Understand the resurgence phenomenon which caused the appearance of large schools of fish on Rio’s beaches

— Yesterday, I had the opportunity to see the fish. I even said to my cousin, “Wow, they’re really close.” I was standing in the shallows. The water was clear, we could see very well. It was in Arpoador — says Maria Sorgilene, a resident of Curitiba in São Paulo who, this Friday (26), was in Pedra do Leme as a tourist with her cousin.

According to Marcelo Vianna, professor at the Department of Marine Biology at UFRJ, these are manjubas or sardines. The species, because they are small and at shallow depths, far from the lenses of cameras looking at the sea, form large shadows.

— What you see is in fact a reflection of the last reproduction period of sardines. At this time of year it is a common occurrence that becomes visible due to the clearer water. This happens because the river water cools during the summer. It’s one of our characteristics. And this cold water that comes in during this time of summer, early spring, is rich in nutrients. Hot water is lighter than cold water, which is dense and sits at the bottom. So, when this cold water rises, it brings back all these nutrients that were at the bottom and enriches the water column: it contains more phytoplankton, which are these microalgae. These nutrients attract schools of small plankton eaters, such as sardines and manjubas, explains the biologist.

Vianna adds that, every year, when the water of beaches like Arpoador, Ipanema, Leme and Coapcabana acquires lower temperatures, it is possible that it is already a resurgence effect that favors the appearance of species. From above, we could see groups of fish whose shadows reached more than a kilometer long. The size attracts the teacher’s attention:

— Every year, we witness an enrichment of the coastal waters of Rio de Janeiro and we see schools of small fish approaching. The difference is that this year, in fact, this school is very large and very close to the coast.

According to the National Environment Institute (INEA), they are not linked to pollution. In reality, these are schools of fish moving close to the surface.”In recent days, several other similar reports have also been published on social media,” the agency informed. Inea attributed the greater visibility of these animals, as well as other marine species, associated with the progressive improvement in water quality, resulting from sanitation works and environmental monitoring.

Cold water last summer

The resurgence phenomenon had already been explained by GLOBO when frozen waters invaded the coast of Rio last summer. According to geographer Flávia Lins de Barros, coordinator of the UFRJ Marine Geography Laboratory, the resurgence phenomenon begins in the Cabo Frio region.

— Upwelling occurs due to a combination of factors, such as wind direction and coastal configuration. The wind blows, the surface water migrates and the bottom water, colder because it does not receive the sun, rises — says the teacher.

Due to the indented design of the coast that runs from Cabo Frio to Arraial do Cabo, southeasterly winds hit the coasts of towns in the Lagos region, and cold water ends up on the coast of Rio with the currents from there to here.