Salmon used to travel up the river Thames, through London, to their breeding grounds in Berkshire. However, the industrial revolution and the pollution that accompanied it killed them all off by 1833.
The water quality slowly improved, and in 1974 a single, stray salmon was found alive. A few were also found in each of the years that followed. These discoveries led to the formation of the Thames Migratory Fish Committee, who were tasked with reintroducing salmon and sea trout to the Thames.Now experts have finally declared the Thames clean enough for the reintroduction to begin. The BBC are reporting that 5,000 2cm long young salmon were released into the Thames tributary, Lambourne river, recently. It's estimated that in 5-10 years there should be a healthy population again.
Salmon are very sensitive to pollution, so the fact that they are now coming back to the Thames is great news. It's just odd that no matter how clean it gets, the Thames still looks filthy.