The Response Of Young Cod (Gadus Morhua) To Environmental Gradients

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The ability of fish to select an optimal habitat allow to respond to inappropriate environmental factors and in doing so to improve the physiological conditions. Fish react to almost every conceivable environmental factor. Environmental temperature, dissolved oxygen concentration, and salinity are significant, because they directly 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. Especially in the areas of the Belt Sea and the Kiel Bay, the water temperatures as well as salinity contents drastically fluctuate within a couple of hours (Gerlach & Kortum 1992); oxygen deficit is a phenomenon that has always been observed (Ruppin 1912; Gerlach 1984, 1988; Ruhmohr 1986; Babenerd & Meyerhöfer 1988) Continuous recordings of temperature, dissolved oxygen concentrations, and salinity in Kiel Bay show a strong coupling of these factors, especially in the stratified water column during summer. Cod react to every of these variables; the preference for one finally determines the distribution of the fish within the area. Studies of Schurmann and Steffensen (1992, 1994) show the response of cod to various combines of temperature and dissolved oxygen concentrations and metabolic response (1997). The hypoxia tolerance was investigated by Chabot et al. (1999) and Plante et al. (1998). However, the results do not explain how cod react to a stratified water column with high temperatures and high dissolved oxygen concentrations in the surface water and low temperatures and dissolved oxygen concentrations in the bottom layer. The purpose of this study was to examine the response of juvenile cod as related to typical changes of temperature and dissolved oxygen in Kiel Fjord (Kiel Bay, Western Baltic Sea) under laboratory conditions. The study aimed at the description preferences (temperature, dissolved oxygen) in order to better understand habitat selection of young cod. This paper presents preliminary results of experimental investigations that will be continued.