164 



Fishery Bulletin 92(1), 1994 



100-1 



%N 



%v 



100 



Night 

 (n = 290) 



%N 



%V 



Legend 



Eup 



100 



100 



"i — i — i — r 

 i = 20% FO 



Figure 4 



Relative importance of prey categories in the diet of alewives, Alosa 

 pseudoharengus, obtained from groundfish research surveys off Nova 

 Scotia ( 1990-91), ranked from highest Index of Relative Importance 

 (left to right) for day and night collections (Symbols as in Fig. 2). 



Atlantic seaboard of the United States consumed 

 relatively fewer euphausiids 137-56% by weight) 

 (Edwards and Bowman, 1979; Vinogradov, 1984) 

 than off Nova Scotia (82-99% by volume). 



Euphausiids represent a large component of the 

 marine zooplankton community and are abundant 

 in the Bay of Fundy (Kulka et al., 1982; Locke and 

 Corey, 1988), Gulf of Maine (Bigelow, 1926), the deep 

 basins of the Scotian Shelf (Herman et al., 1991) and 

 the outer shelf and shelf slope (Sameoto, 1982). 

 Given their two-year life cycle (Hollingshead and 

 Corey, 1974; Berkes, 1976), the availability and rela- 

 tive abundance of euphausiids is more seasonally 



stable than for other prey spe- 

 cies (i.e., chaetognaths, 

 hyperiid amphipods, calanoid 

 copepods, mysids), most of 

 which undergo fluctuations in 

 abundance progressing from a 

 spring low to a summer high 

 before declining in fall and win- 

 ter (Evans, 1968; Sherman and 

 Schaner, 1968; Corey, 1988; 

 McLaren et al., 1989). 



Small seasonal differences in 

 diet composition reflect the op- 

 portunistic foraging behaviour 

 of alewives and the availability 

 of food types from offshore re- 

 gions during winter as com- 

 pared with the Bay of Fundy in 

 summer. During winter, the 

 diet diversity of alewives was 

 greatest on the Scotian Shelf 

 probably because the late win- 

 ter (mid-March) sampling period co- 

 incides with the hatching and occur- 

 rence of the larval forms of Thy- 

 sanoessa spp. (Berkes, 1976) and 

 Ammodytes dubius (Scott, 1972), 

 both of which occurred only in the 

 diet of alewives from the Scotian 

 Shelf. In the Bay of Fundy, alewife 

 consumption of chaetognaths and 

 mysids in the summer reflects their 

 increased abundance and availabil- 

 ity (Sherman and Schaner, 1968; 

 Corey, 1988). 



The increased proportion of eu- 

 phausiids in the diet of alewives 

 from the Scotian Shelf (winter) and 

 the Bay of Fundy (summer) coin- 

 cides with an increased relative 

 abundance of euphausiids with in- 

 creasing depth. In the Scotian Shelf 

 Basins, M. norvegica occur between 170 m and the 

 bottom with highest concentrations generally below 

 200 m (Sameoto et al., 1993). In the Bay of Fundy, 

 M. norvegica is most abundant where bottom depths 

 are between 165 and 200 m, while Thysanoessa 

 inermis occur between 95 and 155 m (Kulka et al., 

 1982). The greater proportion and number of other 

 prey categories at depths less than 101 m on the 

 Scotian Shelf and in the Bay of Fundy likely result 

 from decreased euphausiid abundance (thereby in- 

 creasing the relative contribution of other prey) 

 rather than an absolute increase in the abundance 

 of other zooplankters. Depth-related variation in 



