Ups and downs of non-native and native stream-dwelling salmonids: Lessons from two contrasting rivers

Freshwater rivers and lakes are particularly vulnerable to anthropogenic disturbance because of their proximity to human occupation. Anthropogenic impacts in fresh water include those caused by dams, water pollution, channelization, and invasive species. One common action to conserve freshwater biodiversity involves supplementing or reintroducing extirpated species, though the benefits of these efforts are controversial. Installation of fishways to dams and removal of invasive alien species has also resulted in recovery of native species. Herein results of long-term monitoring of salmonids in two rivers in Japan are presented. The first case involves a river above a dam where native white-spotted charr Salvelinus leucomaenis and masu salmon Oncorhynchus masou declined rapidly in number following dam construction, where non-native rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta increased in number, and where stocking of native masu salmon was unsuccessful. The second case involves a river formerly dominated by non-native rainbow trout, in which native Dolly Varden charr Salvelinus curilus and masu salmon increased in numbers following removal of non-native species and fishway installation. Results reveal natural recovery of masu salmon in a river within which they were once completely extirpated, without stocking. Because salmonids are migratory fish and return to natal rivers, when locally extinct, human intervention to stock or reintroduce them has often been deemed necessary. However, restoration of wildlife would not require stocking or reintroduction if factors that impede the self-recovery capability of species are removed.

Automated summary

Freshwater rivers and lakes are vulnerable to human-caused disturbances due to their proximity to human occupation. Anthropogenic impacts including dams, pollution, channelization, and invasive species can significantly affect these ecosystems. This study in Japan presents the long-term monitoring results of salmonids in two rivers, showing the decline of native species and the increase of non-native species following dam construction and the unsuccessful stocking of native salmon. However, removal of non-native species and installation of fishways resulted in the recovery of native species in a river previously dominated by non-natives. This study highlights the potential for natural recovery of salmonids without the need for stocking or reintroduction if obstacles to self-recovery are removed.