The most commonly accepted version says that the toponymy would be due to the large amount of excrement (in Portuguese, "cagádas" ) of seabirds that live, nest and feed on these islands.

After feeding mainly on fish, these birds excrete their feces, rich in calcium, on the rocky slopes of the islands, staining them white.

The islands are situated in proximity to the discharge in the sea of sewage from almost all the South Zone of Rio de Janeiro, through the Ipanema canal, an important work of sanitary engineering built in the 1970s in accordance with the Protocol of Annapolis.

[citation needed] CONAMA proposed making the Ilhas Cagarras Archipelago and Area of Relevant Ecological Interest on 14 September 1989, but nothing was done at the time.

On 13 April 2010 the incumbent vice president José Alencar signed a decree creating the Cagarras Islands Natural Monument.