Primary stress responses of common carp, Cyprinus carpio, exposed to copper toxicity

Background. Copper is a heavy metal, and an aquatic pollutant, known for its bio-accumulative and non-biodegradable properties. In the aquatic ecosystems, acute and sublethal concentrations of copper may be linked to a variety of effects. Recently, hormones, particularly those regulating vital functions, such as osmoregulation, energy metabolism, and reproduction, may be used as potential biomarkers for sublethal toxicity studies. In the present study, the potential effect of a heavy metal—copper on hormonal changes (cortisol and prolactin) in an economically important fresh-water fish—common carp, Cyprinus carpio, was examined. Materials and methods. The experimental fish were subjected to two experimental regimes (backed by controls). In the first treatment they were exposed to the acute concentration of copper sulphate, amounting to 0.7 ppm. The second treatment featured copper sulphate concentration of 0.07 ppm, constituting 10% of LC50 (24 h). The acute-toxicity trials were carried out in two, 20-L, circular plastic tubs. Twenty fish from the tank were selected randomly and introduced to each tub. Control was maintained in 2 similar plastic tubs with 20 fish per tub. After 24 h, fish from control- and copper-exposure tubs were taken for analyses. To observe the sublethal toxicity four, 125-L, glass aquaria, filled with clean water were used. 200 fish were randomly selected from the stock and 100 of them were added to two aquaria, 50 fish in each, as experimental fish and 100 in two other aquaria, 50 in each, for as control fish. By the end of the stipulated period, 20 fish from control and 20 fish from experimental group were used for the hormone assay. Results. In both acute- and sublethal treatments, both cortisol- and prolactin levels increased. In sublethal treatment, however, plasma prolactin level decreased after 28-day exposure, showing a minimum percentage point decrease of 3.84 by the end of 35-day trial. Conclusion. The increase of the plasma cortisol was probably caused by release of cortisol from the interrenal tissue, as a mechanism of coping with stress. Significantly lower content of prolactin levels in sublethal treatment could be an indicative of a possible restored hydromineral balance or atrophy of the pituitary prolactin cells leading to inhibited prolactin secretion of the fish. These alterations of the above hormonal changes may be used as stress biomarkers in fish.