10,000 sea trout were released into Stockholm Ström on Monday, and will be followed by some 12,000 salmon on Tuesday.
The annual event drew scores of onlookers, who watched the first batch splash into the waters in front of the Royal Palace.
"This is something we have been doing for many years. It is mostly so we can sustain rod fishing here. This is necessary for people to fish here in the middle of the city, which is pretty unique," said Oliver Karlöf, fisheries biologist with the City of Stockholm.