Inter-population differences in growth and energy allocation of northwest Atlantic cod (Gadus morhua L.) revealed by common environment experiments

Purchase, Craig F. (1999) Inter-population differences in growth and energy allocation of northwest Atlantic cod (Gadus morhua L.) revealed by common environment experiments. Masters thesis, Memorial University of Newfoundland.

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Abstract

Geographic variation in life history traits is frequently observed among fishes. Although much of this variation has been shown to be based on environmental variability, genetic differences among populations have been found Atlantic cod (Gadus morhua L) occur throughout much of the north Atlantic Ocean. Factors such as growth rates vary tremendously among stocks, generally with faster growth occurring in warmer water. The relative contributions of temperature and genotype towards growth in cod however, is not known. Knowledge of this information could be useful in the management of cod stocks and in selecting populations for aquaculture. -- This thesis examined growth and energy allocation of cod from different populations, using common environment experiments. In the first experiment, larval cod from the Grand Banks (GB), and the Gulf of Maine (GOM) were reared under identical laboratory conditions to determine the effect of two temperatures on growth. Grand Banks larvae grew faster than GOM larvae at both temperatures tested. This supports the countergradient variation hypothesis, which states that higher latitude populations have greater capacities for growth rates than those at lower latitudes. -- The second and third experiments compared the effects of temperature on growth and energy allocation in juvenile cod from the GB and GOM, and juveniles from two inshore bays on the island of Newfoundland. Temperature significantly affected growth rates, food conversion efficiency, and % liver water content, but did not significantly affect condition factor, hepatosomatic index, or % muscle water content In contrast to larvae, no differences in growth rates were observed between juvenile GB-GOM cod, or between juveniles from the two inshore sites. This rejects the countergradient variation hypothesis in juvenile cod However, population differences in other traits (food conversion efficiencies, energy allocation) were found, suggesting genetic differences between the stocks.

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