Ecosystem effects of invasive domesticated Atlantic salmon (Salmo salar) and global warming

Abstract

Each year large numbers of escaped farmed salmon migrate in Norwegian rivers and some partake in spawning. Offspring with varying degrees of domestication inhabit river systems together with wild conspecifics. As climate warming is another factor that may act synergistically with biological invasions, I studied the impact of domesticated salmon in the context of warmer climate. Although the impact of farmed salmon has been studied extensively, there is little knowledge of the impact of ecosystem functions. In this study, using fully domesticated salmon, I quantified the impact of first feeding salmon in a controlled semi-natural experiment. I estimated the effect of wild and farmed, at todays and the predicted temperature in 100 years, on key stream functions. The effect on leaf-litter decomposition, primary production and macroinvertebrate community was quantified 35 days after salmon emergence. My results indicate overall small effects and sometimes in contradiction to what is expected from other studies. First, I established that farmed salmon are larger than wild salmon and that increased temperature increases final size differences in stream mesocosms. Final sizes were compared to salmon reared in indoor tanks, fed ad libitum. The farmed strain had accelerated growth indoor in comparison to in the more complex habitats, indicating poorer performance in the wild. Second, primary production and microbial decomposition rates were not affected by either treatment, while total decomposition decreased at warmer temperatures. Finally, consumptive or non-consumptive effects on the macroinvertebrate community was not evident in this study. A top-down control of food webs and ecosystem functions could not be established, and the effect of emerging salmon of either origin may be small at this developmental stage. The wild Atlantic salmon is influenced by multiple human stressors and environmental risk assessments should be concluded at ecosystem level. The findings here indicate small and uncertain effects. Yet, an ecological impact can occur later at later developmental stages as Atlantic salmon holds a complex life-history.