The blue dye makes the fish’s red blood cells shrink and get wrinkled, which probably stops them from carrying oxygen properly.

The blue dye causes harmful chemical reactions in fish cells that damage their blood, and the more damage there is, the lower the blood’s oxygen-carrying ability becomes.

Scientists can tell if fish have been exposed to this blue dye by checking their blood — changes in red and white blood cells are clear warning signs.

When rohu fish are exposed to a low but toxic level of a blue dye used in fish farms, their blood loses a lot of its oxygen-carrying capacity, making them very weak.

Fish exposed to this blue dye show a spike in certain white blood cells and a drop in others, which means their immune system is stressed or not working normally.