FISHERY BULLETIN: VOL. 79, N0.4 



Oxygen:Nitrogen Ratios 



The ratio of O2 consumed to N excreted (by 

 atoms) has been used to give an indication of the 

 type of food the fish are metabolizing. The 0:N 

 ratio during the combustion of protein is about 7.4 

 (Kutty 1972), while the ratio for carbohydrate is 

 infinity, and for fat is about 415 (Ikeda 1977). 



The change in the 0:N ratio with time is shown 

 for four phytoplankton experiments (Figure 12). 

 The mean 0:N for all initial prefeeding mea- 

 surements in the phytoplankton experiments was 

 28.2, cr = 9.8. There was a slight increase of the 

 0:N immediately after the beginning of feeding in 

 most of the experiments. This was because the 

 increase in the voluntary swimming speed of the 

 fish during feeding produced an immediate in- 

 crease in O2 consumption, whereas N excretion 

 increased more gradually. Swimming speeds and 

 respiration rates during feeding in the three high 

 ration experiments (nos. 4, 5, 6) averaged about 

 41.3 cm/s and 0.48 mg 02/g wet weight per h, 

 respectively. In these experiments, the 0:N ratios 

 declined to very low levels (between 5 and 10) soon 

 after the initiation of feeding and remained at 

 these low levels for the rest of the feeding period 

 (Figure 12). In the four smaller ration experi- 

 ments, the swimming speed during feeding ranged 

 between 29.3 and 36.5 cm/s and O2 consumption 

 between 0.221 and 0.354 mg 02/g wet weight per h. 



40 

 30 

 20 



10 







E 30 

 o 



■5 20 



10 



.0 



- 

 Z 30 



CD 20 



10 

 

 30 

 20 

 10 





 0600 



1200 



1800 

 TIME 



2400 0600 



hours 



1200 



Figure 12. — 0:N ratios of Brevoortia tyrannus before, during, 

 and after feeding on Ditylum hrightwelli. Oxygen data are from 

 Durbinet al.(1981). 



The decline in the 0:N ratios during feeding was 

 much less than in the high ration experiments. 

 Swimming speeds and O2 consumption rates of 

 nonfeeding fish averaged about 12.2 cm/s and 0.10 

 mg 02/g wet weight per h, respectively. 



In all of the experiments, the lowest 0:N ratio 

 occurred immediately following feeding. This was 

 because after the plankton was gone the fish im- 

 mediately reduced their voluntary swimming 

 speed and O2 consumption, whereas their am- 

 monia excretion remained high. Following this 

 the 0:N ratio gradually increased to the high pre- 

 feeding values. 



DISCUSSION 



Elimination of Food From the Gut 



There has been some controversy concerning 

 whether digestion rates and elimination rates are 

 linear or exponential (Fange and Grove 1979). In 

 the linear model, a constant amount (g) is 

 evacuated per unit time and therefore the instan- 

 taneous evacuation rate continually changes. In 

 the exponential model, a constant proportion of 

 the food present in the stomach is evacuated per 

 unit time; thus, while the exponential rate re- 

 mains constant, the actual amount of food (g) 

 evacuated per unit time continually decreases. 



In some studies the linear model has been 

 explicitly used, by fitting a linear regression 

 through the data points representing the food re- 

 maining in the stomach vs. time (e.g., Swenson 

 and Smith 1973; Bagge 1977). In other studies the 

 time to 100% evacuation of the stomach has been 

 determined (e.g.. Hunt 1960; Molnar and Tolg 

 1962; Edwards 1971; Jobling et al. 1977). Here 

 there is usually an implicit assumption that 

 evacuation is a linear process. Several recent care- 

 ful studies, however, have found that' gastric 

 evacuation was clearly a curvilinear process 

 which was closely approximated by an exponen- 

 tial curve (Brett and Higgs 1970; Tyler 1970; El- 

 liott 1972; Elliott and Persson 1978). Beamish 

 (1972) also concluded that evacuation is exponen- 

 tial over a major part of the digestion period, but 

 that it may deviate at the beginning and near the 

 end of digestion. In the initial stages this may 

 occur if there is a lag between the ingestion of a 

 meal, and the beginning of gastric evacuation. The 

 final stages of evacuation are obviously not expo- 

 nential, since completely empty stomachs are fre- 

 quently seen in fishes. 



610 



