The return of otters to many UK rivers is a conservation success story, which experts say stems from efforts to improve the quality of our watercourses and the habitat around them.

And the increasing populations of otters show just how much better a condition rivers are in now than in previous decades.

Much of the improvement to rivers is the result of changes in farming practices, including the banning of chemical pesticides which polluted rivers and harmed wildlife.

According to Fran Southgate, wetland landscapes officer for Sussex Wildlife Trust, the chemicals suppressed the otters’ immune systems, affected their eyesight and ability to breed and reduced their food sources – mainly fish and other aquatic animals.

Fertilisers washing off fields and dumping too many nutrients in rivers also polluted water courses – while in more built-up areas such as the South East, run-off from highways has been a problem.

But a lot of work has gone into improving water quality, including farming measures such as margins along the side of rivers to stop run-off which have made things better for wildlife.

However, it is not just land management which has an impact on the health of otter populations – it is also how people have treated rivers.

Ms Southgate said: “A lot of fish stocks have crashed, and part of that was because of how appallingly we have managed our rivers.”

Over the centuries, natural wetlands and rivers have been destroyed, with water courses straightened, ditches dug and waterlogged areas drained.

Deeper, straighter channels do not leave fish places to hide from predators and weirs prevent species such as eels from migrating.

River habitats need to be managed in such a way that they provide places for fish and other species such as frogs, crustaceans, mussels, slugs and dragonflies which can all be food sources for otters. And in Sussex, as in other parts of the country, there are efforts to “re-wild” water courses to provide habitat for wildlife as well as deliver benefits for people including water storage in the face of flooding.

Returning a river to its natural state will bring back islands, shallow areas, gravel beds for trout to spawn in and fish nurseries. Many rivers have been stripped of the natural features that would have surrounded them, such as wet woodland, reedbeds and scrub to hide in and old or pollarded trees to provide holts. “For an otter, if they’re breeding, they need a quiet place they can train the young to swim and catch fish.”