Characterisation of Ichthyoplankton Settling in the Gironde Estuary

The Gironde estuary was recently classified for the purpose of the Water Framework Directive in Europe, in poor ecological status according to the ELFI index that considers mainly juvenile fish populations. A question arises: is the low abundance of juvenile and adult fish caused by a failure in settling or by an unsuccessful viability of larvae?

Monthly surveys were conducted to collect ichthyoplankton from 3 intertidal, 3 subtidal mudflats and 2 stations in the main channel of the lower Gironde estuary. A length-frequency analysis was performed to assess the fish larvae growth. Four species (gobies, anchovy, sprat and sea bass), among the 24 captured in 2013, constitute 95% of the total abundance. The Shannon index indicates higher species diversity on mudflats than in the channel. Among sampling stations, abundances display an irregular distribution pattern. Actually, the largest intertidal mudflat of the estuary shows the lowest abundances of larvae. More generally, the sheltered areas showed lower abundances of ichthyoplankton than unprotected zones.

The hydrodynamic conditions of the flood current could partially explain the distribution of the fish larvae.

This study provides essential knowledge about the ecological functions of the Gironde estuary mudflats for the larvae populations.