300 percent for newly -fee ding larvae to 27-31 percent for the smallest larval 

 stages (10-75 jug), and continued to increase slowly for older larvae, ranging 

 from about 18-33 percent for metamorphosed individuals. Laurence (60) 

 predicted a continuous decrease in growth efficiency as prey concentrations 

 were decreased, but so few values of growth efficiency are available for fish 

 larvae that it is not possible to say whether this relationship will hold for other 

 species. 



Some valuable insight into feeding by marine fish larvae recently has been 

 gained by combining results of bioenergetic studies on larvae with studies on 

 feeding behavior and feeding ability of larvae. Blaxter and Staines (23) 

 estimated swimming ability and feeding efficiency of herring, plaice, pilchard 

 Sardina pilchardus, and sole Solea solea larvae. From their estimates they 

 calculated the volume of water that could be effectively searched by larvae 

 when they initiated feeding, and at sizes up to metamorphosis. Because 

 swimming distances and volumes searched per unit time increased rapidly as 

 larvae grew, larvae presumably needed higher prey concentrations during the 

 youngest feeding stages. A similar approach was used by Rosenthal and Hempel 

 (82), who in addition estimated the digestion time for herring larvae. They 

 were then able to calculate the daily ration and required densities of prey 

 {Artemia nauplii) for herring larvae at the end of the yolk-sac stage (10-1 1 mm) 

 and at 13-14 mm length. Estimated ration was 40 Artemia nauphi per day at 

 10-11 mm and 50 per day at 13-14 mm. Required ^rremw concentrations for 

 larvae to obtain the rations at each of those length-classes were 4 to 42, and 2 

 to 25 per liter, respectively. Hunter (46) further extended the method by 

 incorporating metaboHc demands of larvae, and caloric values of prey (the 

 rotifer Brachionus plicatilis and the dinoflagellate Gymnodinium splendens) 

 into the prediction of food requirements. He concluded that first feeding 

 northern anchovy larvae required 105 rotifers per liter or their caloric 

 equivalents (e.g. 1785 Gymnodinium per liter) to just meet metabolic 

 demands. Larvae at 10 days (5.9 mm) required only 34 rotifers per liter. In all 

 of the examples, the relatively poor swimming abiHty and the low prey capture 

 efficiency of first feeding larvae were demonstrated. This implies, as did 

 Laurence's study (60), that food concentration is most critical at the first 

 feeding stage and, when low, could be a significant cause of larval mortality in 

 the sea. 



It is possible to make many conclusions about larval food requirements 

 based on dry weights of larvae, dry weights of prey, prey selection by larvae, 

 digestion time, and estimates of the caloric values of the prey (cal/g ash free). 

 Using these methods, Stepien (92) showed how feeding rates, specific rations 

 and growth efficiency of sea bream larvae varied in relation to larval age, and to 

 temperature for a single prey concentration. At 1000 copepod nauplii per liter, 

 feeding rates for first feeding larvae (2-3 days after hatching) varied from 7.2 



184 



