Massive Fish Schools Darken Rio’s Shoreline as Upwelling Draws Them Near

Overview

• Kilometer‑scale swarms of small plankton‑feeding fish were filmed from the air and underwater along Leme, Copacabana, Ipanema, Leblon, Arpoador and Praia Vermelha, with swimmers moving through the shoals.
• Inea identified the dark areas as fish rather than pollution and linked their visibility to gradual gains in water transparency from sanitation work, with beach water quality monitoring ongoing.
• Marine biologists attribute the aggregation to seasonal ressurgência, as colder, nutrient‑rich water rising off Cabo Frio boosts plankton and attracts manjubinhas and sardine‑like species close to shore.
• Experts note this year’s formations appear unusually large and near the beach, while a concurrent heat wave and calm seas made the phenomenon easier to spot; Inmet issued an orange heat alert with highs near 39–40°C.
• Researchers say the fish pose no risk to bathers but warn that beach litter, plastics and microplastics threaten these species and the coastal food web despite the encouraging sight.