LAURENCE: BIOENERGETIC MODEL FOR WINTER FLOUNDER LARVAE 



maintain an energy balance in the larva's body. 

 Energy expended at a given temperature pro- 

 motes growth and results in a metabolism that 

 produces activity, which in turn acts on the plank- 

 tonic prey to provide an assimilated food intake 

 that supplies energy for metabolism and growth. 

 The whole process at a given temperature is in 

 turn influenced by the size or age of larvae and 

 the planktonic prey concentration. A good exam- 

 ple which depicts the effect of larval age or size 

 on these interrelationships and one which points 

 to a definite "critical period" shortly after hatch- 

 ing around the period of feeding initiation is 

 shown in Figure 15. In this figure the caloric ex- 

 penditures for the important bioenergetic param- 

 eters over the range of weights from 10 to 50 /Ltg 

 are summed for all plankton concentrations. A 

 definite divergence of energy away from growth 

 to metabolism and nonassimilation with a result- 

 ant increased food requirement is shown during 

 early life (10-30 tig). This period coincides with 

 first feeding and is the time when larvae need 

 to grow at a fast rate because of their small size, 

 fragility, and vulnerability to predators. This 

 identified "critical period" is caused by a number 

 of factors and interrelationships including: 1) de- 

 velopmental factors of which reduced visual per- 

 ception and locomotor (swimming) abilities in 



0.060- 



FOOO CONSUMPTION 



METABOLISM 



4 ON ASS IM I I A T ION 



20.0 30.0 



DRV WEIGHT <UG> 



FIGURE 15. — Caloric energy expenditure for the major bio- 

 energetic parameters of winter flounder larvae summed for all 

 prey concentrations over the range of dry weights from 10 to 

 50 /xg at 8°C. 



young larvae prevent efficient prey capture com- 

 pared with older and better developed larvae; 



2) less efficient conversion of food to flesh because 

 of higher metabolic expenditure associated with 

 more searching due to less efficient prey capture; 



3) less efficient digestion in young larvae causing 

 a smaller fraction of the food to be assimilated 

 and be available for potential growth. As the lar- 

 vae grow larger and older, especially during the 

 metamorphosis period (50-1,000 /xg), they 

 become more efficient at converting food to 

 growth. The slopes of the lines connecting the 

 simulated values of the important bioenergetic 

 components summed for all prey concentrations 

 over the weight range of hatching to metamor- 

 phosis in Figure 16 show that the rate of growth 

 accelerates more rapidly towards food consump- 

 tion rate than metabolic and nonassimilation 

 rates with increasing larval size after the critical 

 period. 



In addition to the critical period, plankton den- 

 sity is an important determinant of larval survi- 

 val and, of course, interacts crucially during the 

 critical period. The overall influence of prey den- 

 sity is shown in Figure 17 where the caloric expen- 

 ditures of the important bioenergetic parameters 

 simulated by the model are summed over all 

 weights at each plankton concentration. It can 

 easily be seen that low prey densities strongly 

 affect the dispensation of energy available from 

 food consumption in comparison with high densi- 

 ties. A greater portion of the energy intake is 

 utilized for metabolism and is not assimilated 



0.0 100.0 200.0 300.0 »00. 500.0 600.0 700.0 800.0 300.0 1000.0 1100.0 

 DRY UCIGMT <yG> 



FIGURE 16. — Caloric energy expenditure for the major bio- 

 energetic parameters of winter flounder larvae smiimed for all 

 prey concentrations over the range of dry weights from hatching 

 to metamorphosis at 8°C. 



543 



