FISHERY BULLETIN: VOL. 87, NO. 1 



substantially increase larval mortality due to fish 

 predation. 



Survival of white perch larvae is strongly depen- 

 dent upon food availability and temperature. At 

 13°C, larvae were vulnerable to low food conditions, 

 but survival differences after 8 days among mal- 

 nourished and well-fed larvae were much more pro- 

 nounced at higher temperatures. The metabolic 

 demands of larvae are reduced at low temperatures, 

 allowing relatively low caloric intake to sustain 

 larvae. The food levels of 800 and 50 rotifers/L used 

 in the study correspond to caloric values of 0.64 and 

 0.04 cal/L, respectively (Theilacker and McMaster 

 1971). It was apparent from survival data, par- 

 ticularly at 17° and 21 °C, that 0.04 cal/L was in- 

 adequate for white perch survival, and that critical 

 levels for survival fall in the range of 0.04-0.64 

 cal/L. This estimate falls within the broad range of 

 0.01 to 10.0 cal/L that Houde (1978) summarized as 

 reported critical caloric concentrations for marine 

 fish larvae. 



White perch resemble many small marine and 

 estuarine larvae (e.g., northern anchovy, Engraulis 

 mordax; jack mackerel, Trachurus sym/metricus; 

 spot, Leiostomus xanthurus) in having relatively low 

 survival potential at low food levels (Theilacker and 

 Dorsey 1980; Powell and Chester 1985). For exam- 

 ple, at 17° and 21°C, 8 d survival values for white 

 perch decreased by 60-80% with a 4 d delay in high 

 food levels. At those same temperatures, an 8 d 

 delay resulted in 80-90% decreases in survival. 

 Larger larvae such as sand lance, Ammodytes 

 americanus, (Buckley et al. 1984); Atlantic herring, 

 Clupea harengus harengus, (Rosenthal and Hempel 

 1970; Kiorboe and Munk 1986); and striped bass, 

 Morone saxatilis, (Houde and Lubbers 1986) are less 

 vulnerable to starvation under low-food conditions. 

 When food is scarce, smaller larvae such as white 

 perch are often more vulnerable to starvation be- 

 cause of low frequency of prey contact (Laurence 

 1982). However, comparisons among species should 

 be done with caution because survival potential is 

 species-specific. For example, sea bream, Archo- 

 sargus rhomboidalis, (Houde 1978); plaice, Pleuro- 

 netes platessa, (Blaxter and Staines 1971); and cod, 

 Gadus morhua, (Ellertsen et al. 1981), all relative- 

 ly small at first-feeding, are efficient feeders and 

 exhibit significant survival at low prey levels 

 (<50/L). 



For most species, larval growth variability and 

 stage durations are important aspects of prerecruit 

 survival (Gushing 1976; Houde 1987). Temperature 

 variability resulted in more than fourfold differences 

 in mean weights of white perch larvae after 8 days 



of feeding. Thus, the effect of temperature on feed- 

 ing stage duration would be even more pronounced 

 than its effects on yolk-sac stage duration. Under 

 good feeding conditions, a drop in temperature of 

 2° (from 17° to 15°, for example) would result in 

 a 30% reduction in growth after 8 days (see Figure 

 6). The magnitude of the prolongation of stage dura- 

 tion would be similar. The effects of reduced food 

 on stage duration would be even more pronounced. 

 At 17° or 21°C, food levels need only be reduced 

 for 2 days upon initiation of feeding to produce the 

 same 30% reduction in growth after 8 days (Fig. 6). 



The growth potential of white perch is interme- 

 diate between that reported for temperate and sub- 

 tropical marine and estuarine species (Houde and 

 Schekter 1981). White perch growth at 17°C and 

 higher exceeded that reported for most temperate 

 latitude species, which usually grow at rates of 

 10%/d or less (Houde and Schekter 1981). However, 

 white perch growth rates were less than that of most 

 subtropical species, such as bay anchovy, Anchoa 

 mitchilli, (Florida populations); lined sole, Archirus 

 lineatus; sea bream (Houde and Schekter 1981); and 

 tidewater silverside, Menidia peninsulae, (McMullen 

 and Middaugh 1985), which may grow at 3>20%/d. 

 The specific growth rates of white perch larvae also 

 appear to be slightly lower than those of the larger 

 larvae of congeneric striped bass (Chesney 1986; 

 Houde and Lubbers 1986). 



Springtime densities of microzooplankton in 

 Chesapeake tidal freshwaters usually begin to in- 

 crease when temperatures reach 14° and peak at 

 20°-22°C (Lippson et al. 1980; Martin and Setzler- 

 Hamilton 1981). Temperature and food concentra- 

 tion have important interacting effects on white 

 perch during the first 2-3 weeks of life, with an ap- 

 parent balance struck between hatching success, 

 growth rate, and survival potential. Based on my 

 results and historical patterns of zooplankton abun- 

 dance, the optimum temperatures for white perch 

 development and grov^^th are in the range 15°-20°C. 

 Hatching success was optimal at <17°C. Larvae 

 hatched at 13°C were not as vulnerable to starva- 

 tion, but they grew at <5%/d regardless of food 

 level. At temperatures above 17°C, larvae could 

 grow at >20%/d if high food levels were available 

 at first-feeding. However, at 21°C (and presumably 

 at higher temperatures), hatching success declined 

 and there was greater likeliliood of starvation under 

 suboptimum food conditions. 



Ultimate survival of white perch larvae and poten- 

 tial for recruitment will depend on environmental 

 conditions in the estuary and how they effect subtle 

 changes in growth and mortality rates of prerecruit 



70 



