908 
Transition from pelagic to benthic prey 
for age group 0-1 Atlantic cod, 
Gadus morhua 
Tammy M. Lomond 
Department of Biology 
Memorial University of Newfoundland 
St. John's, Newfoundland, Canada AIC 3X9 
Present address: Greenhouse and Processing Crops Research Centre 
Agriculture and Agri-Food Canada 
Highway 20, Harrow, Ontario, Canada NOR 1G0 
E-mail address: lomondt@em.agr.ca 
David C. Schneider 
Ocean Sciences Centre, Memorial University of Newfoundland 
St. John's, Newfoundland, Canada AIC 5S7 
David A. IWHethven 
Department of Biology, Memorial University of Newfoundland 
St. John's, Newfoundland, Canada A1B 3X9 
Atlantic cod, Gadus morhua L., 
settle to the bottom early in life at 
standard lengths of 30 to 40 mm 
(Bowman, 1981; Hawkins et ah, 
1985; Hop et ah, 1994; Methven and 
Bajdik, 1994). After settlement into 
benthic habitats, Atlantic cod con- 
tinue to feed on pelagic prey but 
then shift to benthic prey. The rate 
at which cod make this transition 
has not been quantified at a level 
usable in trophic dynamic studies. 
To quantify the rate at which this 
shift in diet occurs, the volumetric 
proportion of pelagic prey in the diet 
of 98 juvenile Atlantic cod (age group 
0 and 1) was measured. The data 
were used to evaluate a method that 
enables the calculation of food quan- 
tities with high accuracy for incor- 
poration in trophic studies. 
Materials and methods 
Juvenile Atlantic cod (ages 0 and 1) 
were collected at Bellevue, Trinity 
Bay, Newfoundland, from July to 
December 1989 and from August to 
October 1991. Refer to Methven 
and Bajdik (1994) for additional 
information on fish collection and 
the collection site. A sample of 98 
group-0 (40.2-100.1 mm standard 
length [SL] ) and group-1 (85.1- 
192.0 mm SL) cod were used. Stom- 
ach contents were analyzed by us- 
ing prey volume to quantify the 
transition from pelagic to benthic 
prey. 
For each individual, prey were 
identified to the lowest taxon pos- 
sible, then counted. Displacement 
volume was measured for each 
taxon with a 5-mL graduated cyl- 
inder or a 200-pL micropipette. 
Prey volume was estimated as a 
proportion of the total volume of the 
pipette by using a millimeter ruler 
(200 pL measured 90.5 mm, i.e. 1 
mm = approx. 2.2 pL). If a prey 
group was too small for volumetric 
displacement, measurements of 
length, width, and depth were 
taken for each individual prey item 
by using a dissecting microscope 
with an ocular micrometer. Mea- 
surements were made by using eye 
piece units and converted to milli- 
meters to calculate volumes with 
geometric formulae (Table 1). 
To evaluate the accuracy of cal- 
culated volumes, both displacement 
volume and calculated volume were 
measured for various prey items. 
Calculated volume tended to be 
higher than displacement volume 
by a constant rate, therefore linear 
regression was used to calibrate 
calculated volume against displace- 
ment volume. The intercept was not 
significantly different from zero 
(P=0.1453,F [1 33 j= 1 16.43 ), therefore 
the parameters were re-estimated 
without the intercept (P=0.0001, 
F|i 33 j = 258.02). The regression 
equation was 
Displacement volume = 
(0.72 )(calculated volume). 
A correction factor of 0.72 was ap- 
plied to all calculated volumes to 
obtain estimated displacement vol- 
umes. 
The following references were 
used to identify invertebrate prey 
items and to determine whether 
they were benthic or pelagic with 
respect to habitat: Smith, 1964; 
Allen, 1967; Russell-Hunter, 1969; 
Schultz, 1969; Feeley and Wass, 
1971; Meglitsch, 1972; Naylor, 
1972; Bousfield, 1973; and Gardner 
and Szabo, 1982. Scott and Scott 
(1988) was used to identify verte- 
brate prey. 
Percent pelagic prey in the diet 
was plotted against standard length 
to determine the relation between 
the two. 
Results 
Both group-0 and group- 1 cod fed 
on a broad range of prey. However, 
few occurred in large amounts 
Manuscript accepted 7 January 1988, 
Fish. Bull. 96:908-911 (1998), 
