- Lifespan
- 8–10 years in the wild, up to 15 years in captivity
- Size & Weight
- Body length 57–95 cm (including 35–45 cm tail); weight 7–12 kg
- Habitat
- Fast-flowing rivers, streams, and coastal waters with good water quality and adequate fish stocks, requiring undisturbed bankside vegetation and den sites.
- UK Distribution
- Widespread across Scotland, Wales, and south-west England; expanding range after near-extinction. Resident year-round in suitable habitats throughout the UK.
- Diet
- Primarily fish (trout, salmon, eels, and cyprinids), supplemented with crustaceans, amphibians, and occasionally small mammals and birds.
- Prey
- Fish (especially eels, brown trout, and salmon), freshwater crayfish, frogs, and occasionally water voles and ducklings
- Predators
- Adults have few natural predators; young are vulnerable to foxes, pine martens, and golden eagles; humans historically the main threat
- Mating Season
- December to March, with a peak in January–February
- Breeding
- Litter size 1–4 (average 2–3) kits; gestation 60–63 days; kits remain in den for 8–12 weeks; typically one litter per year
- Behaviour
- Solitary and primarily nocturnal, though increasingly active during daylight. Highly territorial with home ranges of 5–40 km depending on habitat productivity. Excellent swimmers using webbed feet and powerful tail; can hold breath for up to 4 minutes.