Connectivity between estuarine and coastal fish populations: contributions of estuaries are not consistent over time

Otolith elemental signatures (Li: Ca, Mg: Ca, Mn: Ca, Cu: Ca, Sr: Ca, Ba: Ca and Pb: Ca) of Age-0 juveniles of flounder Platichthys flesus and sea bass Dicentrarchus labrax, collected from estuarine systems along the Portuguese coast in 2006 and 2009, were used as baseline signatures to identify the estuarine nursery of Age-3(+) and 2(+) coastal fish matching the juvenile year classes. Otolith elemental signatures were determined via Laser Ablation Inductively Coupled Plasma Mass Spectrometry, and successfully identified the estuarine origin of the majority of coastal fish to characterized baseline estuaries. Relative contributions of individual estuaries to coastal areas, quantified using maximum likelihood estimation, varied over time for the 2 year classes analysed. However, some general patterns were discernible in terms of important estuaries versus those with minor or negligible contributions. Assigned nursery origins varied among species and suggested large scale movements along the coast. For P. flesus the main source for recruits changed between year classes from Ria de Aveiro (69%) to the Douro estuary (59%). The Mondego estuary, located near the latitudinal limit of P. flesus, contributed moderate numbers to coastal populations. The Tejo estuary was the most important estuary for D. labrax in both years sampled (55 and 50%, respectively), whilst the Ria de Aveiro hardly contributed to sea bass coastal populations (<2%). Ultimately, knowledge of which estuaries replenish coastal adult populations, and both local and distant fisheries, is essential for effective management and conservation of these species and their estuarine juvenile habitats.