The Amazon River: High fish abundance in the confluence of two large rivers

In the center of the Amazon River basin, white water of the Amazon River meets with black water of the Negro River, creating a conspicuous visible boundary spanning over 10 km along the Amazon River, which can even be seen from space.

Local fishermen acknowledge the confluence of black and white water rivers is rich in fish abundance, and indeed there are many freshwater dolphins there. So I was wondering what’s going on in the confluence and suspecting there were many prey zooplankton here.

Here, my team and I report a high fish larvae abundance in the confluence zone between black and white water rivers in the Amazon River system.

Our study found that the confluence boundary offers benefits of both high prey concentration (zooplankton) from the black water river and low predation risk from the white water river for fish larvae, explaining the high abundance of fish larvae in the confluence zone.

That’s why the dolphins like the confluence zone!

Nakajima R, Rimachi EV, Santos-Silva EN, Calixto LSF, Leite RG, Khen A, Yamane T, Mazeroll AI, Inuma JC, Utumi EYK, Tanaka A (2017) The density and biomass of mesozooplankton and ichthyoplankton in the Negro and the Amazon Rivers during the rainy season: the ecological importance of the confluence boundary. PeerJ 5: e3308.