[Ectoparasites of the Nile tilapia (Oreochromis niloticus) bred in net-tanks in the Corvo and Guairacá rivers, state of Paraná, Brazil].

Current experiment estimates the prevalence of ectoparasites of the Nile s tilapia (Oreochromis niloticus), strain Chitralada, bred in net-tanks during 130 days, from April to August 2006, and reports on the water's physical and chemical parameters. Three hundred and eighty-seven samples of tegumentary scrapes and branchia of post-reversed males were analyzed. Experiment was conducted in thirty 6.8 m-3 net-tanks (2.0m x 2.0m x 1.7m), filled with 6.0 m-3 water, of which 15 were placed in the Corvo river and 15 in the Guairacá river, with three stocking densities (100, 150, 200 fish per m-3) and five repetitions each. At the start of experiment, total ectoparasites prevalence in 105 fish, average weight 35.4 +/- 19.3g and total length 11.7 +/- 2.1cm, was 87.6%. Prevalence was higher for Monogenoidea (40.0%), followed by mixed parasitism (33.3%) and Tricodinids (14.3%). Whereas in the Corvo river total ectoparasites prevalence in the four collections reached 38.2%, with predominance for Monogenoidea (19.4%), in the Guairacá river prevalence reached 44.2%, with predominance for Tricodinids (17.4%). Water's physical and chemical parameters, with the exception of temperature, fitted conditions for Nile s tilapia breeding.