Some people said it was created by Peruvian scientists, that it gorged on the blood of farm animals, that it was a monster. Many myths have grown up in Bolivia around the Paiche, one of the world’s largest scaled freshwater fish which is native to Amazonian rivers of Brazil and Peru and can grow up to four metres long. But after young fish were accidentally released from a Peruvian fish farm, the Paiche has arrived big time in Bolivian rivers.Every year, it reaches another 40 km of river and is eating all before it, especially smaller native fish stocks including even the deadly piranha. At the same time, the Paiche is proving a boon to many local fisherman who sell it to families and restaurants who are acquiring a taste for it in a land-locked country where meat has always been the favourite form of protein. This gives scientists and the authorities a dilemma. Do they try and control or even eradicate the Paiche from rivers famed for their biodiversity where new species are being identified all the time? Or let its spread continue unabated and provide a useful livelihood for fishermen and a healthy addition to the Bolivian diet? For Assignment, Jane Chambers takes to the rivers of Bolivia