The species breeds on large clear freshwater lakes, pools, the upper reaches of rivers (Billerman et al. 2020) and streams in the boreal, montane (Kear 2005) and temperate forest zones (Johnsgard 1978). It requires waters with a fairly high productivity of fish surrounded by mature hardwood trees with holes excavated by woodpeckers or natural cavities for nesting in (Kear 2005). The species winters on large unfrozen lakes, rivers, lagoons, brackish waters and marshes, generally avoiding highly saline waters (Madge and Burn 1988) although it may move to estuaries, coastal lagoons and sheltered sea coasts with waters less than 10 m deep in particularly harsh winters (Scott and Rose 1996). The species arrives on its breeding areas between March and May (Scott and Rose 1996), actually breeding as early as late-March (although often considerably later in more northerly regions) (Madge and Burn 1988).The species nests in holes excavated by large woodpeckers or natural cavities in mature hardwood trees with entry holes more than 15 m above the ground (Kear 2005). When natural tree-nesting sites are not available the species will use artificial nestboxes (Johnsgard 1978, Billerman et al. 2020) or may nest in rock clefts (Flint et al. 1984), among tree roots in undercut banks, on cliff ledges or in dense scrub or loose boulders on islands (Kear 2005). Clutches are usually eight to twelve eggs. Its diet consists predominantly of fish, but may also include aquatic invertebrates (such as molluscs, crustaceans, worms, and adult and larval insects), amphibians, small mammals and birds (Billerman et al. 2020). Northern breeding populations of this species are fully migratory (Snow and Perrins 1998) although breeders in temperate regions are sedentary or only travel short distances (Billerman et al. 2020).

Although the generation length for both EU and Europe regional assessments were calculated using the same methodology, new information arriving after the EU assessments were undertaken gave rise to an update in the generation lengths. This new information was then used for the Europe level assessments giving rise to a difference between the generation lengths used for the EU and Europe regions.

In Europe, the species breeds primarily in Sweden, Finland and Russia, with significant numbers in the United Kingdom and Norway.

Conservation Actions Underway
CMS Appendix II. EU Birds Directive Annex II. There are currently no known conservation measures for this species.

Conservation Actions Proposed
The erection of nest boxes may encourage the use of local areas by this species (Kear 2005). Future research to inform the conservation and management of this species should include investigations into its breeding requirements, winter habitats and population size (Mallory and Metz 1999). Strict legislation on petroleum drilling and transport should be enforced and important areas protected from drainage and other habitat modifications. Research into the impact this species has on fish stocks and ways to minimise conflict should be established.