The effects of flood regime and fishing effort on the overall abundance of an exploited fish community in the Amazon floodplain

Abstract

Freshwater fishing is highly developed in the Amazonian basin, and the problems of managing the fish resource are becoming increasingly acute. The global model of exploited stock dynamics has proved inadequate for explaining the variations observed in catches from one year to the next. Following the results of recent studies in maritime fishing, this work aims to identify the contribution of environmental variables (hydrological parameters have been selected) to the explanation of variations in the abundance of the exploited fish community. An abundance index is calculated based on the catch per unit of effort statistics and the surface of the considered basin. Using a 12-year time series, the results show that the effect of the fishing effort on the index of abundance is negligible when viewed alongside the flood regime parameters for the years prior to the catch. The effects revealed are, in chronological order of appearance: 1) a positive effect of the water level during the flood regime peak three years previous to the catch, probably associated with recruitment for a number of species; 2) the effect of the water level during its rise two years before the catch, which may well be related to competition phenomena; and 3) the effect of severe low-water levels two years before the catch, possibly due to mass natural mortality during the severest periods of low-water. Because there are complex migratory movements of fish populations either side of low-water periods, only the estimated biomass during the flood appears to be an accurate reflection of the mean biomass in the environment. Linear models are presented, where the abundance index is a function of the hydrological parameters previously identified. The model with 3 variables corresponding to the 3 evidenced effects, explains more than 83% of the variations in the annual abundance indices.