title: Long-term hypo-osmoregulatory capacity in downstream migrating Atlantic salmon Salmo salar L. smolts.
abstract: The duration of hypo-osmoregulatory capacity in downstream migrating Atlantic salmon Salmo salar L smolts previously stocked as startfed young-of-the year (YOY) parr was tested in the River Dalåa from mid-May to late-June 1999. Hypo-osmoregulatory capacity, measured as plasma osmolality and chloride, was assessed after seawater (SW) challenge tests (168 h, salinity = 35). All S. salar exhibited sufficient hypo-osmoregulatory capacity at the initiation of downstream migration in mid-May. Migrating S. salar smolts caught in mid-May and retained in fresh water displayed no signs of de-smoltification as they maintained hypo-osmoregulatory capacity through June. This indicates a physiological smolt window that lasts a minimum of 6 weeks (330 degree days; D°) for hatchery-produced S. salar smolts stocked as YOY parr. Based on the observed river migration speeds, it can be assumed that the S. salar smolts entered SW 2-4 weeks after initiation of migration in the upper parts of the River Dalåa. Hence, based on smolt migration and SW tolerance, it is suggested that stocking of YOY parr is a viable enhancement strategy in the River Dalåa.
authors.affiliation: Norwegian Institute of Water Research, N-7462 Trondheim, Norway.
authors.forename: H A
authors.initials: HA
authors.lastname: Urke
authors.forename: J V
authors.initials: JV
authors.lastname: Arnekleiv
authors.forename: T O
authors.initials: TO
authors.lastname: Nilsen
authors.forename: K J
authors.initials: KJ
authors.lastname: Nilssen
authors.forename: L
authors.initials: L
authors.lastname: Rønning
authors.forename: J B
authors.initials: JB
authors.lastname: Ulvund
authors.forename: T
authors.initials: T
authors.lastname: Kristensen
mesh_terms: D025041:Animal Migration; D000818:Animals; D002712:Chlorides / Q000097:blood; D006854:Hydrocortisone / Q000097:blood; D008274:Magnesium / Q000097:blood; D009664:Norway; D064587:Osmoregulation; D010949:Plasma / Q000737:chemistry; D045483:Rivers; D054712:Salinity; D020317:Salmo salar / Q000502:physiology*; D012623:Seawater
chemical_list: D002712:Chlorides; D008274:Magnesium; D006854:Hydrocortisone
keywords: behaviour; osmoregulation; salmonids; seawater tolerance; smoltification