Sonar counting of the River Neidenelva salmon in 2022

• Main Authors: Kytökorpi, Mikko, Orell, Panu, Pohjola, Jan-Peter, Erkinaro, Jaakko
• Other Authors: orcid:0000-0001-5550-9179, 4100412310, 4100111210, Luonnonvarakeskus
• Format: Report
• Language: English
• Published: Natural Resources Institute Finland (Luke)
• Subjects: Salmo salar; run size; monitoring; Aris; Näätämöjoki; Neidenelva; Luke; Skoltefossen
• Online Access: https://jukuri.luke.fi/handle/10024/553326

Biological reference points in a form of spawning targets have been estimated and established for the whole Neidenelva system in 2022. These spawning targets give now a biological goal, number of female salmon, that should be annually reached to ensure the long-term viability of the Neidenelva salmon populations. For estimating the spawning target attainment, information on the salmon run size together with salmon catches are needed. To estimate the salmon run size, all fish ascending to the River Neidenelva were monitored during the summer 2022, for the first time ever, by using an ARIS-sonar close to the river mouth. Sonar monitoring was a joint Finnish-Norwegian project and was executed by the Natural Resources Institute Finland (Luke). Underwater video cameras were used in parallel with the sonar for species determination. The total salmon run estimate in 2022 was c. 6900 individuals. Proportion of small (50–65cm), medium (65–90 cm) and large salmon (≥90 cm) were 49 %, 47 % and 4 %, respectively. Salmon migration was most active from mid-June to mid-July, whereafter the migration activity decreased significantly. In addition to salmon c. 300 sea trout ≥45 cm was estimated to ascend to Neidenelva. Significant numbers of large whitefish were also detected at the monitoring site, but their numbers were not estimated because of frequent back-and-forth movements. Based on long-term catch statistics and salmon counts at the Skoltefossen fishway, the salmon season 2022 was estimated to be poor. If the current relationship between the sonar count and catch/fishway data was used as a predictor, the salmon run sizes could have been c. 3 times higher (c. 20 000 salmon) in the best years during the period 2006–2022. Overall, the salmon run estimate contains some significant uncertainty. First, high incidence of back-and forth swimming complicated the counting process. Secondly, it was estimated that proportionally more downstream migrating fish are undetected compared to upstream migrating fish, i.e., the total run ...