm dropped with increasing time 

 intervals (Figures 1-4), and there was a leveling 

 off of the time-temperature curves in the lower 

 salinities as time increased. Most of the time- 

 temperature curves have been separated by eye-fit 

 lines into two major segments, the horizontal seg- 

 ment defining the "incipient lethal temperature" 



250 



