‘La Niña’ phenomenon and the relationship between decapod populations and fishes in temporarily isolated shallow lakes

Abstract

The ‘La Niña’ phenomenon causes temporary isolation of shallow lakes in rivers with floodplain. Furthermore, as abiotic conditions become more extreme, and intra–interspecific relationships intensify, the risk of local species extinction increases. The Paraná River in South America is a river with an extensive floodplain with many shallow lakes. In these environments, freshwater prawns and many fish species coexist. The aim of the work was to study the interaction between two species of prawns (Palaemonetes argentinus and Macrobrachium borellii) and fishes during ‘La Niña’ phenomenon. Density variations of prawns and fishes were examined in three shallow lakes at Paraná River. The samples were collected monthly at several points of each aquatic environment with a trawl net. Abiotic and biotic factors were measured. Density and population structure varied in both prawns, and fishes, and the changes have not been simultaneous. These variations might be associated with predation, aggressive behaviour of the prawns and internal micro-migrations. The effect of La Niña creates additional stress on prawn populations when water inflow is delayed. This fact could promote the decrease or the complete disappearance of one or both prawn species in the environments, and modify the trophic web in the next phase of the shallow lakes.