Table 5. — Composition of phytoplankton in 10 water 

 samples taken concomitantly with collections of At- 

 lantic menhaden larvae at Indian River Inlet, Del., 

 November 1960 to May 1961 



Number Number 

 Pennales 



Nitzschia 2 372 



Pleuroaigma 6 559 



Gyrosigma 1 293 



Navicula 3 398 



Diploneis 2 186 



Cocconeis 2 132 



Achnanthea 3 657 



Thalassiotrix - 8 1.310 



Asterionella -.. 7 783 



Centrales 



Bellerochea - - - - 1 483 



Biddulphia . . . _ - _ 1 600 



Corethroii 1 426 



Rhixosolenia 1 2,909 



ActinoptychuB 6 775 



Coacinodiscus -.. 9 3,403 



Thalassiosira 7 2,560 



Cyclotella 5 942 



Skeletonema - - 8 8,689 



Melosira - 7 672 



Diatoms other than 



above genera 6 1,625 



Peridiniales 



Peridinium - . 2 1,525 



Prorocentrales 



Gyrndinium 2 459 



Prorocentrum 1 237 



Exuviella 1 237 



Unidentified 



flagellates 2 604 



Percent 



0.4 

 1.9 

 0.2 

 0.7 

 0.2 

 0.2 

 1.1 

 6.1 

 3.2 



0.3 



0.3 



0.2 



1.7 



2.7 



17.8 



10.4 



2.7 



40.3 



2.7 



0.3 

 0.1 

 0.1 



0.7 



30 55 to ub 50 



FORK LENGTH (mm.) 



»u.^.vij o. — volume ratio ol copepods and amorphous 

 food material (chiefly phytoplanlcton) in alimentary 

 tracts of young Atlantic menhaden in relation to 

 fork length. Open circles represent copepods and 

 solid circles amorphous material. 



removal of the ctenophores, Pleurobrachia and 

 Mnemiopsis , from the samples. Phytoplankton 

 consisted of naked and thecate flagellates 

 (mainly Polykrikos, Peridinium, and Gymno- 

 dinium) , and diatoms (chiefly Pleurosigma and 

 Coscinodiscxis) . Similarities between the color 

 and form of the greenish brown materials 

 found in the alimentary tracts of the fish and 

 the fragile flagellates in water samples sug- 

 gested to us that the amorphous residue was 

 the remnants of these organisms. 



Our studies of the natural food of young 

 Atlantic menhaden confirmed that they are 

 plankton feeders. As larvae they feed only on 

 zooplankton but during metamorphosis they 

 increase their capability to retain smaller phy- 

 toplankters. During its early life stages, this 

 fish directly utilizes the abundant standing 

 crops of both phytoplankton and zooplankton 

 in estuaries. No other marine fish now occu- 

 pies this ecological role in Atlantic Coast 

 estuaries. 



The occurrence of young Atlantic menhaden 

 within the Indian River estuary appeared to be 

 associated with the seasonal cycle of plankton 

 production. Our highest catches of larvae at 

 the inlet were made in December and January 

 (69 and 66 larvae per tow, respectively, as com- 

 pared with to 9 per tow in other months) and 

 coincided with the maximum monthly catches 

 of each of the four dominant copepods repre- 

 sented in our net samples (Centropages, Acar- 

 tia, Pseudodiaptomus, and Temora). Meta- 

 morphosis of menhaden larvae began in April 

 1961 and followed a phytoplankton flowering 

 in this estuary in late March. Jeffries (1964) 

 also found that the occurrence of Atlantic men- 

 haden larvae in Raritan Bay coincided with the 

 seasonal peak of Acartia tonsa in that locality. 

 Apparently, seasonally abundant estuarine pop- 

 ulations of copepods (Deevey 1956, 1960) pro- 

 vide an available food supply for the carniv- 

 orous larvae when they enter the estuary, and 

 high standing crops of phytoplankton (Riley, 

 1967) are available during and following the 

 metamorphosis of larvae into juveniles within 

 the estuary. 



498 



U.S. FISH AND WILDLIFE SERVICE 



