Otters were once one of the most common predators in the UK. However, being apex predators, they are the first sign of contamination in the system. This is because, while each fish may have a low enough dose to not be impacted, when the otter has eaten many fish they are impacted.
The main threats have been chemical runoff from farms, though sewage is also a problem.
By the 1980s, the British otter was virtually extinct from large parts of the country, only surviving in remote places, far from people and pollution. Oddly, otter hunting with special hounds was not banned until 1981, which will not have helped.
No more! Otters are now found within all counties of the UK. Now, it is true that otters are hard to see – they have always been. Generally they are species which require you to be out at strange times of the day, or just very lucky.
There are places to head where this is not the case, such as the Isle of Mull and similar islands around the UK, where the otters are dependent on the tide, so can be easier to find active in daylight.
On mainland Britain, it seems that Cumbria is an increasingly good place to look, with all suitable habitat now occupied.
Time will tell, but hopefully in the near future, this will be the case all over the UK.