LAURENCE: BIOENEROET1C MODEL FOR WINTER FLOUNDER LARVAE 



explained by the increased chance of successful 

 feeding at higher plankton concentrations and 

 concurrent decrease in energy expended to obtain 

 prey. Warren and Davis (1967) concurred with 

 this type relationship, stating that the density 

 of food determines an animal's energy cost in 

 obtaining the food. Decreasing metabolism with 

 increasing food concentration is contrary to re- 

 ported laboratory studies using fish older than 

 the larval stages. Paloheimo and Dickie (1966a) 

 and Beamish and Dickie (1967), examining data 

 from other researchers, concluded that higher 

 average metabolic rates result at higher feeding 

 rates. However, it may be presumptuous to as- 

 sume this type relationship for fish larvae. Most 

 older, nonplanktivorous feeding fishes, such as 

 those referred to in the above citations, are satia- 

 tion or periodic feeders. In fact, most of the experi- 

 mental data cited above were for restricted daily 

 diets at different levels. Larval fish, like the win- 

 ter flounder, are active continuous feeders and 

 the assumption in this model was continuous feed- 

 ing at maintained prey densities. Older fish have 

 more body reserves and can exist on maintenance 

 rations to which they can adjust metabolically in 

 contrast to larval fish which must feed continu- 

 ously and are committed to growth or else die. 

 In fact, the concept of maintenance probably is 

 not relevant to larval fish feeding and energetics. 

 So, it seems logical that fish larvae feeding con- 

 tinuously and committed to relatively high 

 growth rates would optimize growth by reduced 

 metabolic expenditure which would result from 

 the increased contact and efficiency of capture at 

 higher prey densities and resultant feeding levels. 

 The research of Wyatt (1972) with plaice larvae 

 tends to further support this concept. He noted 

 that activity, which he attributed to food search- 

 ing, decreased with increasing prey concen- 

 tration. 



The trends of nonassimilated energy over the 

 range of weights and plankton concentrations in 

 this research are similar to those for metabolic 

 energy expenditure and food consumption (Fig- 

 ure 9). This is not surprising due to the inter- 

 relationships of these factors. The decrease in 

 nonassimilated energy with increasing weight 

 (10-30 /jig) for first feeding larvae is apparently 

 due to their initial inefficient digestion which 

 improves with morphological development. Vi- 

 sual examination of food in the anterior portions 

 of the digestive tracts of young larvae during the 

 digestion rate studies indicated relatively intact 



nauplii. This has been observed for other larval 

 fish species. Rosenthal and Hempel (1970) noted 

 that the efficiency of digestion in Atlantic herring 

 fed Artemla nauplii was very low compared with 

 older larvae. Morphological development of the 

 alimentary tract during the larval stage was 

 studied by Nishikawa (1975) who noted an in- 

 crease in stomach size and extension of the diges- 

 tive tract as a whole in relation to increasing 

 standard length. He postulated that these mor- 

 phological developments cause a rapid increase 

 in the function of the organs during the larval 

 period. The subsequent increase in nonassimi- 

 lated energy with size of winter flounder larvae 

 is merely proportional to the increased ration. 



Daily food requirements of winter flounder lar- 

 vae were initially higher for the period associated 

 with first feeding (10-30 /xg, first 2-3 wk after 

 hatching, Figure 10). These short-term higher 

 requirements were due to the inefficient manner 

 in which newly feeding larvae captured prey and 

 the associated, higher energy expenditure. Re- 

 searchers have reported that young fish larvae 

 are much less adept and successful at capturing 

 prey than older larvae. Braum (1967) showed that 

 freshwater whitefish larvae, Coregonus wart- 

 manni, increased their successful captures from 

 3 to 21% during the first 16 days of feeding. Schu- 

 mann (1965) noted an obvious increase in profi- 

 ciency at capturing food with increased age of 

 Pacific sardine larvae. The reasons for increased 

 success with age are increased visual perception 

 of food organisms and increased locomotor abili- 

 ties with advancing development (Blaxter 1965; 

 Rosenthal and Hempel 1970). The subsequent in- 

 crease in required ration with larval size was 

 the result of normal increased energy demand 

 of growth and metabolism associated with larger 

 sized larvae. An interesting fact is the decrease 

 in rate of food requirement noted in metamorphos- 

 ing larvae (500-1,000 fj.g). This may be associated 

 with the previously mentioned decrease in routine 

 metabolic rate peculiar to flatfish larvae and in- 

 creased efficiency of prey capture during the meta- 

 morphosis period. Riley's (1966) results for an- 

 other flatfish, the plaice, Pleuronectes platessa, 

 substantiate this observation. He noted declin- 

 ing ingestion rates and rations during meta- 

 morphosis. 



Conversion of the caloric values of daily food 

 required into numbers of nauplii or older stages 

 consumed (Figure 11) showed, of course, the same 

 trends for food required. This conversion does, 



541 



