Impacts of predation by Eurasian otters on Atlantic salmon in two Norwegian rivers

The return of the Eurasian otter (Lutra lutra) to western Norway has sparked human-predator conflicts because otters prey on vulnerable Atlantic salmon (Salmo salar) populations. Although predation may not be the ultimate cause of salmon population declines, otters that kill adult salmon in rivers before they spawn impact the salmon spawning stock, with potential consequences for stock recruitment and ecosystem services (especially fisheries). To gain insight into impacts of otter predation on salmon populations, we quantified the predation by otters on adult salmon in two rivers in western Norway using a combination of radiotelemetry and temperature loggers. We tagged 30 salmon in Aureelva and 30 salmon in Sore Vartdalselva, and tracked the salmon until they died or left the river. This method identified the fates of 95% of tagged salmon. Estimated predation rates on adult salmon were 32% in Aureelva and 95% in Sore Vartdalselva. The salmon stock in Sore Vartdalselva was well below the spawning target, partly attributable to putatively additive mortality from predation by otters. Notwithstanding, we found no evidence that otters selectively killed salmon based on sex, length, health status, or activity level. Salmon in Sore Vartdalselva had greater predation risk compared to salmon in Aureelva, possibly due to differences in habitat types such as availability of holding pools. The presence of more holding area in Aureelva probably provided predation refuges for adult salmon that buffered the effectiveness of otter predation. Our findings emphasise that management decisions should be guided by river-specific evaluations of impacts of predation on salmon. Otters are a very visible predator operating at the final phase of the life cycle before spawning, so predation is liable to be controversial given that salmon are now Red Listed in Norway.

Automated summary

The return of Eurasian otters to western Norway has led to conflicts with humans due to their predation on vulnerable Atlantic salmon populations. The predation by otters has a potential impact on salmon spawning stock and ecosystem services, but there is no evidence of selective killing of salmon based on characteristics. The presence of more holding areas in one river may provide predation refuges for adult salmon, suggesting that management decisions should consider river-specific evaluations.