The present study investigated the ultramicroscopic alterations (SEM) on the eye, skin, and fin of spawn of Catla catla exposed to UV B doses of 2.11 × \({10}^{-21} J\) /m2, 1.05 × \({10}^{-21} J\) /m2 and 7.05 × \({10}^{-22} J\) /m2 under different time exposure durations i.e., 5, 10, and 15 min/day for 5 and 10 days. Control limnological parameters were maintained during the total experiment, and there was no mortality during the acclimatization periods. Scanning electron microscopic (SEM) observations revealed no topological alterations in control condition of C. catla. Ultraviolet B radiation (UV B) radiation on eye showed the squeezed and dome-shaped structure of the retinal eyeball, degradation of stratified epithelial cells, washed out retinal pigmented epithelium, existence of guanine crystals, and elimination of collagen fibers and optic nerves. In skin, it displayed injury to the stratified epithelial cells, emergence of microfolds, depositions on mucous pits over the skin surface, hyperpigmentation, presence of guanine crystal, degeneration of body striations and the development of deep channels over the skin surface. Results on fin showed disarrangement of fins and finlets, broken ray formation, necrosis in disorganised stratified epithelial cells, deformation in collagen fibers, and structural degradation on fish body surface. UV B exposure significantly limited /obstracted the growth and survival of C. catla spawn. Application of balanced diets, including natural and some protective measures may be recommended to protect the damage from UV radiations.
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