In seven Danish rivers 125 sea trout kelts were tagged with positively buoyant DSTs. Fifty-three (42%) tags were recovered, enabling a comparison of behaviour in kelts that survived the marine period and kelts that did not. These data were used to estimate kelt survival at sea and geolocate their migration routes. The forthcoming studies showed that Hidden Markov Models can be used for geolocation based on temperature and depth for sea trout.