Stone and Jessop: Feeding habits of Alosa pseudoharengus 



159 



40°N 



70°W 



Figure 1 



Set locations for alewives, Alosa pseudoharengus, obtained from groundfish re- 

 search surveys off the Atlantic coast of Nova Scotia (1990-91) grouped by season 

 and geographic area. Offshore banks are delineated by the 100-m depth contour; 

 the outer edge of the continental shelf is delineated by the 200-m depth contour. 



chaetognaths; 8) crustacean larvae (furciliae of 

 Thysanoessa spp. and some decapod larvae); and 9) 

 pteropods. The percent frequency of occurrence 

 (%FO), percent of total stomach content number 

 (%N), and percent of total stomach content volume 

 (%V) of prey categories were estimated for stomachs 

 containing recognizable food (digestion code >2). The 

 Index of Relative Importance (IRI=(%N+%V) x <7rFO) 

 was calculated for each prey category (Pinkas et al., 

 1971) and used for various diet comparisons. Diets 

 were analyzed by season and geographic area (Win- 

 ter-Fundy, Winter-Georges, Winter-Shelf, Suramer- 

 Fundy), as well as by depth range within season and 

 area, to compare food items from shallow regions 

 and offshore banks <<100 m), mid-depths (101-200 

 m) and the shelf edge or deep basins (>200 m). Diel 

 differences in diet composition (day and night, based 

 on time of gear deployment) were examined for the 

 entire data set. Ontogenetic differences in diet 

 within season/area were examined by grouping fish 

 lengths into four size classes (<151, 151-200, 201- 

 250 and >250 mm FL), which were assumed suffi- 

 cient for detecting shifts in prey composition. Data 

 from 1990 and 1991 were combined for all compari- 



sons because the ranks of IRI values for all prey 

 categories between years were highly correlated 

 (Spearman rank correlation coefficient (r s )=0.67; 

 P<0.05; n=9). 



Predator-prey size analysis 



Total lengths (±1 mm, tip of rostrum to end of tel- 

 son) of undigested, whole M. norvegica in the stom- 

 achs of 55 alewives (>200 mm FL, since most intact 

 prey occurred only in larger fish) from Winter- 

 Georges, Winter-Shelf, and Summer-Fundy cruises 

 were compared with predator size. Thysanoessa spp. 

 were not measured because of poor condition. 

 Lengths of M. norvegica from Emerald Basin col- 

 lected in June 1991, by Sameoto et al. (1993) using 

 the Bedford Institute of Oceanography Net and 

 Environmental Sensing System (BIGNESS) were 

 compared with euphausiid length frequencies from 

 stomach contents to estimate the proportion of the 

 available size range of M. norvegica consumed by 

 alewives. The BIONESS is not considered to be size- 

 selective for euphausiids (Sameoto et al., 1980). 



