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Report2023

Spawning migration of salmon and sea trout in the Tornionjoki river

Huusko, Riina; Hellström, Gustav; Jaukkuri, Mikko; et al.

Abstract

In this collaborative project between the Natural Resources Institute (Luke) and the Swedish Agricultural University (SLU), the migratory behaviour and survival of Tornionjoki (Torneälv in Swedish) salmon and sea trout were studied between 2018‒2021 by the means of radiote-lemetry. Altogether, 227 and 92 salmon were tagged at the Tornionjoki estuary and in the river, respectively. 114 sea trout were tagged in the river. Scale samples and fin clips for age-ing and genetic identification were taken from all the tagged specimens. The external condi-tion of the tagged specimen was also documented (wounds, skin colour, degree of haemor-rhage etc.). Moreover, a separate follow-up of the external condition of salmon caught in trap nets was conducted in 2020‒2021 at sea near the river mouth.
The post-release behaviour of salmon tagged at the estuary was markedly different from that normally expected: a large majority (61% and 83% in 2018 and 2019, respectively) of the salmon which ascended the river after tagging aborted their riverine migration on the lower river and returned to the sea during the summer (i.e., before spawning season). Those salmon which stayed in the river until spawning time predominantly stayed on the lowermost 100 km of the river. More varying migration patterns were observed among the salmon tagged in the river. All specimens caught and tagged during the early summer of 2018 and 2019 started to drift downstream after their release and none of them was alive in the river at spawning time. However, about half of the specimens tagged in the river in early summer 2020 and 2021 continued their upstream migration and were alive in the river at spawning time. Salmon tagged in late summer 2018‒2020 stayed alive in the river and almost half of them also moved further upstream by spawning time. A large majority of salmon overwintered in the river after spawning and returned to the sea in spring. The majority of the salmon caught in the estuary had various external damages (wounds, scale losses, fin damages, and skin haem-orrhage). Most of the damages, however, were regarded as minor. No correlation between the occurrence of damages and the post-tagging behaviour of salmon could be detected.
Based on the data obtained from tagged sea trout, two distinct groups of trout were recog-nised: (1) non-mature trout which ascended the river in autumn and returned to the sea in spring after overwintering in river, and (2) maturing trout which ascended the river in autumn, overwintered in the river, and continued their upstream spawning migration the following summer. Specimens belonging to either of these groups typically overwintered in the same short lowermost stretch of the river, although some of the maturing trout overwintered fur-ther upstream. At spawning season, tagged trout were located both on the main stem (Torni-onjoki and Muonionjoki rivers) and in several tributaries (Naamijoki, Äkäsjoki, Parkajoki, Paka-joki and Merasjoki rivers). After spawning time, trout which were observed in the tributaries usually moved back to the main stem where they overwintered and descended to the sea the next spring. Both the immature and the maturing overwintering trout descended to the sea at almost the same time in spring.
The results of the project highlight the sensitivity of salmon to handling at/around the time of their river ascent in early summer. This sensitivity is likely linked to the recent health problems observed among Tornionjoki salmon and may have induced the unexpected (and seemingly maladaptive) migratory behaviour of salmon observed in the study. The in-river and sea to river movements observed for the Tornionjoki sea trout provides very useful information for efforts to protect this species and strengthen its stock status. In general, mature Tornionjoki sea trout have a two year in-river migratory cycle in connection with spawning, and hence spend a large majority of their life in the river, which underlines the need for good management of the riverine environment and river fisheries.

Keywords

Spawning migration; salmon health; overwintering; anadromous trout; Baltic salmon; arctic river; Kutuvaellus; lohen terveys; talvehtiminen; anadrominen taimen; Itämeren lohi; arktinen joki; Lekvandring; laxhälsa; övervintring; anadrom öring; Östersjölax; arktisk flod

Published in

Natural resources and bioeconomy studies
2023, number: 2023:29ISBN: 978-952-380-651-1, eISBN: 978-952-380-652-8
Publisher: Natural Resources Institute Finland (Luke)