BREWER: THERMAL TOLERANCE AND RESISTANCE 



32' 



30- 



28- 



_ 2*' 

 o 



Q 24- 



S "-I 



16- 

 U- 

 12- 

 10- 

 8- 

 6' 



7 



/ 

 / 



^ • *^ 



/ 



^^ • 



: . 



10 12 u 16 18 20 22 24 26 28 30 32 

 ACCLIMATION TEMPERATURE °C 



Figure 3. — Thermal tolerance polygon for Engraulis mordax 

 juveniles and adults. Those points where the extrapolated LDso 

 levels intersect the diagonal represent the extreme (ultimate) 

 tolerance limits and correspond to 6.5° and 29.5°C. 



29.5° and 6.5°C, respectively. These temperatures 

 represent the maximum tolerance range of E. 

 mordax juveniles and adults sampled from south- 

 ern California and maintained under laboratory 

 conditions as described. Anchovy cannot be accli- 

 mated to temperatures beyond these extremes. 

 Attempts were made to slowly acclimate fish to 

 29.5° and 6.5°C, but they proved futile. 



Figure 4 shows the resistance times to median 

 mortality of juvenile and adult E. mordax, accli- 

 mated to 8°, 16°, and 28°C, upon exposure to tem- 

 peratures beyond incipient lethal levels. These 

 curves were derived by plotting cumulative mor- 

 tality as percentages against exposure time to es- 

 timate the time to LDso for each test temperature. 



Resistance as a Function of Size, 

 Sex, Time, and Photoperiod 



Results of experiments on thermal resistance to 

 30°C in relation to size, sex, and potential diel and 

 photoperiod effects are summarized in Table 1. 

 Analysis of variance (one-way classification) 

 showed that resistance times to lethal tempera- 

 tures of 30°C were not significantly different 

 (P>0.05) for fishes of different sizes (<79 mm; 

 80-99 mm; >100 mm) or for fishes maintained 

 under different photoperiods (8, 12, and 16 h 



HOURS TO MEDIAN MORTALITY 



Figure 4. — Resistance times to median mortality of juvenile 

 and adult Engraulis mordax exposed to high (A) and low (B) 

 lethal temperatures when acclimated to 8°, 16°, and 28°C. 



Table l. — Comparison ofresistance times (minutes until death) 

 for juvenile and adult Engraulis mordax acclimated to 20°C and 

 immediately transferred to aquaria at 30°C. 



Item 



N 



Range 



(min) 



Mean 

 (min) 



SD 



SE 



Length (mm): 



<79 11 



80-99 22 



>100 12 



Sex: 



Male 20 



Female 36 



Time of test: 



Morning 45 



Evening 38 



Photopenod (hours of 



8 14 



12 34 



16 11 



49-285 

 41-302 

 37-343 



6-118 



4-401 



31-343 



8-244 



light): 



6-401 



37-302 



31-343 



133.9 



149.4 

 141.3 



40.9 

 116,8 



143.5 

 72.6 



154.5 

 141.1 

 150.8 



74.9 



62.3 



1029 



33.1 

 113.6 



76.3 

 66.7 



127.0 



67.6 



102.3 



22.6 



13.3 

 29.7 



7.4 

 18.9 



11.4 

 10.8 



34.0 

 11,6 

 30.9 



light). These results should be verified with larger 

 sample sizes. Resistance times showed highly sig- 

 nificant differences (P<0.01) for males compared 

 with females, and for tests conducted in the morn- 

 ing as compared with those conducted at night. 

 Females proved more resistant than males, and 

 animals tested in the morning showed greater re- 

 sistance than those tested in the evening. 



437 



