FISHERY BULLETIN: VOL. 75. NO 3 



varied during the course of the experiment. Ini- 

 tially, approximately 907c of the microzoo- 

 plankters in the containers were T. holothuriae, 

 but by the end of the rearing experiment, B.plica- 

 tilis was the dominant organism (97%). We were 

 unable to determine if the anchovy larvae were 

 selectively feeding on the copepods because the 

 combination culture of microzooplankton which 

 was used to stock the larval rearing containers 

 also experienced a similar succession in species 

 dominance during the experimental period. 



Brachionus plicatilis and T. holothuriae were 

 cultured together in the same vessel using tech- 

 niques described by Hunter (1976). The cultures 

 were filtered through 105- tun screening to remove 

 the largest organisms before inoculating the 

 larval rearing containers. Microscopic examina- 

 tion of the filtrate revealed a predominance of 

 small rotifers and copepod nauplii. 



Fifty anchovy eggs were added to each container 

 the day after spawning and the appropriate dino- 

 flagellate was also introduced at this time. Hatch- 

 ing occurred on the next day, which corresponds 

 to day of the experiment. The number of dead 

 embryos on the container bottom was counted at 

 this time and the percentage hatch was calcu- 

 lated. On day 2, most of the yolk sac was absorbed, 

 the eyes were pigmented, and the larvae initiated 

 feeding. At this time, the microzooplankton were 

 added. The experiments were terminated on day 

 10; standard lengths were measured for each 

 animal; average dry weight for larvae in each 

 container was determined; and the percent sur- 

 vival in each container was calculated. 



Each larval rearing container was sampled 

 daily to monitor the concentration of food organ- 

 isms. Because Gymnodinium splendens and 

 Gonyaulax polyedra tend to form patches, 1-ml 

 samples were taken from three different locations 

 in the tank outside of a patch; the numbers were 

 averaged and an appropriate amount of a dense 

 dinoflagellate culture was added daily to main- 

 tain a concentration of 100 organisms/ml. The 

 density of B. plicatilis and T. holothuriae was 

 maintained in a like manner except that the vol- 

 ume sampled was larger (from 10- to 100-ml sam- 

 ples/container, depending on the stock density of 

 microzooplankton). Also, we were careful to sam- 

 ple a few centimeters away from the container 

 surfaces because T. holothuriae copepodids 

 and adults are thigmotactic. We stocked the rear- 

 ing containers with nauplii (which are less 

 thigmotactic than the older stages). However, 



during the course of the experiments, surviving 

 T. holothuriae developed beyond the naupliar 

 stages and tended to settle out on container sur- 

 faces becoming less available to anchovy larvae. 

 These stages were not included in our counts. 



RESULTS 

 Feeding Experiments 



A total of 518 larvae were presented with four 

 species of diatoms (Table 2). Only one larva fed on 

 diatoms. This single individual ate a narrow (5 x 

 50-75 /um) chain-forming diatom, Leptoeylindrus 

 danicus. 



Most larvae fed on the dinoflagellates Gymno- 

 dinium splendens, Gonyaulax polyedra, Proro- 

 centrum micans, and Peridinium trochoideum. 

 There was no apparent preference by larvae for 

 a particular species of dinoflagellate. Between 72 

 and 89' \ of the larvae tested fed on P. trochoideum 

 (20 /xm), which are as small as the smallest sized 

 particles known to be ingested by first feeding 

 anchovy larvae (Arthur 1976). Peridinium trocho- 

 ideum is a darkly pigmented dinoflagellate. Per- 

 haps this characteristic makes it more visible to 

 the larvae than other particles of a similar size. 

 Lasker (1975) concluded that first feeding an- 

 chovy larvae required a particle greater than 

 40 (iim to fill their gut in 8 h. 



Anchovy larvae did not feed on the smallest 

 prey used in the feeding experiments, the flagel- 

 lates Chlamydomonas sp. ( 10 /urn) and Dunaliella 

 sp. (6 /xm). 



Larval Rearing Experiments 



Growth and survival of anchovy larvae reared 

 for 10 days on different diet regimes are shown in 

 Table 3. The survival rate of larvae reared on the 

 Gymnodinium splendens diet was higher than on 

 the Gonyaulax polyedra diet. The relationship be- 

 tween larval survival and supplementation of the 

 dinoflagellate diet with microzooplankton was de- 

 scribed with linear regressions (Figure 1). The 

 survival of larvae reared in seawater containing 

 100 Gymnodinium splendens/ml did not signif- 

 icantly increase (t for the slope of the regression 

 = 0.1, P<0.20) when microzooplankton were 

 added to their diet as a supplement (Figure 1). 

 Supplementation of the Gonyaulax polyedra diet 

 with microzooplankton did result in a significant 

 increase (t for the slope of the regression = 3.24, 



580 



