Even in rich coastal waters, daily variability in microzooplankton 

 concentrations occurs over order of magnitude ranges. Laboratory studies have 

 shown that larvae deprived of food pass a "point of no return," after which 

 they cannot initiate feeding (21 , 67). This point can occur at only 0.5-2.5 days 

 after yolk absorption for species at 20-32*^ (43, 57). Thus, unstable conditions 

 that lead to temporary low prey concentrations probably are an important 

 cause of mortality, even in areas where mean prey levels are high enough to 

 sustain larvae. 



Growth of larvae in relation to prey concentration can be determined in the 

 laboratory. There are, of course, factors other than density of prey which 

 influence larval growth. The size of prey, their caloric value, their percentage 

 protein, and their digestibility are important. The effect of temperature makes 

 it difficult to compare growth among species of larvae, even when similar foods 

 have been used. Despite limitations in the comparative approach, larval growth 

 responses to changes in food concentration can be demonstrated in the 

 laboratory, and results extended to explain how densities of prey influence 

 grov^rth of wild populations. 



A relationship between size at 16 days after hatching and copepod nauplii 

 concentration was demonstrated for larvae of bay anchovy, lined sole, and sea 

 bream (44, 45). Lengths and mean dry weights of survivors increased rapidly 

 when prey level was raised from approximately 50 to 500 nauphi per Hter. 

 Lengths and weights tended toward asymptotes at food levels higher than 1 000 

 per liter, although significant, additional growth could be obtained at higher 

 prey levels. Laurence's data (58) on haddock larvae at six weeks of age show a 

 similar relationship for prey concentrations in the range 500-3000 copepod 

 nauplii per hter. Weights of winter flounder at 7 weeks of age in relation to 

 copepod nauphi concentration also approached an asymptote at 1000 per liter 

 prey level (60). O'Connell and Raymond (73) also found this type of 

 relationship between length of northern anchovy larvae at 12 days and 

 copepod nauplii concentration, except that prey ranged from 1000-14,000 

 nauplii per Uter and the asymptotic size was not attained until prey level was 

 approximately 8000 nauplii per liter. 



Specific growth rates of marine fish larvae relative to prey concentration 

 have been obtained in only a few instances. Specific growth rates (in dry 

 weight) of haddock larvae were 7 percent, 8 percent, and 9 percent per day at 

 copepod nauplii concentrations of 500, 1000 and 3000 per liter (58). The rates 

 for winter flounder larvae, at the same nauphi concentrations were similar, 6 

 percent, 8 percent and 9 percent (60). Temperature for haddock experiments 

 was 7°C and for winter flounder it was 8°C. Specific growth rates of sea bream 

 and bay anchovy larvae at 26^, and Uned sole larvae at 28^C can be estimated 

 from Houde's data (45). The rates were 16, 20, and 28 percent per day for sea 



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