When the threshold values for feed are stipulated, how much of the substance is discharged to the environment is also taken into account. The substances that the fish cannot efficiently absorb, but that are excreted through fish excrement, can impact the environment. NIFES studies whether the substances in the feed are absorbed by the fish to a lesser or greater extent. Some metals, such as cadmium and copper, are poorly absorbed by the fish, pass through the fish’s intestines and are discharged to the environment.
Change in threshold values
A trial showed that the fish do not absorb copper to any great extent, but that it is discharged to the environment. The maximum limit for copper in feed was therefore reduced. In addition, it was found that too much copper in the feed was negative for fish health.
When the composition of the ingredients in the feed is changed, this will also alter the digestibility of some substances, and the discharges to the environment could therefore also change. This is one of NIFES’s research areas.
Phosphorus and environmental impact
Inorganic phosphorus is added to the feed to cover the nutritional needs of the fish, but because its absorption is inefficient, more phosphorus will be discharged to the environment when more phosphorus is added. It is therefore important to find a balance between the nutrient requirement of the fish and the environmental impact, in addition to increasing our knowledge about what forms of phosphorus the fish can absorb more efficiently. This way, phosphorus in the feed can be optimised so that the fish absorb as much phosphorus from the feed as possible and as little as possible is discharged to the environment.