DURBIN ET AL.: SWIMMING SPEEDS AND RESPIRATION RATES OF ATLANTIC MENHADEN 



Table 3— Metabolic rates among 10 fish species. Except where noted, rates were extrapolated to a 300 g fish, using 

 the appropriate weight-swimming speed-respiration relationships. 



'Actual measured values: x wet weight of rainbow trout = 272 g, and Atlantic menhaden 

 ^Maximum voluntary metabolic rate in this study; presumably less than active rate. 



302 g. 



rate in menhaden remains unknown, though pre- 

 sumably higher than those reported here. Thus 

 the metabolic "scope" (Fry 1947) in menhaden ap- 

 pears to be significantly larger than that of many 

 species. A large metabolic scope is consistent with 

 Hartwell and Otto's ( 1978 ) finding that the critical 

 swimming speeds in juvenile Atlantic menhaden 

 far exceed those reported from other species: at 20° 

 C nonfeeding fish averaging 5.8 cm standard 

 length were able to maintain a speed of 15.8 BL/s 

 for 64 min and a speed of 20.8 BL/s for 2 min. In 

 contrast, at 20° C the critical speed of a 6 cm 

 sockeye salmon, for example, is only about 6.5 

 BL/s (Brett and Glass 1973). 



The respiration during feeding increased sig- 

 nificantly faster per increment in swimming speed 

 in menhaden than in other species which have 

 been studied. In Atlantic menhaden an increase in 

 swimming speed of 1 BL/s caused a 5.8-fold in- 

 crease in the respiration rate (Table 2, Equation 

 (3)); while for eight species reviewed by Beamish 

 (1978) (Oncorhynchus nerka, Lepomis gibbosus, 

 Melanogrammus aeglefinus , Tilapia nilotica, Mi- 

 cropterus salmoides, Liza macrolepis, Cyprinus 

 carpio, Salmo gairdneri), a similar increase in 

 speed caused a roughly 2.3-fold elevation in the 

 metabolic rate. Thus the cost of increasing the 

 swimming speed is 2.5 times higher in Atlantic 

 menhaden during feeding than in these eight 

 species (which were not feeding). 



The very steep slope of the swimming speed- 

 respiration relation for the Atlantic menhaden 

 indicates that during feeding, the loss in stream- 

 lining caused by the expanded opercula and the 

 resistance of the closely spaced gill rakers sub- 

 stantially increase the hydrodynamic drag of the 

 fish. It is likely that the respiratory cost for non- 

 feeding Atlantic menhaden swimming at equiva- 

 lent speeds would be much lower. It would be of 



interest to determine the maximum swimming 

 performance of nonfeeding adult fish, for compar- 

 ison with Hartwell and Otto's (1978) data from 

 juveniles. 



The rapidly increasing respiratory cost of 

 swimming during feeding is perhaps more clearly 

 illustrated when the relationship is plotted 

 arithmetically ( Figure 5, inset). The highest mean 

 voluntary swimming speed in the present experi- 

 ments, 43.4 cm/s or 1.68 BL/s, is very close to the 

 inflection of the curve in Figure 5 (inset), beyond 

 which an increase in swimming speed drastically 

 increases the metabolic rate. Because of the high 

 energy cost it is likely that in nature the voluntary 

 swimming speeds of adult Atlantic menhaden dur- 

 ing feeding will be <2 BL/s for most of the time. 



In conclusion, the Atlantic menhaden, a filter- 

 feeding planktivore, offers an interesting contrast 

 to pelagic predaceous fishes, which have been 

 more widely studied (Durbin 1979). These pred- 

 ators typically consume their daily ration in a few 

 large meals (the}' are "macrophagists"). The time 

 and energy costs of feeding varies in different 

 species, and the energy lost to SDA is a conspicu- 

 ous component of their daily metabolism. In con- 

 trast, a "microphagist," such as the Atlantic 

 menhaden, consumes its food as a continuous 

 stream of very small food particles. While the 

 energy cost associated with feeding is consistently 

 high, in menhaden there is no extended period of 

 elevated respiration following feeding, as ob- 

 served in macrophagists, but rather a continuous 

 and rapid return to prefeeding rates. 



ACKNOWLEDGMENT 



We would like to thank Harold Loftes, skipper of 

 the Ocean State, and Charles Follett, skipper of 

 the Cindy Bett, for their assistance in obtaining 



885 



