Connectivity between estuaries and marine environment: Integrating metrics to assess estuarine nursery function

Natural or anthropogenic induced variations in estuaries and the dynamics of marine fish populations potentially promote differences in connectivity between estuaries and marine areas, i.e. in their importance as nursery grounds. Within this context, an integrated assessment of the differential nursery function of the main estuaries along the Portuguese coast for commercial fish species common sole Solea solea, Senegalese sole Solea senegalensis, flounder Platichthys flesus and sea bass Dicentrarchus labrax was performed through several indicators based on available data. Contribution of individual estuaries to marine subpopulations was measured with potential metrics (juvenile density, habitat quantity, juvenile number and habitat quality within estuaries) and effective metrics (estuarine source of young adults in marine environment measured via otolith elemental fingerprints). The relationship between the two types of metrics was also assessed. Estuaries identified as important nursery and/or effective juvenile habitat (EJH) differed with species and no single estuary was best for all, highlighting species-specific regulation of nursery function. Multiple species assessment of nursery and EJH function differed among estuaries. Management and conservation of estuaries should focus on sites with higher contributions to adult subpopulations of multiple species. The importance of defining precise scientific and management objectives was emphasized by the different rankings of estuaries obtained with nursery or EJH criteria. Potential and effective contribution of estuaries were not significantly correlated, but in a quantitative analysis juvenile densities and number of juveniles seem related with effective contribution in some species. An agreement between potential and effective contributions of estuaries is concurrent with the acknowledged minor role of juvenile stage processes in regulation of recruitment to adult subpopulations.