100 



90 



9/9 



7/7 



UJ 



Q 



Q 

 LlI 



CD 



§50 



O 



40 



OSTR/COCS 



80- 



70- 



CHYDORiDS 



60- 



LlI 

 Q_ 



20 H 



10- 







OTHERS 



CHIRON- 



CYCLO- 

 POIDS 



INSECTS 



FISH EGGS 



OSTRACOCE 



CHYDORIDS 



OTHERS 



FISH EGGS 



4/6 4/12 



COLLECTION DATES (1980) 



Figure 2.— Changes in the composition of the combined diet (both 

 males and females) over the two collection dates at Rocky Hock 

 Creek in April 1980. Numbers above each bar graph indicate 

 number of stomachs with food/total number of stomachs examined. 



too far from the estuary for bluebacks to travel to 

 saltwater for daily feeding. It is important to note, 

 also, that the interface between freshwater and salt- 

 water often extends far out into Albemarle Sound 

 due to the spring discharge of both the Chowan and 

 Roanoke Rivers. This was the case in the spring of 

 1980.'* Moreover, the prey were exclusively of fresh- 

 water origin. There is, therefore, little doubt that 

 these bluebacks were feeding in freshwater. 



The wide diversity of food items consumed was 

 unexpected since bluebacks have previously been 

 reported to be primarily planktivorous (Bigelow and 

 Schroeder 1953; Hildebrand 1963). The limited 1981 

 data suggest that prey other than zooplankters are 

 consumed infrequently if sufficient zooplankters (or 

 large zooplankters such as Daphnia) are present. 

 However bluebacks are also capable, as the 1980 data 

 demonstrate, of foraging opportunistically on other 

 riverine fauna and terrestrial insects, which could 

 also explain Frankensteen's (1976) unusual finding 

 that chironomids were the dominant prey of blueback 

 herring in the Ikr River. Consumption of benthic 

 prey probably accounts for the presence of detritus 

 and sand in the guts. 



My data show a difference between male and 

 female feeding activity. There are two possible ex- 

 planations for this difference First, females may re- 

 quire more energy than males during the spawning 

 migration thus they consume more prey. Neither this 

 study nor other studies of bluebacks have produced 

 data to either support or refute this idea. However, 

 moderate to severe weight loss is common among 

 other spawning anadromous fishes (eg., Atlantic 

 salmon, American shad) with females suffering 

 greater weight loss than males (Belding 1934; Chit- 

 tenden 1976; Glebe and Leggett 1981). Glebe and 

 Leggett (1981) found that development of ovaries in 

 female shad required more energy and time than the 

 male shads' testes. Consequently, female shad enter- 

 ing freshwater, particularly southern rivers, often 

 do not have fully developed ovaries. Thus, not only 

 must females expend energy for swimming but for 

 gonad development as well. The same difference in 

 gonad development may exist between male and 

 female bluebacks and could explain the different 

 levels of feeding activity observed in this study. 



The second explanation for the difference in 

 feeding activity is that all bluebacks, regardless of 

 sex, stop feeding while spawning. However, males 

 might remain on the spawning grounds longer than 

 females. Thus, if females leave the area immediate- 

 ly after they spawn and are replaced by newly ar- 

 rived females with relatively full guts, this could 

 cause the gut samples to be biased. This explana- 

 tion appears to be ruled out by the 1981 data, 

 however, since half the fish with stomachs >1/A full 

 collected at Rocky Hock were males. 



While previous researchers have found food in 

 bluebacks' stomachs (Williams et al. 1975 as cited 



■•R. Holmes, Department of Natural Resources and Community 

 Development, Division of Environmental Management, Raleigh, NC 

 27611, pers. commun. March 1984. 



715 



