| Summary: | Abstract European whitefish is well described and renowned for its polymorphism and faculty to occupy a wide range of freshwater habitats. This includes observations of individuals in some populations that utilize brackish estuarine habitats as feeding areas, although information on these populations is generally scarce. By use of acoustic tracking technology and physiological sampling, the behaviour and salt water tolerance of European whitefish was therefore studied in the estuary and the adjacent fjords of the River Neiden in Northern Norway where individuals have earlier been captured in the estuary area. The results showed that European whitefish utilized the estuary extensively as a feeding habitat, with the capacity to stay for short periods in salinity concentration up to 30 ppt and with a mean water temperature of 15 ˚C. A correlation was found between their horizontal distribution along the estuary axis and the salt water influx following the tidal variations; fish were mainly using the lower estuary at lower salinity concentration following the tidal level, and retreat upstream with the salt water influx under high tide. This study therefore demonstrated that European whitefish at it northernmost distribution area can utilize the estuary extensively as a feeding area. There was also indications of that at least a few individuals could possibly cope with relatively high salinity concentration over some time. This is the first study that has mapped the individual behaviour of European whitefish in an estuarine area over time and therefore provides a better understanding on the ability of the species to cope with salt water habitat. Given that these populations are genetically unique, they should be given special management emphasis.
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