The response of young cod (Gadus morhua) to environmental gradients

The ability of fish to select an optimal habitat allows them to respond to inappropriate environmental factors, and in doing so to improve the physiological conditions. Fish react to almost every conceivable environmental factor. From the beginning, environmental temperature, dissolved oxygen concentration, and salinity are significant, because they di-rectly determine metabolic scope in fish and develop strong gradients in the seas. In the shallow waters of the Baltic Sea strong lack of oxygen exists in areas where young cod (Gadus morhua) grow. Continuous recordings of temperature, dissolved oxygen concentrations, and salinity in Kiel Fjord, Western Baltic Sea, show a strong coupling of these factors. Cod react to each of these variables; however, the preference for one finally determines the distribution of the fish within the area. The purpose of this paper is to examine and look into the response of juvenile cod as related to sudden changes of temperature and oxygen under laboratory conditions. The study aimed at a realistic simulation of both sum-mer and autumn conditions and the description of preferences in order to better understand habitat selection of young cod and the ultimate function of the choice in relation to their growth potential and condition.