The transfer represents a substantial change in the surroundings of the fish, that must go from living in an environment where there is good control over what is in the water it lives in, to an environment with undreamed of quantities of infection agents and environmental influences.
In addition, the fish must completely rebuild its physiology from being adapted to living in freshwater to being a saltwater fish. This requires adaptations throughout the juvenile fish phase, and a precise calculation of the correct exposure time to ensure the best possible transition for the fish.
From nature's perspective, salmonids are adapted to living parts of their life in freshwater, and parts of their life in saltwater. In the wild, they begin their lives in rivers, and live there until smoltification has been completed, when they migrate out to sea where they stay until they reproduce. They then return to the rivers where they grow up to spawn.
In a fish farm situation, we simulate this life cycle by keeping the fish in freshwater in hatcheries and then transfer them to seawater facilities. We do this within a considerably shorter period than in nature, and there is therefore a short period when it is the perfect time to transfer the fish to the sea. We call this period the "smolt window". If this smolt window is missed, the fish will have problems with adapting to their new environment.
If the fish have been exposed to health problems in the juvenile fish phase, they can be poorly equipped to deal with the transition to the sea. For this reason, the operating conditions in the juvenile fish phase have also been important for those who operate fish farms in the saltwater phase.
The process whereby the fish go from being adapted to a life in freshwater to being in seawater.
In freshwater, the inside of the fish will be saltier than the surroundings. This means that water is being forced into the fish all the time, and to maintain its fluid balance, the fish must excrete water via diluted urine, and gather as much salt as possible from the surroundings via pumps in the gills, and from feed via the intestines
In saltwater, the inside of the fish will be less salty than the surroundings. This means that water is constantly seeping out the fish and it is in danger of dehydrating. This means that the fish must drink large quantities of water and excrete concentrated urine. It also actively excretes salts through the gills via the NA/K and chloride channels in the gills. It is the activity in this respect that we measure when we check whether the fish is ready to be put out to sea.