The observations of Engas and Godo (1989b) regarding the greater degree to which larger gadoids are herded into the path of a bottom trawl are probably also applicable in the pelagic zone; one might also expect greater escapement of small fish through the rope wings of the midwater trawl.
Alton (Compiler), A workshop on comparative biology, assessment, and management of gadoids from the North Pacific and Atlantic oceans, part II, p.
Monitoring changes in abundance of gadoids with varying availability to the applied survey technique.
The conclusions of this study cannot be categorically accepted for all species in both commercial and research situations; however, there is no reason to doubt that the conclusions would hold for the majority of gadoid
species along the Atlantic coast.
Examples include dramatic fluctuations in gadoids
(Sirabella et al.
Figure 11 shows how catches of redfish and gadoids
on vertical longlines compare with the results from the acoustic surveys.
Capelin were sampled using an international young gadoids
pelagic trawl (IYGPT) mid-water-trawl towed in a single oblique haul from the surface to a 100-m depth.
(primarily walleye pollock [Theragra chalcogramma] and Pacific cod [Gadus macrocephalus] appear to have little or no herding response to the 83-112 Eastern trawl (Somerton, 2004) and rarely pass under the footrope (Somerton, unpubl, data).
Nocturnal vertical migration behavior has been described for gadoids
such as hake and cod and is considered responsible for the reduction of trawl catches of these fish at night (Beamish, 1966; Bowman and Bowman, 1980).
Monthly growth rates for gadoids
in temperate areas often vary seasonally (Jorgensen, 1992; Hayes, 1993).
1994) and density-dependent changes (Crecco and Overholtz, 1990; Rose and Kulka, 1999) in catchability of gadoids
have been indicated when commercial CPUE is being used as an index of abundance.
Similar size-dependent differences in the distance of seasonal migrations were reported for Pacific hake (Merluccius productus), another gadoid
from the north Pacific (Dorn, 1995).