468 
Fishery Bulletin 106(4) 
Figure 11 
Total acoustic backscatter (bubbles) and catch (noted as individuals) on vertical 
longlines. Upper panel shows total backscatter together with longline catches of 
redfish ( Sebastes mentella ) (■) and gadoides (X); lower panel shows total backscatter 
and longline catches of blue whiting (Micromesistius poutassou) (X) and Greenland 
halibut ( Reinhardtius hippoglossoides) (■)). Data from August 2005. Diagonal line 
represents the bottom, and bubbles beneath this line are acoustic recordings from the 
bottom channel, i.e., from the water column from the bottom up to 10 m off the bottom. 
high vertical activity periods were comparable to depth 
distribution of vertical longline catches. 
Individual and seasonal differences in diurnal activity 
patterns could not be investigated further due to the low 
number of recaptures, and due to the unbalanced sex 
distribution of recaptured fish. However, diurnal vari- 
ability in bottom trawl catches associated with small 
individuals migrating into the water column at night 
time has already been reported off the coast of Labrador 
during summer (Bowering and Parsons, 1986). Such 
behavior is probably related to foraging, and should be 
expected to vary according to prey availability. This 
may explain why diurnal variability was not previously 
observed in bottom trawl catches of Greenland halibut 
from West Greenland waters (Jorgensen, 1997). 
The predominant prey species or prey groups in stom- 
achs of Greenland halibut captured with bottom trawl 
were herring, blue whiting, shrimps, amphipods, and 
the cephalopod G. fabricii. This finding is in agreement 
with that of previous studies from the Barents Sea-Nor- 
wegian Sea continental slope (Michalsen and Nedreaas, 
1998; Bjelland et al., 2000; Hovde et al., 2002). The 
acoustic surveys demonstrated that the vertical distri- 
bution of blue whiting overlapped with the distribution 
of Greenland halibut as determined from vertical long- 
lines. For herring, which were the most important fish 
prey, no overlapping distributions were evident. Herring 
were not recorded below 200 m depth during the trawl 
acoustic surveys — a depth that is shallower than any 
recording of Greenland halibut from vertical longlines, 
but archival tag recordings showed that Greenland 
halibut may be found higher up in the water column, 
up to above 100 m depth. 
A broad overlap could be seen between the vertical 
distributions of redfish and Greenland halibut. Despite 
this overlap, redfish occurred only in stomachs of fish 
larger than 75 cm. This finding may be due to the size 
selectivity of prey as seen in West Greenland waters, 
where Greenland halibut up to 64 cm total length fed 
only on redfish smaller than 15 cm, even though larger 
specimens were present (Pedersen and Riget, 1993). In 
our area, redfish smaller than 15 cm have been vir- 
tually absent because recruitment has been very low 
since 1990. 
