Potential influence of increased sub-lethal UV-A irradiation on aquatic food production (Aquaculture).

In the tropics organisms have permanently adapted to deal with extremely high intensities of light, temperature and other environmental factors. Especially in coastal environments and fish farmings conditions can easily reach and exceed the tolerance limits of an organism. The UV-A band has been shown to be a potent modulator of light induced effects and general physiological functions in organisms. The biological endpoints and underlying mechanisms still have to be investigated. Our experiments were carried out with the easy to handle sub-tropical Convict-Cichlid fish (Cichlasoma nigrofasciatum) as a model organism. In our studies we observed that constant sub-lethal UV-A irradiation had a significant effect on the general metabolism and on the sensitivity towards other environmental stress factors in this fish. We found that, in addition to the depression of the general metabolism, sub-lethal UV-A irradiation in combination with elevated environmental temperature had a deleterious effect on the population. The threshold temperature for a sudden increase in mortality of the fish receiving the additional sub-lethal UV-A irradiation was 32 degree C. The fish were grown and adapted at 27-29 degrees C. A total of 78% of the fish receiving an additional UV-A irradiation died by high temperature stress while the reference population remained at a normal level (3%). The metabolic rate (MR) of the UV-A survivors during temperature stress was less than half the average value obtained in the reference population. After the environmental temperature returned below the apparent upper tolerance limit for this fish, the MR of UV-A population gradually increased to normal. Apparently a constant elevated sub-lethal dose of UV-A leads to the destabilization of an organism, making it more sensitive for other environmental stress factors. These observations seem to be important considerations in fish farming in the tropical and subtropical regions.