Can UV-treated seawater cause cataract in juvenile cod ( Gadus morhua L.)?

Abstract

The results indicate that seawater recently disinfected with ultraviolet radiation (254 nm) can increase the frequency of cataract in juvenile cod. In a 5-month experiment where groups of cod were reared in a recirculating system at two stocking densities (54 and 300 fish/m 3), the frequency of cataract increased after the fish were exposed to UV-treated water. The tank with the water intake closest to the UV lamps (3 m) was affected the most, 39% of the fish developed cataract compared to 5% in the tank located furthest away from the UV lamps (33 m). The results suggest that there was some lag time between turning the UV light on and off and when the frequency of cataract stabilized. For each stocking density, the frequency of cataract decreased exponentially with increased post UV-treatment time, i.e., the time it took the seawater to run from the UV light to the rearing tanks. The calculated half-life of the effect was about 1 min at both densities. The effect of the UV light must have been indirect since the UV lamps were placed inside an opaque steel box. It is hypothesized that ozone or other photoproducts formed by UV radiation may cause cataract in fish. For a given post UV-treatment time, the frequency of cataract was about twice as high at high compared to low density suggesting that perhaps density-dependent abrasion of the cornea may enhance the effect of ozone or other photoproduct on the lens. In three tanks with low stocking density (55 fish/m 3) and clean seawater (flow-through tanks), the frequency of cataract was less than 1% although the incoming seawater was also disinfected with UV light. The high residence time in the flow-through tanks (471 min) compared to the recirculating tanks (37 min) may explain why the cod in them were unaffected by UV-treated water. The results imply that UV lights must be used with great caution in fish farming to avoid the development of cataract.