The traditional conception in Chile of the toxic effect of metals on cultured fish tends to weigh heavily on incoming freshwater in open flow fish farms as the major responsible for mortality in early stages of development in salmon species. This approach is mainly based on two conditions that refer to freshwater quality in Chile, and to the dilutive effect of recirculation fish stocks when a peak of metals appears. For some years, the focus of attention on metal toxicity and the explanation of mortalities was attributed mainly to the entry of metals via the tributary (freshwater) in open-water fish farms and to a lesser extent in fish farms Of recirculation. As a consequence of the above considerations, the toxicity of metals in fish has mostly been evaluated in terms of the quality of incoming water, giving little attention to the effect of hydraulic dynamics within fish farms. In the latter case some advances (unpublished studies) have been made considering the effect of metal storage in recirculation fish farms originated by biomass or fixed organic masses (biofilter and other types of biofilm). At this level, the complexity is greater and relates not only to the quality of the water as a whole and at a given time, but also to microenvironments that are a consequence of hydraulic conditions derived from the design and effects or defects of operation. By way of example, unpublished studies by Aquaknowledge show that the concentration of metals in a fish pond (Recirculation) is higher in the center than in the middle sector where most of the fish population is located.
The same study showed the same trend for the case of Aluminum and also accumulation of Aluminum, Copper, Zinc and Fierro in fixed biomass as the biofilter and in filtration systems. This has positioned the metal stocking as another cause of fish mortality in recirculation fish farms. In this context the application of NaCl and / or seawater in recirculation fish farms plays a positive role since it increases the LC50 of metals facilitating the gill function. The above as it is incorporated without producing mixing zones in the system to avoid mortality risks. Consequently there are a large number of recirculation facilities that have incorporated sea water for these purposes. However, the present study shows the first results in the industry that suggest the need to take greater precautions when using seawater and / or adding salt in recirculation systems. The existence of deficient hydraulic conditions and the incorporation of metals through the addition of seawater or salt (previously considered innocuous) have been found to cause persistent mortalities in recirculation systems due to the presence of anoxic sectors and the appearance of Sulfuric Acid. Studies by Aquaknowledge (unpublished) have shown that the concentration of metals like Cu in some sectors of inland sea can reach 120 ug / L, which means a significant charge of this metal in case of being water entering a facility Of recirculation.