764 



Fishery Bulletin 94(4), 1996 



percentage of urea excreted by larger red drum lar- 

 vae could indicate a metabolic shift away from urea 

 production in larvae approaching transformation or 

 in the secondary loss of the ability to produce it (cf. 

 Anderson, 1995). 



O.N ratios 



Nitrogen excretion data for larval red drum suggest 

 that protein was the major catabolic substrate, with 

 little, if any, energy being stored in, or produced from, 

 lipid reserves. Low 0:N ratios in well-fed red drum 

 larvae as well as in those starved for 24 h indicated 

 a protein fuel source in all cases. Ingested rotifers were 

 protein-rich (Brightman, 1993) which almost certainly 

 contributed to the low 0:N ratios of fed individuals. 

 The slightly lower 0:N ratios for starved larvae sug- 

 gest an even stronger dependence on protein for meta- 

 bolic fuel, combusted at the expense of tissue protein. 



Conclusions 



An interesting outgrowth of the present study is not 

 only the amplitude of change in metabolism with 

 starvation, but the change in the scaling coefficient 

 as well. The drop in "b" from 1.04 to 0.697 as a result 

 of starvation suggests that the larger larvae have 

 sustained a proportionately greater drop in metabo- 

 lism than the smaller ones, which, in turn, under- 

 scores a metabolic "catch-22" situation in young lar- 

 vae. Rapid growth of a larva decreases the number 

 of its potential predators, thereby giving the larva 

 the ecological refuge of increased size. In addition, it 

 simultaneously increases the size range of its poten- 

 tial prey. However, the fact that metabolism scales 

 directly with mass means that larger larvae will also 

 be combusting more metabolic fuel in an absolute 

 sense than will smaller individuals. Because energy 

 storage in red drum larvae is almost nonexistent 

 (Brightman, 1993), the absolute metabolic costs of 

 being larger could prove to be rapidly debilitating 

 during times of starvation (cf. Giguere, 1988). In ju- 

 venile and adult fishes, "6" values typically are close 

 to 0.80 (Giguere, 1988) and weight-specific ^xygen 

 consumption declines with increasing size. Thus, in 

 adult fish, larger size gives a metabolic refugium as 

 well as an ecological one. Because larger adults have 

 a proportionately lower metabolism than smaller 

 adults, starvation will be proportionately less debili- 

 tating for the larger fish. Thus, the "catch-22" of iso- 

 metric scaling (6=1.0) in the very early life history of 

 fish is that it is almost certainly part of the meta- 

 bolic underpinnings of rapid growth, but it also 

 makes larger larvae nearly as vulnerable to starva- 



tion as smaller ones. Rapid growth gives an ecologi- 

 cal refuge but it comes at the expense of physiologi- 

 cal vulnerability. 



Acknowledgments 



The authors would like to thank Bill Falls, Anne 

 Burke, and Dan Roberts of the Florida Department 

 of Environmental Protection's Marine Research In- 

 stitute for providing red drum eggs and for their con- 

 siderable help in teaching us culture techniques. This 

 research was supported by FDEP contract C-7701 

 and NSF OCE 92-18505 to J. J. Torres. 



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