Selectivity of gillnets on the main fish species caught in the floodplains of the Madeira River (Rondônia, Brazil)

Small-scale fishing in the floodplains of the Amazon basin has intensified, with gillnets being one of the main types of tackle used. In this study, we evaluated the length structure of fish species in a floodplain lake of the Madeira River basin, emphasizing the selectivity of the nets in the main species caught. The fisheries were collected in periods of high water and low water, with nets of mesh sizes of 40, 60, 80, 100, 120 and 160 mm. Tests between the periods and also comparing the meshes were performed and the parameters of the selectivity curves estimated with the SELECT calculation code. Records of 17 new maximum lengths are presented. Of the 12 most abundant species, the selectivity of the nets in the modal lengths for Anchovia surinamensis were 13.3, 19.9, 26.6 and 33.2 cm in the nets with mesh sizes of 40, 60, 80 and 100 mm, respectively. In the three smallest mesh sizes, the values for Potamorhina altamazonica were 12.7, 19.0 and 25.3 cm and for Pellona flavipinnis values were 15.4, 23.2 and 30.9 cm. In the two smallest mesh sizes, the following values were presented for the species Potamorhina latior (12.9 and 19.4 cm), Potamorhina rutiloides (11.3 and 16.9 cm), Triportheus elongatus (19.7 and 29.5 cm), Triportheus flavus (14.1 and 21.1 cm) and Pimelodus blochii (15.5 and 23.0 cm). The estimates of selectivity of the nets, in addition to being basic information for fishery sciences, are also fundamental for the management of Amazonian fish stocks.