FISHERY BULLETIN: VOL. 75, NO. 3 



however, give a different perspective in that it 

 shows the actual numbers of organisms that win- 

 ter flounder larvae require on a daily basis. The 

 differences in numbers between nauplii and older 

 stages reflect the differences in sizes providing 

 equivalent caloric intake. Also, winter flounder 

 larvae did not feed entirely on nauplii, but 

 changed in part to larger stage copepods as they 

 grew older. Size selection of prey by larval fishes 

 has been shown to be a factor of mouth size which 

 increases with increased larval size (Shelbourne 

 1965; Blaxter 1969; Detwyler and Houde 1970; 

 Shirota 1970). The numbers of nauplii consumed 

 per day ranged from 19 to 235 over the range 

 of sizes and plankton densities. These values are 

 similar to requirements for other larval species 

 (Chiba 1961; Braum 1967; Rosenthal and Hempel 

 1970), although temperature, larval species and 

 size, and food organisms can account for variable 

 results. 



Decrease in percent food eaten per day with 

 body weight (Figure 13) is in accordance with re- 

 sults of other researchers and was due to the rel- 

 ative decrease in the rate of food intake compared 

 with the growth rate with larval development. 

 Pandian (1967) observed decreases in percent 

 eaten per day with increases in body size of Mega- 

 lops cyprinoides and Ophiocephalus striatus, as 

 did Laurence (1971b) for larval largemouth bass 

 and Stepien (1974) for larval sea bream. 



The percentages of body weight consumed per 

 day predicted in this research were high from over 

 300% at the smallest larval sizes and lowest prey 

 concentration to 27-31% at the higher prey con- 

 centrations and largest larval sizes. Percent body 

 weight eaten per day is typically much greater 

 for larval and juvenile fishes as compared with 

 adults since there is a much higher energy 

 demand for growth purposes (Winburg 1956). 

 Stepien (1974), in the only other known compar- 

 able research on marine larvae, also reported high 

 percentages. His results for sea bream at 29°C 

 were from 222.4% for 2-day hatched larvae to 79% 

 for 7-day-old larvae. Sorokin and Panov (1965) 

 reported 40-60% body weight eaten per day by 

 larval freshwater bream. 



The gross growth efficiencies recorded in this 

 research increased rapidly with size for the small- 

 est larvae (10-75 /u.g) and then increased at a 

 decelerated rate for the remainder of the larval 

 period to metamorphosis (Figure 14). Increased 

 gross growth efficiency at greater body weights 

 observed in my experiments is contrary to the re- 



sults of research with older fishes. Parker and Lar- 

 kin (1959) stated that within any growth stanza 

 the gross efficiency must decline with increasing 

 size, as a greater portion of the food must be used 

 in maintenance. This may not be true for larval 

 fishes, as their development is so rapid that a 

 large portion of the energy derived from food in- 

 take is used in growth. It is my opinion that larval 

 fishes could not exist on a maintenance ration. 

 Rapid growth is a definite prerequisite for success- 

 ful survival in the environment of larval fishes, 

 and they must either consume food at high levels 

 with resultant rapid growth or die. The ability 

 of larvae to increase their feeding efficiency with 

 increased size could also contribute to greater 

 growth efficiency. 



Divergent opinions have been expressed by re- 

 searchers concerning the relationship between 

 growth efficiency and feeding level or prey concen- 

 tration. Paloheimo and Dickie (1966b) stated that 

 growth efficiency declined with increasing ration. 

 Warren and Davis (1967) showed that growth 

 efficiency increased to two-thirds the maximum 

 feeding level and then decreased. Finally, Davies 

 (1964) demonstrated that efficiency of digestion 

 and absorption of food by goldfish, Carassius 

 auratus, was improved by increasing food input 

 over a given weight range. He postulated that 

 secretion of digestive fluids was stimulated by the 

 effects of increased food. In all cases the studies 

 and analyses were done with adult fishes. Winter 

 flounder larvae increased their gross growth effi- 

 ciencies with increased plankton density similar 

 to Davies' results. However, the causative mech- 

 anism was most likely the increased efficiency of 

 prey capture with increased prey encounter at 

 higher densities with resultant metabolic savings 

 for growth rather than increased secretion of 

 digestive fluids. Growth efficiency is most likely 

 a dynamic factor not subject to generalizations 

 and dependent on life stage, type of feeding strat- 

 egy, or prey type. 



The range of values of growth efficiency for 

 larval winter flounder on this research were from 

 5 to 33%, depending on larval size and plankton 

 concentration. These values are similar to those 

 for other young fishes (Ivlev 1939a; Sorokin and 

 Panov 1965; Edwards et al. 1969; Laurence 1971a; 

 Frame 1973; Stepien 1974). 



The above discussions have revealed that there 

 are interrelationships between the bioenergetic 

 parameters simulated by the model and that the 

 whole system works in a circular pathway to 



542 



