Larvae were exposed to the patch concentration 

 twice during each 13-h feeding period to obtain the 

 total desired time at the patch level. For example, 

 for a 6-h patch exposure, the prey concentration 

 was adjusted to 500/1 from 0800 to 1100; it was 

 then quickly reduced to 25-50/1, where it was 

 maintained until 1800, when the prey concentra- 

 tion was readjusted to 500/1 for the remaining 3 h 

 of the light cycle. 



Results 



Table 2. — Survival and standard lengths of Ardiosargus rham- 

 boidalis larvae at 10 days after hatching, based on 140 eggs and 

 variable patch exposure time. A patch is a prey concentration of 

 500/1. Nonpatch levels were 25-50/1. Patch conditions were pre- 

 sented to larvae on days 2-6 after hatching. 



Anchoa mi t chilli 



'Food concentration was tneld constant at 25-50,'l during days 2-6. 

 ^Food concentration was held constant at 500/1 dunng days 2-6, then re- 

 duced to 25-50/1 during days 7-10 



Percent survival ranged from 0.36% at 0-h 

 patch exposure to 22.86% at 13 h (Table 1). The 

 steady increase in survival as patch exposure time 

 was increased was described by an exponential 

 function, Y = 0.3038 e034i9x^ where Y = percent 

 survival and X = hours at 500/1 prey concentration 

 (coefficient of determination, r^ = 0.98). For the 

 500/1 patch concentration, there was no minimum 

 time of exposure above which larval anchovy sur- 

 vival increased sharply or equalled the survival 

 obtained when larvae were exposed throughout 

 the day to the 500/1 prey concentration. 



Surviving bay anchovies at 10 days after hatch- 

 ing differed significantly in mean standard 

 lengths (Table 1) among patch exposure times 

 (analysis of variance, P<0. 001). Mean lengths at 

 3-, 6-, and 9-h patch exposure times were sig- 

 nificantly greater than those at 11 and 13 h 

 (Student-Newman-Keuls test, P <0.05). 



Table l. — Survival and standard lengths of Anchoa mitchiUi lar- 

 vae at 10 days after hatching based on 280 eggs and variable 

 patch exposure times. A patch is a prey concentration of 500/1. 

 Nonpatch levels were 25-50/1. Patch conditions were presented to 

 larvae on days 2-6 after hatching. 



'Food concentration was held constant at 25-50/1 during days 2-6. 

 ^Food concentration was held constant at 500/1 during days 2-6. then re- 

 duced to 25-50/1 durino davs 7-10. 



Archosargus rhoviboidalis 



Survival ranged from 3.57 to 66.43% for sea 

 bream larvae over the range of patch exposure 

 times (Table 2). The relationship between percent 



survival and patch exposure time was described by 

 a power function, 7 = 25. 07395s: 2878^ where Y = 

 percent survival and X = hours at 500/1 prey con- 

 centration. Although the power function described 

 the relationship reasonably well (coefficient of de- 

 termination, r^ = 0.94), an asymptotic regression 

 might be better to describe the relationship be- 

 cause sea bream larvae exposed to a 500/1 patch 

 density for between 3 and 6 h daily apparently 

 survived as well as when the 500/1 prey concentra- 

 tion was offered throughout the day. The power 

 function is retained here because fits to the data by 

 asymptotic regressions gave lower coefficients of 

 determination, due to the relatively high variabil- 

 ity in observed survival as patch exposure times 

 increased. 



Mean lengths of survivors at 10 days (Table 2) 

 differed significantly among patch exposure times 

 (analysis of variance, P <0.001 ), but there was no 

 clear relationship between the mean lengths that 

 differed significantly (Student-Newman-Keuls 

 test, P<0.05) and the time of exposure. 



Discussion 



There was a marked difference in response of 

 bay anchovy and sea bream larvae to the simu- 

 lated patch conditions. Sea bream survival im- 

 proved greatly when larvae were presented with 

 prey at 500/1 for more than 3 h/day, the observed 

 survival then equaling that when they were of- 

 fered a constant 500/1 prey concentration. Bay an- 

 chovies were less successful in using the patch 

 conditions to improve their survival, although in- 

 creased survival rates did occur when larvae were 

 exposed for more than 6 h to the patch concentra- 

 tion. Results imply that first feeding bay anchovy 

 may require a high and stable prey density to 

 attain best survival in the sea, but that sea bream 



485 



