Interpopulation differences in growth rates and food conversion efficiencies of young Grand Banks and Gulf of Maine Atlantic cod (Gadus morhua)

Geographically separated Atlantic cod (Gadus morhua) stocks in the northwes t Atlantic exhibit life history variation and have been shown to differ gen etically. The genetic and environmental contributions to phenotypic differe nces, however, have not yet been measured. We used common environment exper iments to evaluate the importance of temperature on the observed growth var iation between Grand Banks (GB) and Gulf of Maine (GOM) cod stocks. Larvae from the GB grew faster than GOM larvae at both 7 and 12 degreesC. Growth r ates of juveniles were not different, but GB juveniles had higher food conv ersion efficiencies than those from the GOM (at both ambient and warm tempe ratures). The results indicate that faster growth of GOM cod in the wild is not due to a higher genetic capacity for growth rate in GOM than in GB fis h. The findings give evidence of genetically based phenotypic variation, wh ich is in agreement with molecular studies on population differentiation in cod, and support the theory of countergradient variation in growth rates o f larval fish.