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 
growth 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 nauplii per liter. 
Lengths and weights tended toward asymptotes at food levels higher than 1000 
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 liter. Weights of winter flounder at 7 weeks of age in relation to 
copepod nauplii 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 liter 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 nauplii 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 lined 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 
182 
