rapidly toward the school and rejoined it. Some 

 larvae ingested as many as 15 copepods (av- 

 erage 10) before they stopped feeding. They 

 ignored or rejected Artemia or Balantis 

 nauplii, dead copepods, chopped clams, and 

 finely ground particles of fish meal when in- 

 troduced with copepods, but readily ingested 

 Artemia or Balanus nauplii alone. Unsuccessful 

 attempts at feeding and the occurrence of lar- 

 vae with empty alimentary tracts during our 

 observations suggest that some individuals 

 may be ineflScient feeders when the food sup- 

 ply is limited. Atlantic menhaden larvae thus 

 appear to select preferred foods visually but 

 will accept substitutes when no preferred 

 foods are present. 



Prejuveniles and juveniles reacted differently 

 than larvae to the introduction of food. A 

 school of fish suddenly reduced its forward 

 swimming speed and turned in the direction of 

 the food. Individuals differentially increased 

 their swimming speed until they were in con- 

 tact with the food; then, with mouth agape, 

 they swam slowly upward and downward 

 through the water as they appeared to suck in 

 the food particles. Throughout feeding, their 

 opercula were flared and their bright red gill 

 filaments were clearly visible. At times some 

 of the prejuveniles pursued individual cope- 

 pods, but, in general, their feeding behavior 

 was the same as that of juveniles. During feed- 

 ing the fish spread out, but as the food supply 

 diminished they gathered in small groups. 

 These groups, enlarged by the addition of in- 

 dividuals, eventually joined to reestablish the 

 school. 



Our observations generally agree with those 

 of Breder (1959), but indicate that filter feed- 

 ing, rather than particulate feeding, is the 

 principal means by which both prejuvenile and 

 juvenile Atlantic menhaden obtain food. 



Influence of Light on Feeding 



Larvae fed at a minimal light intensity of 

 25 lux (table 9). At lower intensities, the lar- 

 vae remained inactive and randomly distrib- 

 uted near the bottom of the bowl. At higher 

 light intensities, the rate of feeding was in- 

 creased and maximum intensity (about 500 



lux) had no apparent adverse effect. We did not 

 study the possible effects of the changes in 

 wave length that accompanied reduction of 

 light intensity on the feeding response. 



Our laboratory observations thus indicate 

 that Atlantic menhaden larvae do not feed at 

 low intensities of artificial light; however, re- 

 sults of our field studies suggested that larvae 

 fed more actively on moonlit nights than on 

 dark nights. The incident light intensity of 

 moonlight probably does not exceed 0.5 photo- 

 topic lux (Urey, 1961), but this natural light 

 level may be sufficient for larvae to capture 

 swimming copepods silhouetted at or near the 

 water surface. 



Table 9. — Numbers of larvae feeding at different light 

 intensities 



[Light intensities in parentheses] 



Feeding began. 

 - Alimentary tract full or nearly full. 



Digestion Time 



Artemia nauplii passed through the esopha- 

 gus to the posterior alimentary tract — where 

 they often could be seen wriggling — within 1 

 to 3 seconds. At water temperatures ranging 

 from 16.8° to 17.5°C., 80 percent of the larvae 

 with apparently full posterior tracts had voided 

 all contents between 8 and 10 hours after feed- 

 ing; those estimated to be one-quarter to three- 

 quarters full, between 6 and 7 hours. One larva 

 retained food for 14 hours. 



Rapid uptake of radioactive zinc (Zn'^^) by 

 the body tissues of larvae fed labeled copepods 

 indicated that assimilation of food began al- 

 most immediately. Nearly two-fifths of the 

 radioactivity was in the body tissues within 

 11/2 hours; three-fourths, after 8I/2 hours; and 



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U.S. FISH AND WILDLIFE SERVICE 



