development of a functional branchial filtering 

 apparatus which promotes a grazing of phyto- 

 plankton and suspended detritus. Late juveniles 

 and adults are primarily herbivores also but retain 

 the ability to eat zooplankton. 



The stimulus for this investigation was a shore- 

 line observation of adult menhaden grazing di- 

 rectly on the benthic microbial communities cover- 

 ing the rocks in the headwaters of a Massachusetts 

 estuary. The fish were observed to bite or rip off 

 chunks of the benthic community film and swallow 

 them. This film was composed primarily of dia- 

 toms and detritus. Subsequent gut analyses of the 

 fish and the epilithic diatom assemblage confirmed 

 the field observations. Additionally, ingestion of 

 these benthic primary producers and their as- 

 sociated detritus by juvenile menhaden is pos- 

 tulated from a reinterpretation of previous 

 reports on their diet. 



Methods 



In the early afternoon of 19 September 1974, 

 nine adult menhaden (25-34 cm fork length) were 

 collected in the oligohaline region of the Slocum 

 River estuary, Mass. (Hoff et al. 1969). The fish 

 were sampled with a 10-m, 64-mm mesh haul seine 

 from a school of about 150, which was observed 

 feeding on the bottom within a 500-m'- area about 

 1 m deep for the 15 min prior to collection. The 

 pyloric stomachs were excised, opened, and their 

 fullness visually estimated. The stomach contents 

 of each fish were maintained and examined 

 separately; they were preserved in 3*^ formal- 

 dehyde solution. A preliminary microscopic ex- 

 amination of the contents was made to determine 

 the presence of diatoms and other components of 

 the diet. Diatoms were prepared for detailed 

 examination by a nitric acid-dichromate oxidation 

 of an aliquot of the sample followed by washing of 

 the cleaned frustules and mounting in Hyrax^ 

 (Hohn and Hellerman 1963). Diatom populations 

 in each sample were identified and enumerated 

 from a random sample of about 200 frustules, 

 which were observed using oil-immersion phase- 

 contrast optics at a magnification of 1000 x . 



On 21 September 1974, a 20-cm diameter rock 

 was removed from the same region of the estuary 

 in which the menhaden had been observed feed- 

 ing. The diatom assemblage on the rock was 

 air-dried, then scraped off and subjected to the 



'Reference to trade names does not imply endorsement by the 

 National Marine Fisheries Service, NOAA. 



same procedures of preparation and examination 

 as those derived from the stomachs. 



All samples and slides have been deposited in 

 the Hellerman Diatom Herbarium at Southeast- 

 ern Massachusetts University according to the 

 following collection numbers: HH918-HH926 

 (stomach samples) and HH927 (epilithic sample). 



The diatom populations were classified as 

 freshwater, brackish, or marine based on the 

 habitat in which they grow optimally. This clas- 

 sification was derived primarily from the works of 

 Hustedt (1937-1938, 1939), Patrick and Reimer 

 (1966), Foged (1947, 1954), and Cleve-Euler 

 (1951-1955). Only those populations identified 

 without reservation to the species level were 

 classified ecologically. Additionally, in an 

 ecological classification of diatoms, identification 

 of populations to the level of variety is desirable 

 among multivarietal species, because frequently 

 different varieties of the same species have differ- 

 ent optimal habitats. 



The terms "common" and "rare" as employed in 

 this paper, differentiate diatom populations hav- 

 ing greater than 1% or less than 1% mean relative 

 abundance, respectively, in the stomach samples. 



Results 



All fish stomachs were completely full or nearly 

 so. Amorphous detritus and diatoms composed the 

 bulk of the material with the detritus accounting 

 for the greater portion, but as estimated micro- 

 scopically, from 5 to 25^ of the volume was dia- 

 tomaceous. Most larger diatom cells were broken 

 and without contents, but many smaller diatoms 

 retained their chromatophores in structurally 

 intact frustules. Other microorganisms, par- 

 ticularly filamentous blue-green algae and nema- 

 todes, were evident, and the remnants of some 

 microcrustaceans were noted in a few stomachs. 



The examination of about 1,800 diatom in- 

 dividuals from the stomachs revealed 163 popula- 

 tions of which 134 were identified to species or 

 variety. Twenty-three populations were common 

 and only three of them were not assignable to a 

 particular species (Table 1). The rare populations 

 which were unidentified constituted less than 2% 

 of all individuals. Practically all the populations 

 are benthic. Eight of them, particularly Skeleton- 

 ema costatum and Thalassiosira spp., are con- 

 sidered planktonic, but they contained less than 

 7% of all individuals and were found also on the 

 rock. Freshwater populations composed 50*^ of the 

 common and nearly 70% of all populations (Table 



690 



