160 



Fishery Bulletin 92(1). 1994 



Diel feeding periodicity and daily ration 

 estimate 



Diel feeding periodicity and daily ration were exam- 

 ined separately for winter (Bay of Fundy, Scotian 

 Shelf, and Georges Bank combined) and summer 

 (Bay of Fundy) collections because of seasonal dif- 

 ferences in photoperiod. Stomach fullness data from 

 tows within each successive 3-hour (winter cruises) 

 and 4-hour (summer cruises) interval were grouped 

 and assigned to the midpoint of the time period. 

 Small sample sizes precluded grouping of summer 

 collections into 3-hour intervals. 



Daily ration (DR) of alewives during winter and 

 summer and by size class during winter (<151 mm, 

 151-200 mm, 201-250 mm, >250 mm) was esti- 

 mated in terms of % body weight from the model of 

 Elliott and Persson (1978): 



c _ (*-*■-) a . 



1-e 



■Rt 



where the consumption of food (C t ) during the time 

 interval t to t t is calculated from the average 

 amount of food in the stomach at time t (S„), the 

 amount in the stomach at time t t (S,) and the instan- 

 taneous evacuation rate R. The estimates of C t cal- 

 culated for each time interval are then summed to 

 give the total daily ration (DR). Feeding is assumed 

 constant within each time interval. R is assumed 

 exponential and temperature dependent (Elliott, 

 1972), as 



R = ae hT . 



The slope (6) may be fairly constant for different 

 prey types and fish species (mean=0.115), but the 

 intercept (a) changes with prey type and can be 

 estimated from gastric evacuation experiments 

 (Durbin et al., 1983). Gastric evacuation rate data 

 are unavailable for anadromous alewives; therefore, 

 an intercept (a=0.0406) was obtained from Durbin 

 et al. based on values for a variety of small 



invertebrates fed to several freshwater and marine 

 fishes. High fat levels in the prey may retard evacu- 

 ation (Durbin et al., 1983) but the principal food 

 item in this study (M. norvegica) has a low lipid 

 content (Ackman et al., 1970). Average bottom tem- 

 peratures for winter (mean=7.16°C) and summer 

 (mean=7.43°C) collections were used to estimate R. 



Statistical analysis 



Differences in the rankings of IRI values for prey 

 categories (n =8 1 between three or more groups were 

 tested for significance with the Kendall coefficient 



of concordance (w) (Siegel, 1956); for paired groups, 

 the Spearman rank correlation coefficient (rj was 

 used (Fritz, 1974). Euphausiids, which consistently 

 ranked highest in importance in all comparisons, 

 were excluded from correlation analysis to reduce 

 bias and emphasize correlations among remaining 

 prey groups. 



One-way ANOVA was used to examine feeding 

 activity, represented by the index of fullness (arc- 

 sine Vp transformed) by season and geographic area, 

 by depth range within season and geographic area 

 and by diel sampling period (winter and summer 

 collections) and to compare total lengths of eu- 

 phausiid prey. Paired means, adjusted for unequal 

 sample sizes, were compared with the Tukey- 

 Kramer test (Sokal and Rohlf, 1981). The relation 

 between predator fork length and mean prey length 

 was examined by linear regression for alewives with 

 three or more M . norvegica present in their stomachs. 



Results 



Alewives examined for stomach contents measured 

 95 to 305 mm FL (mean=213.6 mm, n=l,215); fish 

 from summer cruises in the Bay of Fundy were 

 larger on average than those from other collections 

 (Table 1). Capture depths ranged from 36 to 269 m, 

 although most (75%) specimens were obtained from 

 regions 101 to 200 m deep. 



Recognizable prey from over 20 different taxa oc- 

 curred in 55% (668 of 1,215) of stomachs examined 

 (Table 2). Over 95% of the total prey number, vol- 

 ume, and frequency of occurrence were crustaceans 

 (Table 2). Euphausiids were the most prevalent (91% 

 by volume); Meganyctiphan.es norvegica were domi- 

 nant by volume (61%) and furcilia larvae of 

 Thysanoessa spp. were dominant numerically (32%). 

 Other prey, including hyperiid amphipods, calanoid 

 copepods, crustacean larvae, mysids, polychaetes, 

 chaetognaths, pteropods, and fish larvae contributed 

 little and varied temporally and spatially in relative 

 importance. 



Diet composition by season and area 



Euphausiids were the most important food of ale- 

 wives during winter and summer for all areas (Fig. 

 2). During winter, alewives from the outer Bay of 

 Fundy and Georges Bank fed almost exclusively on 

 euphausiids (99% and 95% of total volume, respec- 

 tively). On Georges Bank, small (%V<3) proportions 

 of calanoid copepods, hyperiid amphipods, and ptero- 

 pods were also consumed. Prey diversity was great- 

 est for alewives from the Scotian Shelf; euphausi- 

 ids dominated by volume (82%) but were numeri- 



