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Fishery Bulletin 97(2), 1999 



swimming speeds during routine activity and 

 feeding (Durbin and Durbin, 1975; Hettler 

 1976; Durbin et al., 1981). Swimming capac- 

 ity is unusually high in Atlantic menhaden; 

 Hartwell and Otto (1978) reported that 5.8- 

 cm-SL juvenile menhaden sustained a critical 

 swimming speed of 15.8 BL/s for 64 min at 

 20°C. The critical swimming speed of similarly 

 sized sockeye salmon at 15°C is about 7 BIVs 

 (Brett and Glass, 1973). 



Standard metabolism (Fry, 1957) is a mea- 

 sure of basic maintenance costs and as such 

 may be a useful reference level for interspe- 

 cific comparisons. The magnitude of standard 

 metabolism also seems to reflect species life 

 styles with respect to their general activity 

 levels. Standard metabolic rates for menha- 

 den (Table 3) were lower than those for active 

 predators like the bluefish, Pomatomus 

 saltatrix (0.156 mg 0._,/(g wet wt • h), for 217-g 

 fish at 15 C (Freadman, 1979), but higher than 

 those for the sedentary flatfish Limanda 

 limanda (0.015, 0.029, and 0.042 mg OJ(g wet wt • 

 h) at 5°, 10=, and 15 C for 236-400 g fish (Duthie, 

 1982)). Standard metabolism in menhaden was simi- 

 lar to that of sockeye salmon (Brett, 1964) at 15^^C, 

 but lower at 10" and 20 = C (Table 3). Durbin et al, 

 ( 1981) previously estimated standard metabolism for 

 menhaden at 20''C to be only 0.036 mg O,^ Ag wet wt 

  h), from the regression of feeding metabolism as a 

 function of swimming speed. However we believe that 

 the present higher estimates are more reliable be- 

 cause the data include a broader range of swimming 

 speeds and require less extrapolation to zero swim- 

 ming speed than the earlier estimate. 



The QjQ values (Prosser, 1973) provide a measure 

 of the effect of environmental temperature on meta- 

 bolic rates. The Qj,, for standard metabolic rate in 

 our study was very close to that of Hettler for rou- 

 tine metabolism in juvenile menhaden (Table 3). The 

 decline in Q,„ between 10°-15°C and 15''-20°C thus 

 appears to be real. A similar pattern was obsei'X'ed 

 in dab, Limanda limanda, where the Q,,, over 5°- 

 10°C was higher than over 10 -15°C (3.7 and 2.1 

 respectively ( Duthie, 1982 ). The more extensive data 

 of Hettler ( 1976) and Brett (1964) indicate that over 

 a wide temperature range (Hettler: 14-24°C; Brett: 

 5-24°C), the Q,,, for metabolic rate is close to 2.0, 

 but within this range the Q,,j declines as the ther- 

 mal optimum is approached and increases towards 

 the thermal extremes for the species. 



Hettler's data on routine metabolic rate in juve- 

 nile menhaden, when extrapolated to 300-g fish 

 (Table 3 ), yield relatively high routine metabolic rates 

 compared with those observed by Durbin et al. ( 1981 ) 



(0.10 mg O.J\ g wet wt • h ) at 20°C ). Because the maxi- 

 mum weight of Hettler's fish was only 74.3 g, the 

 extrapolated values for 300-g fish are most likely 

 overestimates. The calculated routine rates in Table 

 3 correspond to swimming speeds of 18.8, 25.7, and 

 27.4 cm/s at 10°, 15°, and 20°C, respectively which 

 were faster than the routine speed of 12.2 cm/s at 

 20' C reported by Durbin et al. (1981 ). 



Routine metabolic rates are measured directly at 

 spontaneous activity levels. Thus routine rates re- 

 flect real world activity levels, because few fish ac- 

 tually sleep or totally cease movement that would 

 depress metabolic rates to basal or standard levels. 

 Our present observations do seem to be comparable 



