Fishes are sensitive to metals because of their osmoregulatory tissues on skin, gills, and kidney. Some of the metals facilitate the osmoregulation and others hinder it(especially heavy metals).

Salmon is full of toxins such as mercury, and fish farmers maintain it in unhealthy ways. It is known that fish are highly sensitive to bacteria and die as soon as they reach them. To avoid farmers, fish are given large amounts of antibiotics that penetrate the human body and cause many problems. . Cadmium and zinc alloys

Thanks salam

In a general way, fish are pollutant sensors. But it is the carnivorous fish, located at the end of the food chain, which have the capacity to concentrate in their organism the greatest quantity of chemical substances (bioconcentration), which are several times higher than those present in water and some of which are very harmful. This is possible by the progressive accumulation (bioaccumulation) of these substances in the tissues, which, once stored, are not eliminated. Thus, as we move up the food chain, heavy metal concentrations are increasingly important since they will correspond to the metals ingested from the first link in the food chain to the last one.

In this case, large piscivorous fish species are obviously the most sensitive.

In these fish, the bioaccumulation of heavy metals seems to influence the immune defense mechanisms by inhibiting, among others, phagocytosis whose role is to eliminate certain endogenous elements (dead cells, lesions) and especially exogenous (pathogens). These fish become very vulnerable especially on fish farms where overcrowding and containment promote the development of pathogens.

Thanks Oum for an explicit answer..