1974, 1977). Houde (in press) recently demon- 

 strated that survival of three species of marine fish 

 larvae from hatching to metamorphosis was 109c 

 or higher when mean prey concentrations were 

 only 34-130/1. But, he also found enhanced surviv- 

 al when food concentrations were increased. For 

 significant numbers of larvae to survive the tran- 

 sition stage from yolk nutrition to active feeding, 

 some researchers believe that dense patches of 

 prey must occur in the sea (O'Connell and 

 Raymond 1970; Hunter 1972). Such patches might 

 occur at densities of 10 to 1,000 times above the 

 mean prey density. Lasker (1975) has discussed 

 the dense patches of the dinoflagellate Gym- 

 nodinium splendens, which serves as prey for lar- 

 val northern anchovy, Engraulis mordax, in the 

 California Current and their possible relationship 

 to larval survival. Hunter and Thomas (1974) de- 

 monstrated that larval northern anchovies were 

 able to remain in patches of G. splendens that were 

 artificially created in laboratory experiments. 



In two series of laboratory experiments we have 

 examined the effect of two simulated patches of 

 prey on survival in the bay anchovy, Anchoa 

 mitchilli, and the sea bream, Archosargus rhom- 

 boidalis. Patches were simulated during the first 6 

 days after hatching, when these larvae are most 

 susceptible to starvation mortality. The purpose of 

 the experiments was to determine if prey at high 

 density that were offered for more than some 

 minimum period would result in survival rates of 

 larvae that approached those obtained at a high, 

 constant prey concentration. This would indicate 

 that the larvae were able to obtain a daily ration 

 suitable for maintenance and growth by increas- 

 ing their feeding rate during the period of expo- 

 sure to the patch concentration of prey. At the low 

 prey concentrations usually found in the sea, a 

 relatively great expenditure of energy would be 

 required by larvae to obtain the minimum daily 

 ration for maintenance and growth. Such larvae 

 might weaken or fail to grow and thus be more 

 susceptible to starvation or predation. 



Methods 



Larvae were hatched from fertilized eggs that 

 were collected in plankton nets from Biscayne 

 Bay, Fla. In each experimental trial 140 sea bream 

 eggs were stocked (2.0/1) and 280 bay anchovy eggs 

 were stocked (4.0/1) in a 76-1 glass aquarium. Lar- 

 vae were reared for 10 days at 26±1°C. Salinities 

 ranged from 30.0 to 32.5%o for bay anchovy and 



484 



33.0 to 33.5%o for sea bream. Lighting was pro- 

 vided at 2500-2800 Ix by 40-W, cool-white fluores- 

 cent tubes. A 13 h light-11 h dark schedule was 

 maintained. Tanks were isolated in a black plastic 

 enclosure and all light was extinguished during 

 the dark periods. Sea bream and bay anchovy lar- 

 vae do not feed in the dark. At the end of experi- 

 ments, survivors were preserved in 5% Formalin^ 

 and measured using an ocular micrometer. 



Prey were the nauplii and copepodid stages of 

 copepods, approximately 50-100 /xm in diameter, 

 that were collected in 53-/Ltm mesh plankton nets. 

 Prey concentrations were determined by counting 

 organisms in 100- to 200-cm'^ aliquots from the 

 rearing tank (Houde 1975, 1977) several 

 times per day during the 13-h feeding period. 

 Background (i.e., nonpatch) prey levels were set at 

 25-50/1; this concentration was maintained when 

 patch concentrations were not offered from 2-6 

 days after hatching and continuously from 7-10 

 days after hatching. The patch concentration was 

 500 prey/1. Patches were provided for periods rang- 

 ing from 1.5 to 11 h (Tables 1,2). Both Oh, at which 

 no patches were provided, and 13h, at which a 

 constant 500/1 prey concentration was main- 

 tained, also were included in the series of experi- 

 ments for each species. The patch schedules were 

 maintained for only the first 5 days of active feed- 

 ing because larvae that survived that period had 

 greatly increased their searching ability and were 

 less dependent on high prey concentrations for 

 successful feeding. 



Patches were created by adding prey to obtain 

 the 500/1 concentration. After larvae had fed at the 

 patch concentration for the desired period, prey 

 were reduced to 25-50/1 by siphoning them out of 

 the system through a 280-/>tm mesh screen and 

 replacing the siphoned water with 26°C filtered 

 seawater from a 150-1 header tank. Sea bream 

 larvae had no difficulty avoiding the siphon and its 

 screen during water exchanges, but precautions 

 were necessary for bay anchovy larvae. A 280-)U,m 

 mesh partition was used to "herd" anchovy larvae 

 toward one end of the tank prior to each siphoning 

 procedure. Siphoning procedures and water ex- 

 changes also were carried out in the 0-h and 13-h 

 patch period experiments to insure that those lar- 

 vae were exposed to the same procedural distur- 

 bances as larvae in experiments where prey con- 

 centration was being varied. 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



