It's really not an unclean river - for a city river - overall. Upriver of London the Thames is really quite clean - it doesn't look like the Thames in London in any respect - and any fish caught there would definitely be safe to eat.
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Although it may be hard to spot through the thick silt, mud and sand, the River Thames is home to some 125 different species of fish. Stretching from Teddington Lock in the west out to the Thames Estuary, some of the more well-known varieties include seabass, sole and even trout.
Second, even if you don't see warning signs, call your local or state health or environmental protection department and ask for their advice. Ask them if there are any advisories on the kinds or sizes of fish that may be eaten from the waters where you plan to fish.
Crocodiles definitely aren't native to London's waters, but this is the strange beast that city workers spotted in the Thames yesterday afternoon. They panicked after thinking there was a crocodile resting on a floating pontoon, only yards from where children were paddling.