Percentages of larvae %vith food organisms 

 in the alimentary tracts were generally higher 

 in daylight and evening (fig. 2). Feeding activ- 

 ity tended to be higher on moonlit nights than 

 on dark nights (50 percent of the larvae caught 

 on moonlit nights contained food compared 

 with 31 percent on dark nights). 



The composition of the net plankton and the 

 alimentary tract contents of larvae showed 

 similarities and differences. For example, cope- 

 pods accounted for 98 percent of the net zoo- 

 plankton and 99 percent of the tract contents 

 (table 4). Four genera, Centropages, Acartia, 

 Pseudodiaptonuis, and Temora, were common 

 to both the net zooplankton and the alimentary 

 tract contents, but their order of rank differed. 

 With the exception of the single sample col- 

 lected in May, in which the genus Temora pre- 

 dominated, Acartia consistently was the most 

 abundant organism in the net zooplankton. 

 Pelecypods were rare in both the net samples 

 and the alimentary tracts. 



Centrate diatoms dominated the phytoplank- 

 ton numerically in 10 samples taken at random 

 throughout the sampling period (table 5). Of 

 all larval alimentary tracts examined, only one 

 contained a single phytoplankter, Coscino- 

 discus. 



Samples Within the Estuary 



The contents of the alimentary tracts of lar- 

 vae and smaller prejuveniles consisted of zoo- 

 plankton, but changed to phytoplankton and 

 unidentifiable material in larger prejuveniles 

 and juveniles. Only 29 (20 percent) of the 

 tracts examined were empty. As was found in 

 the inlet collections, the tract contents of larvae 

 consisted exclusively of copepods (chiefly 

 Acartia) . The contents of progressively larger 

 prejuveniles gradually shifted from copepods 

 to a mixture of greenish brown amorphous ma- 

 terial, diatoms, and flagellates (fig. 3), and at 

 least 80 percent (by volume) of the tract con- 

 tents of juveniles consisted of these latter 

 items. Diatom genera identified, in order of 

 their frequency of occurrence, were Pleuro- 

 sigma, Navicvla, Nitzschia, Cyclotella, Melo- 

 sira, Amphora, Gyrosigma, and Surirella. 

 Flagellates identified included Peridinium, 

 Gymnodinium, and Polykrikos. 



The composition of the plankton resembled 

 the contents of the alimentary tracts examined 

 for the size range of fish sampled. Copepods, 

 chiefly Acartia, accounted for nearly 100 per- 

 cent (by volume) of the net zooplankton after 



Table 4. — Composition of fauna in 23 tow-net samples taken concomitantly with collections of Atlantic menhaden 

 larvae at Indian River Inlet, Del., November 1960 to May 1961 



Organisms 



Samples 

 containing 

 organisms 



Mean 



-■number of 



organisms 



per cubic 



meter 



Percentage 



of total 

 organisms 



Size of organisms 



Range 



Length 



Width 



Mean 



Length 



Width 



Chaetognatha ... . 



Copepoda 



Acartia 



Te-mora 



Centropagea 



Pseudodiaptomua 



ToTtanua 



Labidocera 



Rhincalanua 



EucalanuB 



Unidentified nauplii 



Oatracoda 



Malacostraca 



Mysidacea 



Isopoda 



Decapoda 



Amphipoda 



Pelecypoda 



Miscellaneous - - 



Number 



23 

 10 

 ZS 

 20 



4 



4. 



2 



1 



2 



21 

 7 



13 

 9 

 8 



13 



Number 

 3 



2.228 



2,194 



457 



312 



3 



23 



1 



<1 



4 



1 



92 

 1 

 3 

 3 

 6 

 6 



Percent Mm. Mm. Mm. Mm. 



<0.1 4.60-14.60 0.22-0.7B 9.41 0.46 



BB.6 0.89- 1.65 0.26-0.37 1.20 .83 



23.8 1.40- 1.76 0.40-0.59 1.49 .60 



11.4 0.88- 1.92 0.30-0.68 l.Bl .43 



6.8 1.09- 1.86 0.28-0.68 1.36 .40 



<0.1 1.68- 1.96 0.44-0.65 1.86 .61 



0.1 2.07- 2.75 0.46-0.74 2.43 .64 



<0.1 3.03 0.52 _ 



<0.1 2.25 0.60 -- 



<0.1 0.81 0.30-0.37 .33 



<0.1 0.63- 1.18 0.37-0.48 0.86 .48 



2.1 2.56- 6.76 0.30-1.60 6.87 .63 



<0.1 1.80-21.00 0.68-7.00 



<0.1 1.32-62.00 0.30-1.80 



<0.1 1.32- 6.70 0.31-1.50 3.56 .68 



<0.1 0.36- 1.20 0.20-0.60 0.66 .38 



<0.1 0.36-5.25 0.36-0.90 -- 



FOOD OF YOUNG ATLANTIC MENHADEN IN RELATION TO METAMORPHOSIS 



497 



