756 



Abstract.— Oxygen consumption 

 and nitrogen excretion rates were de- 

 termined by direct measurement in fed 

 and starved red drum, Sciaenops 

 ocellatus, larvae between the ages of 

 two and eighteen days. Oxygen con- 

 sumption rates (7, nL0 2 /ind./h) scaled 

 isometrically with mass (X, mg) accord- 

 ing to the equation }' = 4.58X 1 04 

 (^=0.80) in larvae fed 5 prey/mL, re- 

 sulting in a dry mass-specific oxygen 

 consumption (Q0 2 ) of 4.18 uL 2 /mg 

 DM/h at a mass of 100 ug. In contrast, 

 oxygen consumption of starved larvae 

 scaled with mass according to the rela- 

 tionship y = 1.00.X 697 (r 2 =0.50), result- 

 ing in a Q0 2 of 2.01 uL 2 /mg DM/h at 

 100 ug. The 50% drop in Q0 2 between 

 fed and starved larvae over the mass 

 range addressed in the study was as- 

 sumed to represent the metabolic cost 

 of growth (specific dynamic action). 



Nitrogen was excreted in the form of 

 ammonia and urea. Ammonia excretion 

 (Y, ngNH 3 /ind./h) varied with mass (X, 

 mg) in fed individuals according to the 

 relationship Y = 0.277X 0728 (r^O.58), 

 resulting in a dry mass-specific ammo- 

 nia excretion rate of 0.52 ug NH.,/mg 

 DM/h at a mass of 100 ug. Like oxygen 

 consumption, nitrogen excretion in 

 starved individuals dropped to 50% of 

 that in fed animals, suggesting a gen- 

 eral metabolic slowdown during star- 

 vation. Urea production as a percent- 

 age of total N-excretion was inversely 

 related to ammonia production in 

 starved individuals and increased with 

 time of starvation; it may be an indica- 

 tor of starvation in very young fish. 



The isometric scaling of metabolism 

 with mass in young larvae suggests 

 that physiological vulnerability, i.e. 

 susceptibility to starvation, does not 

 decline rapidly with increasing size in 

 young fish larvae. Larger larvae are 

 nearly as vulnerable to starvation as 

 smaller ones though their ecological 

 position has improved, i.e. their preda- 

 tor spectrum has declined and their 

 prey spectrum has increased. 



Energetics of larval red drum, 

 Sciaenops ocellatus. 

 Part I: Oxygen consumption, specific 

 dynamic action, and nitrogen excretion 



Joseph J. Torres 

 Ross I. Brightman 

 Joe Donnelly 

 John Harvey 



Department of Marine Science, University of South Florida 

 140 Seventh Ave South, St Petersburg, Florida 33701 

 e-mail address jtorres@manne usf edu 



Manuscript accepted 10 June 1996. 

 Fishery Bulletin 94:756-765 (1996). 



Energy utilization during a fish's 

 early life history reflects its strat- 

 egy for survival. Ingested energy 

 must be apportioned between meta- 

 bolic requirements and the need for 

 growth in size; the remainder is lost 

 as fecal and nitrogenous waste. The 

 need for rapid growth in very young 

 larvae is particularly critical: in- 

 creasing size rapidly decreases the 

 spectrum of potential predators 

 even as it increases the spectrum of 

 items available for forage (Weather- 

 ly and Gill, 1987). The two factors 

 that most profoundly influence the 

 amount of ingested energy available 

 for growth are the energy lost to 

 respiration and excretion (Brett and 

 Groves, 1979; Houde, 1989). 



Respiration in eggs and larvae is 

 believed to occur through cutaneous 

 diffusion (de Silva, 1974). At hatch- 

 ing, most species of fish lack respi- 

 ratory pigments and are almost 

 transparent. The blood of these lar- 

 vae becomes pink weeks or months 

 later upon transformation, which 

 marks the time of blood pigment 

 development and advanced gill fila- 

 ment formation (Weihs, 1981; Blax- 

 ter, 1986). Energy demands of res- 

 piration in fish larvae are tempera- 

 ture-dependent, ranging from 48% 

 of the gross ingested food energy at 

 10°C to 31% at 30"C (Houde, 1989). 



A portion of the food energy in- 

 gested by an individual is indigest- 

 ible and lost as feces (up to 20% in 

 older fish; Brett and Groves, 1979). 

 In larval fishes, feces are small, dif- 

 ficult to collect, and, as a conse- 

 quence, are rarely measured. The 

 fraction of ingested energy lost to 

 fecal excretion is therefore usually 

 computed by the difference between 

 the sum of the energy devoted to 

 growth and metabolism and that 

 ingested. Of the remaining food en- 

 ergy that is digestible, a portion is 

 lost as nonfecal nitrogen, mainly as 

 ammonia and urea. Brett and 

 Groves (1979) have stated that en- 

 ergy lost through nonfecal excretion 

 ranges from 37c to 10% of total in- 

 gested calories in adults. However, 

 data describing nitrogen excretion 

 in young fish larvae are very scanty. 

 Available figures suggest that total 

 excretion ranges from 23% to 40% 

 of the gross ingested energy (Houde, 

 1989). 



Data collected simultaneously on 

 oxygen consumption and ammonia 

 excretion can be used to determine 

 the biological substrate that is com- 

 busted as fuel by using the atomic 

 ratio of oxygen consumed to nitro- 

 gen produced (0:N ratio) (Daven- 

 port et al., 1983). The 0:N ratios of 

 8 or less indicate that pure protein 



