Hello,

Fish cultured in open-pen sea cages or recirculating aquaculture systems are presumed to be free of stress for the piscifauna cultured when that is far from truth. The primary cause of cataract in aquatic organisms such as fish is the over exposure to ultraviolet and other electromagnetic radiation. The nature of cataract whether sub-capsular or nuclear however, depends on more specific reasons and it hasn't been specified in the question.

In sea cages particularly the fish are more exposed to UV rays from the sun compared to their natural habitats where they could dive below the depth of maximum attenuation of UV radiation (few millimeters to meters based on the clarity of the water and the position of the Sun). And the fish are under stress continuously for not being able to move to a different location which they easily decipher owing to their innate geomagnetic guidance systems.

The RAS provides them with biologically filtered ammonia reduced, highly oxygenated water but the residual increase in the dissolved nutrients slowly builds up toxicity in any well aerated/ filtered and circulated aquaria and the stress caused by the confinement and the chemicals may contribute to cataracts and other signs of stress.

In both sea cage and RAS the water clarity is mainatained at a high and the the absence or removal of suspended particulate matters may also contribute to lesser attenuation of the radiation through scattering. In both the systems reduced activity or repetitive actions may also trigger a rise in the body cholesterol in the fishes which also is a well known medical cause of cataract formation.

Stress combined with UV exposure according to my understandings play key roles in cataract developments in cultured fishes.

Regards,

Dr. Abhishek Mukherjee

Thank you for your informative answer. We raised seabream and seabass fish in the same conditions and same water but still the cataract is appeared only in seabass. Does that mean seabass is more sensitive to these factors?

Do histidin amino acid or iron and zinc had any role in ameliorate this symptom?

They could have, but without proper evidence nothing shoud be left for speculation