Stone and Jessop: Feeding habits of Alosa pseudoharengus 



165 



euphausiid species composition in the 

 diet of alewives from all regions 

 matches differences in the bottom 

 depth preferences of M. norvegicci 

 (>150 m) and Thysanoessa spp. (100- 

 150 m) (Berkes, 1976; Kulka et al., 

 1982; Sameoto et al., 1993). 



Diel differences in the diet of ale- 

 wives may reflect the influence of vary- 

 ing light intensity on prey availability 

 and on their relative success in locat- 

 ing and capturing prey. Consumption of 

 microzooplankters (crustacean larvae, 

 calanoid copepods) was greater at night 

 perhaps because of increased filter- 

 feeding activity (Janssen, 1978b). Con- 

 versely, ingestion of macrozooplankters 

 (euphausiids, hyperiid amphipods) may 

 be highest during the day because visual 

 cues favour a particulate-feeding mode. 



Large size, darkly pigmented eyes, 

 and a habit of forming large concentra- 

 tions (Mauchline and Fisher, 1969) 

 may make M. norvegica easily detect- 

 able by alewives during daylight 

 whereas at night, photophores along 

 the abdomen of M. norvegica may as- 

 sist detection. Most euphausiid species 

 migrate vertically over the diel period, 

 rising from deep water (150-200 m) 

 towards the surface at dusk, remaining 

 near surface throughout the 

 night, and then migrating to 

 the depths at dawn (Mauch- 

 line, 1984). Alewives also have 

 a diel pattern of vertical migra- 

 tion in the marine environment 

 (Neves, 1981; Stone and 

 Jessop, 1992) and may encoun- 

 ter sufficient light higher in the 

 water column at night to par- 

 ticulate feed on euphausiids. 



Ontogenetic differences in 

 diet composition were not ap- 

 parent; euphausiids dominated 

 the diet of alewives ranging in 

 length from 95 to 305 mm. Ale- 

 wives switch from feeding pri- 

 marily on microzooplankton to 

 macrozooplankton at some 

 point during their marine de- 

 velopment and like other simi- 

 larly sized clupeids, concentrate 

 their feeding at intermediate 

 trophic levels (James, 1988). 



Winter- 

 Shelf 



100 



E 



O 



> 

 CD 



1) 

 u 



n. 



80 



GO 



40 



20 



Winter- 

 Georges 



13 65 34 3S 



Summer- 

 Fundy 





Legend 



Eup 



Cop 



Ptar 



Amp 



CruLar 



FIshLar 



Chaet 



Mys 



Poly 



B C 



C D 



Predator length group 



c D 



Figure 5 



Percentage of total volume of prey categories in the diet of ale- 

 wives, Alosa pseudoharengus, for different size classes (mm FL) 

 obtained from groundfish research surveys off Nova Scotia (1990- 

 91). Euphausiids were the only prey category in Winter-Fundy 

 cruises. A: <151 mm; B: 151-200 mm; C: 201-250 mm; D: >250 

 mm; Eup = euphausiids; Cop = calanoid copepods; Pter = ptero- 

 pods; Amp = hyperiid amphipods; CruLar = crustacean larvae; 

 FishLar = fish larvae; Chaet = chaetognaths; Mys = mysids; Poly 

 = polychaetes; n = number of stomachs with food. 



Winter-Georges 

 Winter-Shelf 

 Summer-Fundy 

 BIONESS 



(n = 257) 

 (n - 269) 

 (n = 178) 

 (n = 785; 



25 30 35 



Total length (mm) 



45 



Figure 6 



Size frequency distributions of M. norvegica consumed by alewives, Alosa 

 pseudoharengus, obtained from winter (Georges Bank. Scotian Shelf) and 

 summer (Bay of Fundy) groundfish surveys off Nova Scotia (1990-91) and 

 from BIONESS samples in Emerald Basin (Spring, 1991 ). n = sample size. 



