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Fishery Bulletin 94(4), 1996 



with a Cahn Electrobalance for dry mass measure- 

 ments. It should be noted that the number of repli- 

 cate experiments for larvae starved for 24 h was lower 

 than that for fed individuals because many of the 

 larvae that survived the period of starvation were 

 not hardy enough to be good experimental subjects. 

 Oxygen consumption as pL 9 was converted to 

 calories (cal) by using the oxycalorific equivalent of 

 0.00463 cal/pL 2 (Brett and Groves, 1979). Calories 

 were then converted to joules (J) by multiplying by 

 4.1868 J/cal (Pennycuick, 1988). 



Nitrogen excretion 



Nitrogen excretion rates were determined for larvae 

 of the same ages and feeding states as those used in 

 the oxygen consumption experiments. Larvae aged 

 3 to 14 d were split into 2 feeding groups: those fed 

 to satiation and those starved for 24 h. Five sets of 

 10 larvae were removed from each feeding group and 

 each set was placed in a 10-mL screw-top centrifuge 

 tube filled with seawater that had been filtered 

 through activated charcoal. To measure any ammo- 

 nia addition caused by transfer of the larvae, an ad- 

 ditional tube was filled with charcoal-filtered seawa- 

 ter, and a volume of water from the incubation bowl 

 equivalent to that associated with transfer of the lar- 

 vae was added. A second additional tube was filled with 

 10 mL of charcoal-filtered seawater to serve as a blank. 

 All tubes were sealed and incubated for 4 h at 25°C. 

 Larvae were subsequently filtered from the tubes 

 with 50-pm mesh netting, dried at 60°C for 24 h, and 

 weighed. The remaining sample water was split into 

 10-mL aliquots and immediately capped and frozen 

 for later analysis. Concentrations of ammonia and 

 urea for each sample were determined colorimetri- 

 cally with an Alpkem autoanalyzer (Price and 

 Harrison, 1987). 



O.N ratios 



0:N ratios (mol/mol) were calculated by using mass- 

 specific oxygen consumption rates and our data for 

 mass-specific total nitrogen excretion (ammonia and 

 urea). Rates of mass-specific ammonia excretion were 

 used to obtain 0:NH., ratios. 



Results 



Oxygen consumption rates 



Absolute oxygen consumption rates for individuals 

 (ind.) fed 5.0 rotifers/mL increased with increasing 

 dry mass and, as a consequence, with the age of the 

 larvae as well (Table 1: Fig. 1). Rates ranged between 

 0.05 (day 3) and 0.14 pL Cyind./h (day 7) for the first 

 week of growth. During the second week of growth, 

 values increased sharply from 0. 14 to 0.34 pL O./indTh 

 at day 14, reaching 0.77 pL Og/ind./h at 18 d after hatch- 

 ing. The slope (6) of the oxygen consumption versus 

 mass curve was 1.04 ±0.08 ( x ±SE), indicating that 

 metabolism was directly proportional to mass (isomet- 

 ric scaling) for the first two weeks of life. Mass-specific 

 oxygen consumption (QO.,) varied between 2 and 6 pL 

 Cymg DM/h with a mean of 4.23 ±0.17 pL 0,/mg DM/h 

 ( x ±SE). The slope (b) of the regression line for oxygen 

 consumption versus age was 1.28 ±0.12. 



Individuals starved for 24 h also had absolute oxy- 

 gen consumption rates that increased with mass and 

 age, but with lower b values (Fig. 2). Slope (6) for the 

 oxygen consumption vs. mass curve dropped to 0.697 

 ±0.230 ( x ±SE ); that for oxygen consumption vs. age 

 dropped to 0.975 ±0.400 ( x ±SE). Absolute and mass- 

 specific oxygen consumption and, as a consequence, 

 energy utilization of starved larvae was one-half or 

 less that of fed larvae at all ages up to day 17 (Table 2). 



