The pacu-caranha inhabits the waters of the Pantanal, along with other rivers in southern Brazil, Uruguay, northern Argentina, Paraguay and Bolivia. In some areas such as dams and ponds, where not naturally occurring, it has been introduced to support local fishing. It can often be found underneath fruiting trees - waiting for the fruits, seeds, or other vegetable matter to fall into the water. During the annual floods, pacu leave the main river channels - entering the waters around the flooded gallery forests, where higher waters sometimes enable them to eat the fruits directly off the trees - and has the ability to search for food in very shallow water if required. Aside from fruits, pacu will also feed on aquatic insects, plankton, snails, crabs and other small fish. It's omnivorous and opportunistic diet has resulted in the local nickname "porco do rio" (pig of the river), suggesting that it will eat anything which comes its way. Research has shown that pacu have an important role with the dispersal seeds, with at least 43 pantanal plant species (representing about 25 percent of gallery forest diversity) being reliant on pacu, at least in part, for their propagation. Many local fruits contain an outer pulp surrounding a hard inner seed. The seeds, when ingested aren't always crushed by the pacu's jaws. They are scarified by the fish's digestive system, then excreted, often far from the parent tree. This pattern of seed dispersal is likely true also for riverine Amazon plant species.
The behaviour and diet of the tambaqui and pirapitinga of the Amazon region are broadly similar. However, these prefer the warmer tropical waters of northern Brazil, northeastern Bolivia, Peru, Colombia and Venezuela.