Seasonal variation in the occurrence of Araguaian river dolphins, Brazilian Cerrado

River dolphins of the genus Inia are strongly influenced by seasonal hydrological dynamics, yet
information on Inia araguaiaensis remains limited. We investigated spatial and seasonal patterns
of dolphin occurrence along a 240 km stretch of the middle Araguaia River (Brazil) during the
dry (100 transects) and wet (91 transects) seasons. Boat-based transects were used to estimate
sighting rates and the number of individuals per sighting. We applied a Generalized Linear
Model (GLM, quasi-Poisson) to test for seasonal differences in group size and a Generalized
Linear Mixed Model (GLMM, Tweedie distribution) to evaluate variation in sighting rates.
No significant effect of season was detected on group size, although slightly higher values
were observed during the rainy period. On the other hand, sighting rates were significantly
higher in the rainy season. Channel depth and width had no effect. These results likely reflect
atypical hydrological conditions, highlighting the importance of interannual variability when
interpreting seasonal patterns. The work was carried out in 2023.