910 
Fishery Bulletin 96(4), 1998 
cod 60-100 mm SL was 39% pelagic 
prey, and the diet of cod greater than 
100 mm SL was 13% pelagic prey by 
volume. A simple explanation for this 
rapid shift is that the mouth opening of 
smaller size classes is too small to en- 
able predation upon many benthic prey 
species. However, it is important for 
juvenile cod to settle to benthic habi- 
tats as soon as possible to find protec- 
tion against predators. Therefore, they 
quickly shift to a benthic diet as soon 
as their gape size is large enough. 
A series of hypotheses were examined 
to decide whether the rapid shift in diet 
was associated with conditions of the 
study: time of day, time of year, and 
breakdown of the thermocline. The pat- 
tern of change in Figure 1 was not due 
to time of day, because cod were cap- 
tured primarily at night (18:10 to 04:30 
hours). Data were then replotted by 
month of capture, which proved not to 
be responsible for the rapid shift in diet. 
Data were then replotted by date to in- 
vestigate relation to the breakdown of the ther- 
mocline in mid-October. It was hypothesized that the 
thermocline keeps pelagic and benthic prey well sepa- 
rated (i.e. pelagic prey are unavailable to demersal 
juveniles). After the breakdown of the thermocline, 
the pelagic prey may occur lower in the water col- 
umn. However, the change in percentage of pelagic 
prey did not change suddenly at the time of ther- 
mocline breakdown. 
Conclusions 
The diets of group-0 and group- 1 Atlantic cod at Trin- 
ity Bay, Newfoundland, are comparable to those of 
other areas (Daan, 1973 [northeastern Atlantic]; 
Arntz, 1974 [western Baltic]; Palsson, 1980 [Iceland]; 
Bowman, 1981 [western Atlantic]; Keats et al., 1987 
[eastern Newfoundland]; Paz et al., 1991 [Flemish 
Cap]; Keats and Steele, 1992 [eastern Newfound- 
land]; Hop et al., 1992, 1994 [northeastern Atlantic]). 
Demersal group-0 cod fed on both pelagic and benthic 
organisms, whereas group-1 cod fed more on benthic 
organisms as the principal components of the diet. The 
diets of group-0 and group- 1 cod overlapped within a 
narrow range of body size (85.1 to 100.1 mm SL). Small 
pelagic prey, such as calanoid copepods, became less 
important in the diet of group- 1 cod, whereas larger 
benthic prey items, such as decapods, amphipods, te- 
leost fish, and polychaetes, became more important. 
Table 3 
Percent pelagic prey in the diet of three size groups of ju- 
venile Atlantic cod. 
Size group 
(mm) 
Mean % 
pelagic prey 
Standard 
error 
Number of 
stomachs 
40-59 
97.56 
2.44 
8 
60-100 
38.5 
5.32 
45 
>100 
13.06 
4.43 
41 
Results of the present study indicate that the on- 
togenetic shift from pelagic to benthic prey occurs 
within a narrow size range (from 60 to 100 mm SL) 
and hence more rapidly than previously believed 
(Daan, 1973; Bowman, 1981). This rapid shift may 
have been overlooked in previous studies that did 
not examine the diet of small cod in as much detail 
as in our study (i.e. volume in microliters of indi- 
vidual prey taxa). However, results of our study do 
not indicate that all cod less than 100 mm rely solely 
on benthic food. For example, it is well documented 
(e.g. Kohler and Fitzgerald, 1969) that, at particular 
times of the year, large juvenile and adult cod can 
forage far off the bottom on pelagic prey. One impli- 
cation of this finding is that any investigation of cod 
recruitment variability, in relation to food supply, 
should include postsettlement stages. 
