MAY: EFFECTS OF ACCLIMATION 



those acclimated to 357oo. There was little change 

 in the lower TLm's between 24 and 48 h, but a 

 slight rise occurred between 48 and 72 h in all but 

 the 30"/ 00 acclimation group. 



DISCUSSION 



Fry et al. (1946) define the "zone of tolerance" as 

 the range of any environmental factor within 

 which an animal can live indefinitely, and the 

 "zone of resistance" as the range within which the 

 animal can live for only a finite period of time, 

 depending on the level of the factor. The zone of 

 tolerance is bounded by the upper and lower "in- 

 cipient lethal levels." In work on the upper thermal 

 tolerance of fishes, the incipient lethal level is 

 defined by an abrupt flattening of the time- 

 temperature curve at a temperature below which 

 less than 50% of the exposed individuals succumb 

 (Brett 1956). Some of the curves generated in the 

 present study (Figures 1-4) suggest that the in- 

 cipient lethal level has been reached, but curves 

 from the higher salinities lack a horizontal seg- 

 ment. This points up a difficulty in working with 

 early larvae: at 24°C the larval yolk supply is 95% 

 consumed by about 40 h after hatching (May 1974), 

 and this occurs even sooner at higher tempera- 

 tures. The 48- and 72-h TLm's therefore apply to 

 starving larvae. Unlike adult fish, larvae which 

 hatch from pelagic eggs are extremely sensitive to 

 food deprivation (e.g., Lasker et al. 1970) and 

 begin dying of starvation soon after yolk absorp- 

 tion if food is not provided for them, and unfed 

 bairdiella larvae die sooner at high temperatures 

 and salinities (May 1975). Therefore, prolonging 

 these tests would not have helped in defining the 

 upper incipient lethal temperature for larvae in 

 the higher salinities-the TLm would simply con- 

 tinue to fall. Even at the lower salinities, the TLm 

 would decline after a sufficient period of time; the 

 curves for a salinity of 35'*/oo (Figures 1, 3) show 

 how a flat segment is reached, only to be followed 

 by another drop in TLm. A further difficulty in 

 estimating tolerance limits for warmwater larvae 

 is that these larvae develop morphologically at an 

 extremely rapid rate and are very different or- 

 ganisms 1 or 2 days after hatching than they are at 

 hatching. Newly hatched bairdiella are poorly 

 developed and rather inactive (May 1975), whereas 

 by 45 h after hatching (at 24°C) they have acquired 

 functional eyes and an open mouth and are quite 

 active. In this situation, consideration of the TLm 

 at a more or less arbitrary time after exposure to 



the test conditions, such as 24 h, is at least a useful 

 approach for comparative purposes. 



Larval bairdiella are more sensitive to high 

 temperatures when the salinity is also high, as are 

 bairdiella gametes and developing embryos (May 

 1975). This adds further weight to the suggestion 

 (May 1975) that in nature, eggs spawned late in 

 the season at high temperatures will have a 

 reduced chance of contributing recruits to the 

 population when natural salinities rise as they are 

 doing in the Salton Sea. The survival of bairdiella 

 larvae in the Salton Sea would be significantly 

 reduced at temperatures above 31°C, and 

 temperature data from the Salton Sea (May 1975) 

 indicate that some larvae could be exposed to 

 thermal stress of this level or greater. The highest 

 TLm is reached in W/m, the lowest salinity in 

 which larvae were tested and the nearest to being 

 isosmotic with larval body fluids. Older fishes of 

 various species are also most tolerant of high 

 temperatures in isosmotic or nearly isosmotic 

 salinities (Aral et al. 1963; Strawn and Dunn 1967; 

 Garside and Jordan 1968; Simmons 1971). The add- 

 ed burden of osmotic work seems to reduce the 

 ability of both larval and adult fish to tolerate 

 extremely high temperatures. 



It is clear that acclimation can alter the 

 tolerance of early bairdiella larvae to both 

 temperature and salinity, even though the rapid 

 developmental rate of bairdiella eggs restricts the 

 period of acclimation to between 20 and 40 h (the 

 time between fertilization and transfer to test 

 conditions, which is a function of incubation 

 temperature). Incubation of bairdiella eggs at 

 higher temperatures produces larvae with a higher 

 upper thermal TLm. However, increasing the 

 acclimation temperature from 27° to 30°C does not 

 increase the upper TLm, even though the TLm's 

 are generally above 30°C. Hence the lethal levels 

 determined for an acclimation temperature of 

 27°C may represent "ultimate" incipient lethal 

 temperatures (Fry et al. 1946), but here again one 

 must consider the unique problems of working 

 with early larvae. If the effect of thermal 

 acclimation on the tolerance of yolk-sac larvae is to 

 be studied, acclimation must take place during 

 embryonic development, but the embryos are more 

 sensitive to temperature than are the larvae to 

 which they give rise (cf . May 1975). A temperature 

 of 30°C is extremely stressful for developing eggs, 

 and the larvae produced at this temperature sur- 

 vive poorly, a trait magnified at higher salinities. 



253 



