During the breeding season, the species occurs below 900 m in forested, clear, fast-flowing mountain rivers and rapid streams, generally far from human habitation (BirdLife International 2001, Kear 2005, Carboneras and Kirwan 2020). The most important factors in the selection of breeding habitat are water transparency and the presence of natural tree cavities for nesting (Solovyeva et al. 2017). The full extent of the species' movements are still not understood, but the main migration routes appear to pass through the Korean Peninsula (where some individuals remain for the winter) and over the Yellow Sea to mainland China (Solovyeva et al. 2012, Dong-Ping et al. 2014).

Habitat use is slightly more varied in the non-breeding range, with the species also occurring on lakes, reservoirs, and slow-flowing rivers and lagoons, with a small percentage of the population also undergoing moult migration in brackish and marine waters; in China and the Korea Peninsula, it often remains in mountainous, fast-flowing rivers typical of the breeding habitat (Brazil 2009, Barter et al. 2014, Solovyeva et al. 2014b).

The species feeds on various fish, frogs, and insect larvae (Solovyeva et al. 2017).

The majority of the population breeds in Far Eastern Russia, primarily Primorsky Krai and Khabarovsk Krai, with the remainder breeding in Heilongjiang, and the Changbai Mountains, Jilin, in China, likely extending over the border to DPR Korea (Solovyeva et al. 2014a, Solovyeva et al. 2017, eBird 2025). Most of the non-breeding range occurs within the Yangtze River catchment in central mainland China, with smaller numbers occurring in DPR Korea and South Korea, as well as Japan, Taiwan (China), and as far south as Hong Kong (Barter et al. 2014, Solovyeva et al. 2017, Chikara 2019, Moores 2022, eBird 2025). The species largely passes through peninsula Korea and over the Yellow Sea during passage, but records indicate it may also be found in northern Japan on passage, and potentially throughout the Chinese mainland and along the eastern coastline (Solovyeva et al. 2012, Dong-Ping et al. 2014, eBird 2025).

Conservation Actions Underway
CMS Appendix II. There are 28 protected areas containing highly suitable habitat for this species in Russia (8 local and 11 national) and China (4 local and 5 national; Xu et al. 2021). An international action plan for the conservation of this species has been adopted within the framework of the East Asia-Australasian Flyway Partnership.

An artificial nest programme in Russia, involving the provision of over 300 artificial nest sites (Solovyeva 2021) has shown positive results, increasing habitat capacity along rivers with logged flood-plains (Solovyeva et al. 2013, D. Solovyeva in litt. 2018). The programme involves the continued maintenance of artificial nests, liaison with hunters and fishermen, and collaboration with local communities, including information and education activities and the construction of a research and visitor centre (D. Solovyeva in litt. 2018). This programme has already resulted in a change in fishing practices by local people, and has also facilitated the capture of females for tagging with geolocators, allowing the identification of staging and wintering sites (Cranswick 2010, Solovyeva et al. 2012). Collaboration with General Motors has resulted in the re-use of Chevrolet Volt battery covers as successful nesting boxes in the Changbai Mountain National Nature Reserve in China (General Motors Green 2015). In 2015, 11 ducklings hatched from the Chinese nest box programme (Cranswick in litt. 2015).

The Scaly-sided Merganser project has been running in Primorye since 2000 and has collated information on the threats facing the species (Solovyeva 2013). An International Action Plan (2016-2025) has been developed for this species (Solovyeva et al. 2017).

Conservation Actions Proposed
Gap analysis revealed that 10% (5,967 km²) of the global highly suitable breeding habitats were within existing protected areas, leaving 90% (51,651 km²) unprotected (Xu et al. 2021). Setting a goal of 40% of highly suitable habitats to be protected, with an additional 10,414 km² of protected areas in Russia and 6,596 km² in China is suggested. Among the priority areas to be protected in Russia are the Pavlovka and Zhuravlevka river catchments and watershed in Chuguevsky Districts of Primorye (c. 3,600 km²) and the enlarging of the existing Udygeiskaya Legenda National Park with upper reaches of the Iman River. In China, to minimise impacts of human activities, prioritising conservation action and land management along the Fuer, Songjianghe and Manjiang rivers (i.e. key habitats overlaid with human developments) will likely be required (Xu et al. 2021).

Recommendations for research priorities and conservation actions are provided in Solovyeva et al. (2017), including: research into breeding numbers, range, and staging and wintering areas in DPR Korea, habitat use and key wintering sites in China, the impacts of dams on winter distribution and foraging, annual survival rates, and the impacts of pollution; as well raise awareness of all threats, reduce the use of dams in sensitive areas, promote legislative action to reduce the impacts of pollution, reduce dredging in sensitive areas, increase knowledge of the species with hunters and prevent all hunting including with increased enforcement, and take steps to minimise bycatch and reduce disturbance (e.g. better guidance for boaters) during brooding, and manage a captive-breeding programme to maximise genetic diversity, including support for zoos for in-situ conservation. Research and monitoring into population sizes/trends should also be continued.