reported by Houde (45). O’Connell and Raymond (73) estimated that more 
than 1000 nauplii per liter were required by northern anchovy larvae. Haddock 
Melanogrammus aeglefinus , larvae required 500-3000 per liter (58) and winter 
flounder, Pseudopleuronectes americanus , required 300-3000 per liter (60). It 
is possible that some reported prey concentrations required by larvae could be 
too high. Saksena and Houde (84) needed 1500-2000 nauplii per liter to 
successfully rear about 10 percent of bay anchovy larvae, but more recent 
experiments, with refined culture methods (45), have demonstrated that only 
100 nauplii per liter are necessary to attain that survival rate. In some research, 
such as toxicological studies to determine effects of pollutants on larval 
survival, potential survival rates higher than 10 percent are required. For those 
studies, copepod nauplii concentrations of 1000 per liter or higher should be 
routinely employed (44, 45) (Table 13-1). 
For six cases where copepod nauplii were fed to similar-sized larvae, the 
relationship between percent survival and nauplii concentration can be 
compared (Table 13-1). Haddock larvae had the highest required prey 
concentration, more than 2000 nauplii per liter being required for 10 percent 
survival. Winter flounder and northern anchovy larvae had an expected survival 
of 10 percent when nauplii were available at approximately 1600 and 1000 per 
liter, respectively. But, bay anchovy and lined sole required only about 100 
nauplii per liter and sea bream needed less than 50 per liter to attain 10 percent 
survival. All of these species consume prey of similar types and sizes; the 
differences in requirements among species have not been explained. 
Temperature may play a role because the three species with lowest required 
prey concentrations were reared at 26-28°C, while the three with higher 
requirements were reared at 7-17°C. If searching ability and capture efficiency 
are enhanced at higher temperatures, required prey concentrations may 
decrease accordingly. 
Rotifers, Brachionus plicatilis, are often used at high densities by 
aquaculturists to successfully rear fish larvae, but the minimum concentration 
required by larvae usually has not been determined. Hunter (46) estimated that 
105 rotifers per liter were required by newly-feeding northern anchovy larvae 
to meet metabolic demands, a number that must be exceeded for larvae to 
grow. Lined sole larvae required from 60-120 rotifers per liter for 10 percent 
survival to metamorphosis (Houde, unpublished data). 
Concentrations of microzooplankton in marine waters are not frequently 
reported, but when suitable collection techniques have been used observed, 
concentrations often are in the ranges of required prey densities determined in 
the laboratory (Table 13-2). Concentrations of suitable prey are exceptionally 
low in oceanic waters compared to coastal waters, and larval survival may 
depend upon the occurrence of relatively dense prey patches in oceanic areas. 
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