well as probably those of shallow marine coastal 

 waters, are utilized directly as a food resource by 

 adult and juvenile menhaden. Our field observa- 

 tions of their grazing habits, the preponderance of 

 benthic diatoms in their stomachs, and the taxo- 

 nomic and ecological similarity of the diatom 

 assemblages in their stomachs with that of the 

 benthos support this conclusion. The composition 

 of the stomach and epilithic samples is commen- 

 surate with the expectations of random sampling 

 of the benthos in this region of the estuary. The 

 quantitative characteristics of estuarine benthic 

 diatom assemblages can be extremely variable 

 within a small space, even on similar substrates 

 (Mclntire and Overton 1971; Round 1971; Main and 

 Mclntire 1974), and so the expectation of quaniti- 

 tative identity among random samples is low. But, 

 much greater quantitative similarity is expected 

 of samples from similar substrates in the same 

 area. 



The data of other investigators, but not their 

 conclusions, support our findings. In a study of the 

 diet of juvenile menhaden collected between April 

 and June 1961, in Delaware, June and Carlson 

 (1971) found most frequently eight genera of 

 diatoms present in their guts: "Pleurosigma, 

 Navicula, Nitzschia, Cyclotella, Melo^ira, Am- 

 phora, Gyrosigma, and SurireUa.'" All these gen- 

 era are characteristically benthic in marine and 

 estuarine ecosystems. Compared to the list of 

 diatom genera they reported from the phyto- 

 plankton, which they sampled between November 

 1960 and May 1961, in the same area, the eight 

 genera accounted on the average for less than 10% 

 of the total number of diatom phytoplankters. 

 Furthermore, they reported that Skeletonema, 

 Coscinodiscus, Rhizosolenia, Thalassiosira, and 

 Thalassiothrix composed on the average 75% of 

 the diatom phytoplankton, but all were unrecorded 

 from their gut analyses of the fish. We conclude 

 from their data that the juvenile menhaden they 

 collected were not grazing primarily on the 

 plankton but rather on the benthos. Likewise, 

 Mulkana (1966) reported six diatom genera from 

 the stomachs of juvenile menhaden collected in 

 Rhode Island estuaries, and four of the six are 

 usually benthic: Gyrosigma, Grammatophora, 

 Achnanthes, and Navicula. Although the diatoms, 

 whether planktonic or benthic, appear to consti- 

 tute a less significant portion of the diet of 

 juveniles and adults in estuaries than does detritus 

 (Jeffries 1975; Peters and Kjelson 1975), they 

 accurately reflect the immediate source of the 



detritus, because they are good habitat labels 

 (Round 1964, 1971). 



Both juvenile and adult menhaden tolerate 

 salinities of less than T/oo (Reintjes 1969), but we 

 know of no records other than our own of their 

 feeding on primarily freshwater or oligohaline 

 resources. 



The Atlantic menhaden is among the commer- 

 cially most important species in the United States 

 fishery, and consequently, the factors which 

 regulate its population size are of considerable 

 interest. Assuming that human and other preda- 

 tors are prudent, trophic energy availability is 

 likely to be limiting. McHugh (1967) has postulated 

 that "the rate of plankton production will limit the 

 numbers of menhaden . . . that a particular body of 

 water can support." If we interpret the concept of 

 the plankton liberally, including the living organ- 

 isms plus the suspended detritus, the idea is 

 certainly tenable; however, it is conditional upon 

 the menhaden's grazing being restricted to the 

 plankton. Also, adult menhaden's minimum-size 

 threshold for filtration of particles appears to be 

 around 15 p.m with the consequence that a sub- 

 stantial portion of the phytoplankton will be 

 unavailable to them (Durbin and Durbin 1975). 

 But, considering the productivity of benthic 

 primary producers and the quantities of 

 sedimented detritus in shallow estuaries (Darnell 

 1967; Odum 1971; Smayda 1973), the menhaden's 

 exploitation of the benthos, potentially, at least, 

 doubles the energy available to it. Unfortunately, 

 the quantitative significance of their benthic 

 grazing habits and their ability to assimilate the 

 ingested materials during the estuarine portions 

 of their life cycle are unassessed. 



Jeffries (1975) has characterized the menhaden 

 as an adaptable species capable of switching from 

 one food resource to another, and thus compen- 

 sating for the variability in the availability of 

 estuarine food resources. In general, this apparent 

 switching in juveniles and adults is more the 

 product of a fine-grain feeding in resource-dif- 

 ferent habitats than of coarse-grain feeding on 

 the plankton. Our observations extend this mode 

 of feeding in menhaden to include benthic 

 habitats. 



Literature Cited 



Cleve-Euler, a. 



1951-55. Die Diatomeen von Schweden und Finnland. K. 

 Sven. Vetenskaps akad. Handl., Ser. 4, 1,2(l):l-163, (1951); 



692 



