Linking hydrodynamics and fish larvae retention in estuarine nursery areas from an ecohydrological perspective

Abstract

Global change is affecting freshwater flow into estuarine nursery areas, with effects on fish larvae retention. Until now, related studies usually considered all early phases of fishes as passive drifters. However, the swimming behaviour of temperate fish larvae in these areas with strong tides and freshwater inflow is poorly described. The goal of this study was to review the swimming abilities of temperate fish species through ontogeny, the influence of nutritional conditions and how they are able to cope with hydrodynamics changes in nursery areas (including extreme freshet events resulting from dam management), using the Guadiana Estuary as a case study. The ontogeny of the swimming behaviour of estuarine fish larvae from different taxonomic groups was analyzed using the maximum sprint or critical swimming speed (Ucrit). Those results were compared with current observations performed under various tidal and river discharge conditions. In addition, the effect of larval nutritional condition on swimming abilities was also discussed. Overall, most of the species presented skills to be retained inside nursery areas, using distinct strategies. Those involve short term directional horizontal swimming against the axial currents, vertical migration and the use of residual upstream flows, especially over shallower areas. Nevertheless, during high freshet events all the studied species larvae will be flushed out of the estuary with consequent survival threaten. Extensive investigations are needed to thoroughly explore the relation of in situ behavioural responses of fish larvae and small scale estuarine hydrodynamics, towards a sustainable ecohydrological management of nurseries in flow regulated estuaries.