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To test the countergradient variation (CnGV) hypothesis and determine any genetic dif- ferences among populations, we conducted an experiment with juvenile cod originating from 3 regions in the NW Atlantic, 3Ps (Placentia Bay, Newfoundland, Canada; 48° N, 54° W), 4T (southern Gulf of St. Lawrence; 46° N, 61° W), and 4X (44° N, 67° W), at 2 temperatures (7 and 11°C) for 15 wk. Results indicated that temperature influenced the growth rates of individuals in all 3 populations, and juveniles reared at 11°C were significantly bigger (length and weight) than those reared at 7°C. No significant differences in growth and food conversion efficiency were found among the 3 populations. Although 3Ps juveniles were significantly smaller in length and lighter in weight than 4T and 4X juveniles at the start of the experiment, length and weight did not differ among populations at the end of the experiment. Survival of both 3Ps and 4T juveniles was significantly higher than that of 4X cod in both high and low water temperature treatments. Although statistical analyses showed no sig- nificant differences in growth and food conversion efficiency among juveniles from the 3 populations, reaction norm analysis showed a strong genotypic influence on growth and survival (no genotype × environment interactions) but a genotypic × environment interaction in food conversion efficiency. Our results suggest that Atlantic cod juveniles from these 3 regions do not show CnGV in growth and food conversion efficiency. However, performance differences among populations may be stage specific, and this may have masked CnGV in traits among Atlantic cod populations in our study.

Possible countergradient variation in growth of juvenile cod Gadus morhua from the northwest Atlantic