Predation by Eurasian otters on adult Atlantic salmon

• Main Authors: Sortland, Lene, Lennox, Robert, Vollset, Knut, Velle, Gaute
• Format: Master Thesis
• Language: English
• Published: Universitetet i Bergen 2022
• Subjects: Norway; Atlantic salmon
• Online Access: https://hdl.handle.net/11250/3050886

The return of the Eurasian otter to western Norway has sparked human-predator conflicts as otters prey on vulnerable Atlantic salmon populations. Although predation may not be the direct cause of salmon population declines, otters that kill salmon in rivers before they spawn may impact the salmon spawning stock, with potential consequences for stock recruitment. Concerns of local people and stakeholders suggest that otter predation inhibits recovery of salmon populations. However, there is limited information on mortality caused by otter predation on adult salmon. To gain insight into impacts of otter predation on salmon populations, I quantified the predation by otters on adult salmon in two rivers in Western Norway using a novel combination of radiotelemetry and temperature loggers. I tagged 45 salmon in Aureelva and 30 salmon in Søre Vartdalselva and tracked the salmon until they died or left the river. This method identified the fates of 95 % of tagged salmon. Otters killed 9 tagged salmon in Aureelva and 20 tagged salmon in Søre Vartdalselva. I found no evidence that otters selectively killed salmon based on sex, length, health status or activity level, which suggests that predation mortality on pre-spawners was additive. Otter predation contributed in reducing both salmon populations below their spawning targets, and without otter predation both populations would have been closer to reaching their spawning targets. However, the magnitude of predation differed greatly between rivers. Salmon in Søre Vartdalselva had greater predation risk compared to salmon in Aureelva, possibly due to differences in the number of holding pools between the two rivers. The findings from this study emphasise that management decisions should be guided by river-specific evaluations of impacts of otter predation on salmon, for which the combination of radiotelemetry and temperature loggers can provide a valuable tool. Predation by Eurasian otters on adult Atlantic salmon publishedVersion