FISHERY BULLETIN. VOL. 78. NO. 3 



The Qio's [oxygen consumption at (T + 10) 

 ° C/oxygen consumption at T° C] are presented in 

 Table 9. Although there are minor differences at 

 different temperature ranges, the average Q,,j's 

 are nearly equal and very close to the average 1.7 

 obtained by Scholander et al. (1953) for P. 

 brasiliensis tested at 25° and 30° C. Wolvekamp 

 and Waterman (1960) stated that generally Q^, 

 values increase as the temperature decreases, but 

 an increase was not obvious in this study. 



Table 9. — Qio's for two sizes of Penaeus aztecus; oxygen con- 

 sumption data averaged over all test salinities. 



Table lO. — Calculation of oxygen available to Penaeus aztecus 

 and consumed at 20 and 30%o salinity (S) and 33° C; m = mass. 



Size (g) 



Temperature ( C) 



Q,n 



3.7 

 3.7 



6.7 



6,7 



Mean 



Mean 



18-28 

 23-33 



18-28 

 23-33 



18-28 

 23-33 



1-59 

 1 63 



1.71 

 1.63 

 1.65 

 1.63 



Temperature effects at tested salinities were not 

 uniform. In 10 and 20%o S, oxygen consumption 

 increased significantly as temperature increased 

 (Figure 1, Table 2), but in 30%o S, oxygen consump- 

 tion peaked at 28° C and decreased at 33° C. This 

 reduction indicates a possible detrimental effect 

 on P. aztecus when both salinity and temperature 

 are high. The osmoregulatory abilities of P. az- 

 tecus are reduced at 33° C (Figures 2, 3), and salin- 

 ity effects appear to become increasingly impor- 

 tant. Other studies have also indicated reduced 

 responses of P. aztecus tested at high tempera- 

 tures. Survival of juveniles (10-50 mm TL) was 

 <80% at temperatures >28° C at 25%o S (Zein- 

 Eldin and Aldrich 1965; Zein-Eldin and Griffith 

 1969). Rates of growth ( mass) of postlarvae in salin- 

 ities >25%o were less at 32° C than at 25° C (Zein- 

 Eldin and Aldrich 1965). Brown shrimp acclimat- 

 ed to 32° C were more sensitive to temperature 

 change than those acclimated to 18° or 25° C and 

 showed reduced osmoregulatory abilities in salin- 

 ities <10%o ( Venkataramiah et al. footnote 4). 



The possibility exists that oxygen consumption 

 rates at 33° C and 30%o S are a reflection of reduced 

 dissolved O2 concentration. That is, at 33° C, oxy- 

 gen is less soluble in 30%o S than in 10 or 20%o S, 

 and the shrimp's oxygen consumption may be pro- 

 portional to the oxygen concentration. To test this 

 hypothesis, the difference between the average 

 oxygen consumption in 20 and 30%o S at 33° C was 

 calculated and compared with the difference be- 

 tween the oxygen available in the test chamber at 

 20 and 30%o at 33° C (Table 10). The decrease of 



total oxygen consumption between 20 and 30%o 

 was <0.1 mg h~' and is not of similar magnitude to 

 the oxygen-available difference of 1.56 mg h~' ; the 

 differences indicate that P. aztecus is an oxygen 

 regulator. Also the saturated oxygen concentra- 

 tion at 30%o S and 33° C is well above the stress 

 level of 2 ppm obtained by Egusa (1961) for P. 

 japonicus. Therefore the decrease in dissolved 

 oxygen resulting from the increased salinity does 

 not appear to be responsible for the reduced rate of 

 oxygen consumption of brown shrimp in 30%o S 

 and 33° C. 



As temperature increased to 33° C, hemolymph 

 osmolality tended toward that of the external 

 medium for shrimp tested in 10 and 30%o S ( Figure 

 3). Williams (1960) found the osmoregulatory 

 abilities of P. aztecus were significantly less at 8.8° 

 C than at 28° C. Thus it appears that as tempera- 

 ture approaches environmental extremes, os- 

 moregulatory abilities are impaired, and shrimp 

 tend toward osmoconformity. Penaeus aztecus was 

 able to maintain homoiosmoticity at 20%o over the 

 tested temperatures (Figure 3), indicating that at 

 high temperatures ( 33° C ) and a moderate salinity, 

 osmoregulatory processes are not adversely af- 

 fected. 



Energy Considerations 



The metabolic energy expenditure of shrimp can 

 be calculated from knowledge of their oxygen con- 

 sumption rates and their metabolic substrate (in- 

 direct calorimetry). Because shrimp are omnivo- 

 rous (Williams 1955; Mistakidis 1957; Eldred et 

 al. 1961; Perez Farfante 1969; Moriarty 1977), a 

 combination of carbohydrate, lipid, and protein as 

 the shrimp's metabolic substrate should give a 

 reasonable estimate of the oxygen-consumption/ 

 energy-expenditure relationship. At standard 

 conditions, combustion of 1 g of carbohydrate, 

 lipid, or protein with 1 1 of oxygen yields 5,007, 



752 



