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Cocaine being washed into the River Thames is "another" problem which eels do not need, a wildlife expert said today.

Mike Waller, a conservation ecologist for the London Wildlife Trust, told of his concerns for the freshwater fish's population after reports suggested drugs were making them “hyperactive”.

“It’s just another thing on top of the fact that they have really struggled with general population,” Mr Waller told the Standard.

“This is just one example that grabs attention, if you think about the sheer amount of chemicals that get flushed, that’s all mixing up.”

He added: “People don’t give it a second thought. What we would say really is, ‘Think about what you’re putting down the toilet’.”

Mr Waller has urged people to consider how they dispose of items, including non-illegal drugs, as even things such as painkillers can cause problems.

Items put down the toilet “can go far and wide” and create issues for all wildlife, he said.

Further on the topic of eels, he said the fact that they are “out of sight and out of mind” means people are less likely to consider their welfare over other animals.

“Fish do struggle and people are more likely to care about more charismatic and cuddly animals,” added Mr Waller.

Species of eel are “really important to the eco system” he added, stating that there has been a lot of work to improve water in the Thames and elsewhere to aid them.

However, he conceded more work needs to be done. “It’s important we try and ensure they don’t decline any more," he said.

The International Union for Conversation of Nature lists the population of the European Eel as “declining”.

Scientists also state that across Europe there number has declined to less than five per cent of the number there was in the 1980s.

Issues related to their migration, pathogens, parasites and climate changes have also been listed as issues for them, according to the Environment Agency.

The details expanding upon cocaine’s adverse impact on them was highlighted by a King’s College London study, which stated that Londoners use of the class A drug throughout the week meant levels were consistently found in waste water.