metabolic rate of accumulated excretory products and other gases, particularly when large 

 numbers of fish are used. This aspect should be investigated further. Geyer and Mann (1939a) 

 are of the opinion that a decrease in oxygen concentration with greater numbers of fish is respon- 

 sible for the "group effect. " This factor can probably be neglected in the present work since, as 

 will be pointed out in the next section, oxygen consumption in the iao is independent of concentra- 

 tion down to at least 2. 5 cc. /I. In the flow-rate experiments, the oxygen concentration of the out- 

 let samples in no case fell below 2. 5 cc. /I. 



It is of interest to evaluate these laboratory findings in terms of the problems involved 

 in keeping bait cilive in fishing vessels. The lowering of metabolic rate with large numbers of 

 fish might be construed to mean that increasing the number of fish in a live-well leads to more ef- 

 ficient maintenance. This may be true, but more data are necessary to substantiate such a view. 

 Furthermore, if too many fish are present, the oxygen content of the water obviously may be re- 

 duced to near the lethal value, which, as will be seen shortly, is about 1.0 cc. /I. for iao. For 

 nehu, the preferred baitfish, the limited data available indicate a lethal value of about 2, cc. /I. 

 (table 7). Those boats having pumps could handle more fish by increasing the flow rate through 

 the live-wells, but this in itself causes an increase in activity and oxygen consumption (fig. 5). 

 In order to arrive at the best combination of flow rates and aggregation size the present laboratory 

 experiments should be followed up by field tests, possibly using the same flow rates and densities 

 of fish. 



METABOLIC REGULATION IN THE IAO 



In summarizing the literature pertaining to the relationship between rate of oxygen 

 consumption and the dissolved oxygen content of the water, it is apparent that some fish are 

 markedly affected by chauiges in external oxygen while others are not. Gaarder (1918) found a 

 very slight dependence of oxygen consumption on oxygen concentration in the carp. Powers and 

 Shipe (1928) claim that for herring, coho salmon, and viviparous perch a decrease in the amount 

 of dissolved oxygen caused a proportional decrease in the rate of oxygen consumption. According 

 to Hall (1929), the degree of metabolic regulation depends on whether the fish is an active or 

 sluggish species, the former showing metabolic independence and the latter complete dependence 

 on the oxygen concentration of the medium. In amy event, small changes from the normal oxygen 

 content of the water probably do not aiffect oxygen uptake in nnost fish (Keys 1930a; Wells 1935a; 

 Lindroth 1940; Fry and Hart 1948), but an oxygen level is eventually attained below which meta- 

 bolic adjustment fails and the rate of oxygen uptake begins to decrease. This "critical level'' 

 varies with species and with temperature (Lindroth 1940; Fry and Hart 1948; Graham 1949). 



One means by which fish accomplish metabolic regulation is an increase in the frequen- 

 cy of breathing movements. This compensates for the fall in oxygen concentration by permitting 

 more water per unit time to flow past the gill surfaces. Several workers (Westerlund 1906; Win- 

 terstein 1908; Wells 1913; Gardner and King 1922; Belding 1929; Wilding 1939) have incidentally 

 observed an increase in the rate of opercular nnovement, to a greater or lesser degree, with a 

 decrease in the oxygen content of the water. On the other hand, there are few works in which both 

 the rate of oxygen consumption and breathing movements are analyzed amd their interrelations dis- 

 cussed. Notable exceptions are papers by Meyer (1935), who worked with Uranoscopus scaber , 

 and van Dam (1938), who worked on eel and trout. Both workers determined not only breathing 

 rate, but breathing depth and ventilation volume in relation to oxygen concentration. Van Dam 

 found that the eel (Anguilla vulgaris ) showed much better metabolic regulation thjin the trout (Salmo 

 Shasta ). 



The above discussion points to the importance of determining the degree of metabolic 

 regulation in any fish whose oxygen relations are being investigated. In this section the relation- 

 ship of metabolic rate to oxygen concentration is analyzed, and to supplement these data the rate 

 of opercular beat is determined at various oxygen values. 



13 



