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

Abstract

1. 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).2. 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 Søre Vartdalselva, and tracked the salmon until they died or left the river.3. This method identified the fates of 95% of tagged salmon. Estimated predation rates on adult salmon were 32% in Aureelva and 95% in Søre Vartdalselva.The salmon stock in Søre 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. 4. Salmon in Søre 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. 5. Our findings emphasise that management decisions should be guided by riverspecific 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.

The paradox of predation studies

“…This usually leads to establishing predation studies that attempt to document how large the 'predation problem' is. One such example is seen in Scandinavia, where the resurgence of Eurasian otter (Lutra lutra) populations has raised concerns among anglers over their potential impact on Atlantic salmon (Salmon salar) populations [27]. Another example is in western Canada, where there has been considerable debate over the culling of grey wolves (Canis lupus), driven by concerns about their impact on terrestrial ungulates [28].…”