FISHERY BULLETIN: VOL. 74, NO. 2 



Resistance to Acute Temperature 

 Changes 



Resistance to high temperatures is surprisingly 

 great for embryos and larvae when exposure is of 

 short duration (Figure 9). Blastodisc stage em- 

 bryos are least resistant while yolk-sac larvae are 

 most resistant; LDso values for the 60-min expo- 

 sure period for the larvae are within 1.3°C of the 

 extrapolated 24-h tolerance limits determined 

 from Figure 8. Engraulis mordax embryos and 

 larvae appear to be insensitive to abrupt tempera- 

 ture decreases down to 0.5°C for short periods 

 (Brewer 1975b). 



Table 4. — Comparison of the maximum size attained by En- 

 graulis mordax larvae in the yolk-sac stage before shrinkage due 

 to starvation. 



40- 



cH 38- 



36' 



32- 



30 



28-^ 



3 J 5 a 8 10 20 



MINUTES TO MEDIAN MORTALITY 



Figure 9. — Minutes to median mortality for the blastodisc 

 stage (1), blastopore closure stage (2), and yolk-sac stage (3) 

 subjected to abrupt temperature increases from 16°C (original 

 data in Brewer 1975b). 



Development and Growth 



Table 4 summarizes data on the growth of lar- 

 vae at constant temperatures between 10° and 

 26°C. The maximum size of larvae attained at any 

 temperature before shrinkage due to starvation 

 was 4.16 mm. This is considerably smaller than 

 the value of 4.8 mm given by Lasker (1964) for jB. 

 mordax larvae reared under similar conditions. 

 Variability in egg size may be responsible for this 

 discrepancy; egg size of the Argentine anchovy, E. 

 anchoita, is known to vary by season and location 

 (de Ciechomski 1973). 



The highest mean growth response was ob- 

 tained for larvae reared at 18°C (3.94 mm). Mean 

 larval lengths less than 3.78 mm were considered 



significantly smaller than the maximum response 

 at 18°C (Least Significant Difference, Sokal and 

 Rohlf 1969). It seems reasonable to assume that 

 larvae reared at temperatures of 12°C or lower and 

 24°C or higher converted yolk into body tissue at 

 suboptimal levels. Analysis of variance showed 

 that maximum mean lengths attained by larvae 

 reared at 14°, 17°, 18°, and 20°C were not sig- 

 nificantly different (P>0.05). 



DISCUSSION 



Figure 10 shows a graphic summary of various 

 field and laboratory-deduced temperature ranges 

 and limits for the distribution and survival oi E. 

 mordax. A temperature range of about 4.5°C lies 

 between the highest temperatures that anchovy 

 adults have been found in nature (25°C, Baxter 

 1967) and the experimentally determined upper 

 lethal temperature for juveniles and adults 

 (29.5°C). Anchovy had been maintained in the 

 laboratory at 28°C for weeks with no apparent ill 

 effects. The fish are extremely active at this tem- 

 perature and their metabolic requirements are un- 

 doubtedly considerable. Anchovy maintained at 

 28°C and fed the standard ration lost weight. The 

 upper environmental temperature limit and 

 southern distributional limit of £. mordax may be 

 dictated by metabolic demands which outweigh 

 the ration supplied by the environment. 

 Maximum temperatures off Cabo San Lucas, 

 which is the southern range limit for E. mordax, 

 exceeds 25°C (Lynn 1967). Interestingly, 25°C cor- 

 responds to the highest temperature that juvenile 

 E. mordax would venture into when tested in 

 laboratory thermal gradients (Brewer 1974). 

 Moreover, the plateau in the thermal tolerance 

 polygon (Figure 3) shows that acclimation tem- 

 peratures of 24°C and above have little effect on 

 increasing the incipient upper lethal temperature. 

 Apparently the anchovy's overall mechanisms for 



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