Survival, Migration Speed and Swimming Depth of Atlantic Salmon Kelts During Sea Entry and Fjord Migration

Abstract

In contrast to most species of Pacific salmon (Oncorhynchus spp.), Atlantic salmon (Salmo salar L.) is an iteroparous species such that it may survive and return to spawn repeatedly. Little information exists on these survivors (kelts) even though they might contribute significantly to salmon production when returning as repeat spawners. In order to estimate survival, timing of migration, swimming progression and swimming depth of Atlantic salmon kelts during sea entry and fjord migration, 60 individuals were captured, tagged with acoustic transmitters (of which 20 with depth sensors) and released in the River Alta, Northern Norway. In addition, 172 kelts were also tagged with external Carlin tags to obtain reliable recapture rates in the fisheries. Nearly all (95%) kelts tagged with acoustic transmitters were recorded during their outward migration at four transects of acoustic receivers deployed across the river mouth and Alta Fjord. Most of the kelts migrated through the 30 km long fjord in only 1–2 days (mean time 33 h, range 7–138 h) and generally stayed close to the surface during the fjord migration (individual mean depth of 2 m, range of individual means 0–15 m). Ninety-two percent (55/60) were recorded at the outermost transect 30 km from the river mouth, indicating a high minimum survival rate. The high survival rate and fast progression of kelts throughout the fjord indicate that sea-entry and early sea migration is not a critical phase for Atlantic salmon kelts, despite their weakened condition after spawning and overwintering in the river. Even though kelt migration overlapped partly with the fishing season both in the river and the fjord, and the kelts migrated in shallow waters exposed to several types of fishing gear, reported recapture rates were small (3%, 6 of all 232 tagged kelts).