2.1. Study area and fish A total of 304 farmed Nile tilapia was analyzed in fish farms that showed historic of chronic mortality. The fish were cage-reared in reservoirs at the hydroelectric power station in southeastern Brazil, which comprises Rio dasVel- has (19° 08'31.8" S; 47o 41'05.6" w), Nova Ponte, Minas Gerais and the Parana River reservoir (20° 16'39.8" S: 51 03'38.2" W), Santa Hé do Sul, Sao Paulo. In northeastern Brazil, the fish were cage-reared in a small reservoir in the Vale do Juliana River (13° 43'57.56" S; 39° 09'00.79" W) and earth pond hatchery in the city of Itubera, Bahia.