Abiotic factors influencing piranha attacks on netted fish in the upper Paraná River, Brazil

Abstract

The influence of selected abiotic variables on the frequency of attack by piranhas Serrusalmus spilopleura and S. marginatus on gill-netted fish was evaluated to identify fishing strategies suitable for environments with high piranha densities. The percentages of attacked fish were recorded during monthly gill netting in the main channel and two lagoons, during both dry and wet periods, in the lower Ivinheima River (22º40 -22°50 S; 53º10 -53°40 W), Paraná River basin, Brazil. In the channel, the catch was removed three times per day, and in the lagoons it was removed every 3 h. Attack frequency on netted fish was correlated positively with temperature, and was more intense at temperatures higher than 25°C. Most piranha attacks were diurnal, and attacks were most intense between 1200 and 1500 hours. To minimize attacks on netted fish, we recommend that gill-netting strategies focus on avoiding areas and periods with high water transparency and temperatures.