Some fish, such as the salmon, are born in freshwater in the upstream areas of rivers. Soon after they are born, however, they travel down the river to live in the sea. This change of habitat must also be accompanied by physiological changes allowing the fish to adapt to sea water (especially its salinity). This is smoltification.
Smoltification also results in physical changes (change of colour) and behavioural changes (such as grouping into schools of fish) which are all anti-predatory strategies.
Fish adapt to the saline environment using smoltification. © ZaNiaC, Flickr, cc by nc sa