Assessing the long-term causal effect of trout invasion on a native charr

Abstract

Many studies have indicated that invasive species can drive declines in native species through interspecific competition. In freshwater ecosystems, brown trout (Salmo trutta) and rainbow trout (Oncorhynchus mykiss) have been implicated in reducing native species, especially congeneric salmonid fishes. However, demonstrating causality in the wild is problematic because non-natives might replace natives in areas where the former were extirpated by other environmental changes. Using the recently developed technique of multispatial convergent cross mapping (CCM) and a long-term monitoring data set (2002–2017) with spatial replication on both introduced and native salmonids in a Japanese stream, I tested whether an increase in non-native trout caused a decrease in native charr (Salvelinus leucomaenis). Native charr decreased their population density over time in contrast to the non-native brown trout and rainbow trout. The dominant species changed from native charr (64%) in 2002 to non-native trout (97%) by 2017. Multispatial CCM identified significant causal forcing of the native charr by both rainbow trout and brown trout, lending support for the hypothesis of displacement, rather than replacement, of a native species by non-native species. These results therefore suggest that eradicating the invasive trout species may aid the recovery of native charr in the region.