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Sonic tagged age-3 cod (28-33 cm) rested almost exclusively in rocky areas at night during winter (Clark and Green 1990). Between June and September, however, individuals were active nocturnally and wide ranging (>3 km/day), moving daily between deep (30 m) cold water, where they were inactive in rocky areas, to shallow (<15 m) sandy substratum where they were active at night in relatively small feeding areas (=540-2,580 m2). When the water column became isothermal in September, age-3 cod remained in the shallow water during daylight leading researchers to speculate that the switch from nocturnal to diurnal feeding might be an antipredator strategy, i.e., to avoid being cannibalized at night when adult cod are seasonally active in relatively shallow water. Other common predators of juvenile cod off Newfoundland are pollock (Pollachius virens) and shortfiruied squid (Illex illecebrosus).

Spatial Depth Gradient of Juveniles
For three years following stock collapse, Methven and Schneider (1998) undertook extensive sampling of the Newfoundland coastal zone to a depth of 55 m and by a variety of gears. Finding consistent spatial and diel changes in catch across gears, they interpreted results as characteristic of cod distribution. Catch rate of age-0 cod was inversely related to depth each year, highest at night, and higher at 4-7 m, the center of 0-group distribution during autumn.

There was a sharp decrease in catch rate at 20 m (Schneider et al. 1997). Demersal age-0 cod were found almost exclusively alongshore within the northeastern coastal bays of Newfoundland; yearlings extended further offshore and older juveniles were widely distributed on the continental shelf confirming an ontogenetic pattern of movement to deeper water with increasing size. Age-dependent distribution was also obvious from trawl station catches on survey transects extending from the coast to hundreds of kilometers offshore (Dalley and Anderson 1997). When the stock was more robust, demersal age-0 cod were distributed more widely onto the shelf.

The only coastal region of eastern Canada where the seasonal pattern of distribution for young cod appears to be different is the coastal portion of southern Gulf of St. Lawrence where water temperatures might be too warm during summer months (Hanson 1996). Fine scale distribution studies with trawls found that cod did not occupy water 2-12 m deep along shores of Prince Edward Island during summer. They were mostly absent from shallow waters (<20 m deep) in the Miramichi estuary and the contiguous Shediac Valley coastal shelf during any time of year Yearlings and 2-year-olds, but not age-0 cod, were almost exclusively found in 15-35 m depths of the Gulf from June to early October before joining older age-groups in an extensive migration to deep (>100 m) offshore water for winter.

The spatial depth gradient of juvenile cod from all other areas of eastern Canada seems consistent with published information from the Northeast Atlantic. The depth of highest age-0 cod abundance using a beam trawl off the British Isles was 6 m (Riley and Parnell 1984). Greatest density of age-1 cod sampled with gill nets off Greenland was <20 m (Hansen and Lehmann 1986; Hovgard and Nygaard 1990). Acoustic surveys off the Norwegian coast showed most juveniles at depths <35 m and highest densities of demersal0-group cod very close to rocky shores where the research vessel could not survey (Olsen and Soldal 1989).

Density-dependent Habitat Use and Mortality

Contraction or expansion of geographic range with decreasing or increasing population size has

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