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Fishery Bulletin 97(2), 1999 



20°C, we first calculated from Table 1 the metabolic 

 rate for forced swimming at the routine speed ( 12.2 

 cm/s) = 0.111 mgOg/ig wet wt-h) and subtracted the 

 standard metabolic rate from Table 3 (0.087 mg O.J 

 (g wet w • h) to obtain the net cost of swimming = 

 0.024 mg 0./(g wet wt • h). The net cost of routine swim- 

 ming (Durbin et al, 1981) is 0.1 - 0.087 = 0.013 mg 

 0.^(g wet wt • h). The ratio of the two (= swimming 

 co"st ratio [SCR] of Boisclair andTang, 1993) = 0.024/ 

 0.013 = 1.8. This value is much lower than Boisclair 

 and Tang's (1993) average SCR value of 9.4 for the 

 ratio of routine to forced swimming costs and is more 

 comparable to their average SCR value of 1.6 for the 

 ratio of directed to forced swimming costs. This sug- 

 gests that routine activity in menhaden, although 

 energetically more expensive than sustained swim- 

 ming, is relatively economical. Metabolism associated 

 with routine activity is, in fact, only slightly elevated 

 above standard metabolism in menhaden. Menha- 

 den thus may be an exception to the rule (Webb, 1991 ) 

 which states that routine metabolic rates of swim- 

 ming menhaden are not quasi-steady but are sub- 

 stantially higher than forced-swimming rates. 



The cost of swimming in menhaden increases ex- 

 ponentially with a linear increase in swimming speed 

 (Fig. 5, this study; Durbin et al., 1981), a pattern 

 characteristic offish in general (reviewed by Brett 

 and Groves, 1979). The relation between the meta- 

 bolic cost of swimming and temperature is similar 

 for unfed Atlantic menhaden and sockeye salmon 

 (Oncorhynchus nerka) (Fig. 6; Table 2). In both spe- 

 cies, the rate of increase in respiration rate per in- 

 crement of swimming speed is highest at the lowest 



temperature. With increasing temperature the slopes 

 of the relationships decrease, indicating that the 

 metabolic cost of swimming declines. The cost of 

 swimming to unfed Atlantic menhaden is close to 

 Beamish's ( 1978 ) mean value for eight species, where 

 metabolism increased by 2.3-fold for a 1 BL/s increase 

 in swimming speed. Thus we conclude that the meta- 

 bolic cost of swimming in nonfeeding menhaden is 

 similar to that of other pelagic fish species. 



Conversely, the cost of swimming to menhaden 

 while filter-feeding is much higher, where the meta- 

 bolic rate rose five-fold per BL/s increase in swim- 

 ming speed (Eq. 2 in Table 2, Durbin et al., 1981), 

 compared with a 1.65-fold increase in unfed fish at 

 20 = C (Table 2, our study). Durbin et al. ( 1981) specu- 

 lated that the very high cost of swimming during fil- 

 ter feeding was related to the increased hydrody- 

 namic drag of the expanded gill opercula and the long 

 gill rakers that strain plankton from the water. 

 Present results permit the separation of metabolic 

 rate during feeding into its component parts at 20°C 

 (Table 4; Fig. 7). We observed that during filter feed- 

 ing, menhaden swam at a preferred speed of about 

 41.3 cm/s over a wide range of plankton concentra- 

 tions. However, after prey concentrations had been 

 reduced to subthreshold levels, feeding ceased and 

 activity and respiration both returned to routine lev- 

 els where there was no postfeeding respiration peak 

 ( Durbin etal., 1981). Standard metabolism accounted 

 for only 18.1'^ of the total metabolism, and the en- 

 ergy demand for swimming was about 22.9%. The 

 remaining 599^ of metabolism was associated with 

 the energy cost of feeding, which includes the in- 



