It’s hard to find salmon in the Thames these days, and it’s even harder to find untagged salmon, which likely originate not from restocking efforts but from the wild. If this is the case, it may mean that the animals are finally repopulating the Thames after having been extirpated from the river in the 19thcentury—an impressive feat considering that teams of conservationists have been unable to achieve this goal despite decades of efforts.

The River Thames is one of the most famous polluted habitats in the Western world; urbanization, coupled with the industrial revolution, left the river foul and uninhabitable not just for the Atlantic salmon, but for many animals. Runs of salmon up the Thames were well-known as far back as 1215, when they were mentioned in the Magna Carta; fish numbers were previously so high that there was a fishery on the river until the early 19th century. But once the waters became polluted, the salmon gradually disappeared; the last record of a"natural" Thames salmon was made in 1833.

Over the years there have been various attempts to restore the species to the river, not just because it is an important keystone species and can be used as a bioindicator, but also because it can provide substantial economic benefits thanks to its popularity among avocational and vocational fishermen alike. Ultimately, none of the previous attempts met with much success. During the intense rehabilitation efforts performed in the 1970's and 1990's, the river was stocked predominantly by a mixture of captive-bred fish from Scotland and "supportively-bred" (in other words, with only one generation of captive breeding) fish from two rivers in Ireland. Despite these efforts, the number of adults in the Thames diminished steadily until 2005, when no fish were captured at all.

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