170 R. J. H. Beverton and S. J. Holt 



Catla catla, and Carassius carassius, all of which have rather 

 low values of K, have a low oxygen consumption of about 

 0-2 ml./g.^^^/hr. (Blazka, 1958). Over the temperature range 

 5-35°, oxygen uptake by another cyprinid, the goldfish, 

 Carassius auratus, ranges from 0-05-0-46 ml./g.^^^/hr. (Fry 

 and Hart, 1948); this species has a K value of about 0-3. 



Metabolic rate has been estimated, in connexion with ex- 

 periments on the transport of live fish in closed containers, 

 from the rate of carbon dioxide accumulation; in one such 

 case Tilapia mossambica, which has a higher K value than 

 Cyprinus carpio, respired faster than the latter, though kept 

 at the same temperature (Vaas, 1952). Further evidence of 

 relative metabolic rates comes from studies of the rate of 

 uptake and loss of radioactive substances by fishes. Thus, in a 

 review of this subject. Boroughs, Chipman and Rice (1957) 

 quote results indicating that the exponential loss coefficient 

 of orally administered strontium 89 from the body of Tilapia 

 mossambica is two and a half times that of skipjack {Euthynnus 

 yaito), yellow-fin {Neothunnus macropterus) and "dolphin" 

 (Coryphaena hippurus). These latter fishes are more active 

 species than Tilapia and, from the scanty data available, 

 appear to have higher K values. We have not found any 

 published data to indicate whether, in fish for which K is 

 higher in males than in females, the respiratory rate of males 

 is also higher, as might be expected. 



Natural death and reproduction 



The last line of evidence we shall mention is that concerning 

 the connexion, in fish, between natural death and reproduc- 

 tion. We have previously mentioned that in the short-lived 

 species where there is an abrupt end to the lifespan, death 

 usually occurs at or soon after spawning. What has been 

 called "reproductive drain" may also become apparent in 

 other ways. For example, the ratio of the weight of a fish to 

 the cube of its length (called the "condition factor" or 

 "ponderal index" in fisheries literature) varies seasonally, 



