Otters numbers declined dramatically from 1957 due to chemicals such as PCBs which were released in river systems and accumulated in the food chain, poisoning otters. Sport hunting and discrimination by farmers and fishermen also played a role.
To prevent extinction those chemicals and hunting were banned, less water was taken from rivers where known otters lived and safe passages were built beneath roads (they have no road sense!).
Today estimates put the UK population at up to 12,000, its highest for over 20 years, and they are once more seen in every county. They are definitely doing well here on the west coast of Scotland!