Spawning season movements of transported landlocked Atlantic salmon in a newly restored river habitat

Certain spawning areas of the critically endangered Lake Saimaa landlocked Atlantic salmon (Salmo salar m. sebago) have been recently restored by excavator- and helicopter-scattered gravel, but the success of applied methodologies has remained open. Here, we monitored the spawning-related movements and redds occurrence of transported semi-wild and hatchery-reared salmon in the restored River Ala-Koitajoki, eastern Finland, using radiotelemetry and snorkeling. Nearly all fish accepted the newly restored spawning habitats in the river that maintains a low flow rate (only 4 [m.sup.3] * [s.sup.-1]). The movements of females released up to 2 weeks before spawning time were restricted mainly to the closest rapids from the release site, and most of the movements were directed downstream. Semi-wild and hatchery background fish exhibited similar mobility, which was more variable among males. A majority of redds were found in restored areas, and both helicopter- and excavator-scattered gravels were accepted as spawning substrates. Our results indicate that natural reproduction of the studied landlocked salmon population can be successfully returned to its original spawning river by restoration of habitats and transfer of spawners. Resume : Certains lieux de frai du saumon atlantique de lac (Salmo salar m. sebago) du lac Saimaa, qui est en voie de disparition, ont ete restaures a l'aide de gravier repandu par excavatrice et par helicoptere, mais le succes des methodes employees n'est pas encore etabli. Nous avons surveille les deplacements associes au frai et la presence de nids de frai de saumons semi-sauvages et eleves en ecloserie transportes dans la riviere Ala-Koitajoki, une riviere restauree de l'est de la Finlande, en utilisant la radiotelemetrie et la plongee avec tuba. Presque tous les poissons ont accepte les habitats de frai nouvellement restaures dans cette riviere qui maintient un faible debit (seulement 4 [m.sup.3] * [s.sup.-1]). Les deplacements des femelles relachees jusqu'a 2 semaines avant le moment du frai se sont principalement limites aux rapides les plus proches du site de lacher, et la plupart des deplacements etaient vers l'aval. Les poissons semi-sauvages et issus d'ecloseries presentaient une mobilite semblable, qui etait plus variable parmi les males. La majorite des nids de frai trouves etaient dans des zones restaurees, et tant les graviers repandus par helicoptere que par excavatrice etaient acceptes comme substrats de frai. Nos resultats indiquent que la restauration d'habitats et le transfert de geniteurs peuvent permettre le retour de la reproduction naturelle dans la riviere de frai d'origine pour la population de saumons de lac etudiee. [Traduit par la Redaction]
Introduction Restorations of the spawning and nursery habitats are widely applied to enhance ecological state of regulated and degraded rivers (Pennell et al. 2007; Marttila et al. 2019; Pulg et [...]