Annual bottom‐trawl surveys (1990–2010) were used to examine associations between environmental conditions, spatial distribution, and size‐specific abundance of Greenland halibut Reinhardtius hippoglossoides in the estuary and Gulf of St. Lawrence (EGSL), and to test the influence of hypoxic conditions on habitat selection. Size classes representing juvenile, immature and adult fish were used for the analyses. The highest concentrations of fish were found in the St. Lawrence estuary at both high and low levels of stock abundance. The areas occupied by 50, 75, and 95% of juvenile fish expanded with higher population abundance. However, contrary to our predictions, densities in marginal habitats did not increase at a higher rate than in optimal habitats. Fish longer than 32 cm were distributed over a broader area than juvenile fish. Their abundance explained a limited proportion of the variability in spatial distribution. The spatial dynamics of Greenland halibut in the EGSL is best described by a proportional density model where the rate of increase in local density is associated with population abundance. Habitats selected by Greenland halibut were characterized by low dissolved oxygen (DO) levels. The strong association between high fish densities and low DO concentrations indicates a high tolerance of Greenland halibut to hypoxia. It also indicates that negative effects, if present, could be compensated by other factors such as food availability and/or refuge from predation. The results of this study also clearly define the St. Lawrence estuary as the major nursery area for the EGSL population.

Greenland Halibut (Reinhardtius hippoglossoides), especially juveniles, are abundant in the St. Lawrence estuary, where oxygen levels are very low (18 25% saturation). Current data suggest that juveniles may be sedentary in this region. We investigated the relative importance of oxygen for juvenile growth in different areas occupied by juveniles in the estuary and Gulf of St. Lawrence (EGSL). More specifically, we examined the mean size-at-age for 1-and 2-year-old juveniles as well as the growth rate in different areas in relation to oxygen, depth, temperature, and juvenile density. Overall, oxygen concentration was found to affect juvenile Greenland Halibut growth: growth rate varied inversely with dissolved oxygen levels and significantly decreased when oxygen conditions were below 80 mol/L (~ 25% saturation). Temperature did not affect juvenile growth rates within the range found in these areas (4.95-and 2-year-old juveniles (17 and 27 cm, respectively) as well as length increment estimates from 1 to 2 years old in the EGSL were much higher than those observed in other populations of Greenland Halibut. Length increment from ages 1 abundance of juveniles in bottom waters characterized by low oxygen concentrations and also found that there was continuous juvenile growth over the year; these observations suggest that the negative impacts of dissolved oxygen should be limited and/or largely compensated by other physical or biological characteristics of the EGSL, such as food abundance, food availability, and/or predator density.

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