Effect of annual flood and drought cycles on arapaima growth in the Brazilian Amazon

Background/Question/Methods

Hydrological dynamics, such as flooding events, have major impacts on riverine fishes throughout the world. This is particularly true in Amazonian floodplains, where the life cycles of countless organisms, including that of Arapaima, are influenced by annual high and low water seasons. During low water, fishes are concentrated in enduring water bodies, such as river channels and lakes. As waters rise and peripheral wetlands become inundated, fish, like Arapaima, move into the surrounding landscape where they feed and reproduce. It is likely that the extent and magnitude of these flooding events has an impact on the life cycle and growth of fishes and other organisms. This work aims to explore the relation between Arapaima growth and the intensity of flood and drought in the Mamirauá Sustainable Development Reserve, Amazonas State, Brazil. Annual growth rates of Arapaima were studied through an analysis of ring patterns on scales collected from the same location across nearly two decades (mid-1990s, early 2000s, and min 2000s). The response of growth (i.e., length at age) to various hydrologic parameters (e.g. including the length (days), height (m), and volume (m3) of annual flood and drought cycles) will be tested. This work tests the null hypothesis that the growth of Arapaima is not affected by flood duration and intensity. A correlation matrix was used remove hydrological variables with correlation coefficients ≥80. A stepwise linear regression (performed in JMP 5.1.2; probability to enter: 0.05; probability to leave: 0.1) was used to test the impact of flood and drought variability on Arapaima growth age classes. 

Results/Conclusions

A stepwise linear regression showed that annual flood volume was significantly related to annual growth rates of the first four age cohorts.  R²’s of the model were highest for the age 0 fish.  Growth of older age cohorts (≥4 years) was not significantly influenced by flood or drought parameters.  These data suggest that the flood and the drought cycle influence growth of young Arapaima and not older Arapaima.