Failed protective effort of ex situ conservation of River Vistula trout (Salmo trutta) in Sweden

Abstract

Ex situ conservation comprises some of the oldest and best-known conservation methods and it has been applied for different fish stocks. This study describes attempts undertaken to preserve the long-migrating River Vistula (Poland) brown trout by a sea-ranching programme in the River Dalälven (Sweden). This focal strain was threatened in its native river and a major reason for selecting the River Dalälven was that its river mouth faces north as does that of the River Vistula. In the Swedish river, the Vistula trout in the 1970s and early 1980s returned early in the autumn and were generally large-sized (compared to the native strain of the River Dalälven). Over the study period (1978–2012), the size of returning adult fish declined, the return date becoming later in the year, and the number of spawners also declined. The reasons for these changes were probably caused by several interacting factors: (1) the most likely being high mortality of early returning fish which were held for longest in captivity before being used in the artificial spawning process; (2) the fact that the hatchery process was adapted to the Dalälven strain and not the Vistula one; (3) that the Vistula fish were marked by removing both pelvic fins (possible reducing subsequent survival); and (4) low genetic variation (as a result of few returning individuals). Thus, for several reasons, this ex situ conservation attempt for Vistula trout in Sweden is thought to have failed.