Evidence for meta-population structure of Sardina pilchardus in the Atlantic Iberian waters from otolith elemental signatures of a strong cohort

The European sardine is one of the most important clupeoid fish captured by the Portuguese and Spanish purse-seine fleets in the Atlantic waters of the Iberian Peninsula. Despite the importance of this species, and substantial decrease in catches over the last two decades, knowledge about sardine movements and connectivity between their juvenile recruitment areas and the adult fishing grounds is scarce. In this study otolith elemental fingerprints (both whole and core) of juvenile recruits (ages 0+ and 1+) and adults (age 3+) collected along the Portuguese coast (North, Central and South regions) were used to demonstrate regional population structure. The whole and otolith core analyses represented the entire life-history prior to capture and the larval/recruitment phases (first 2 months of life), respectively. Molar ratios of Sr/Ca, Mg/Ca, Mn/Ca and Ba/Ca provided location-specific elemental signatures, although otolith elemental compositions were not temporally stable within a year-class, particularly for Mg/Ca and Ba/Ca. Classification accuracy rates obtained from quadratic discriminant function analysis of core and whole otolith chemistry data of adults were relatively high (63% and 79% mean accuracy percentages, respectively). The results support the hypothesis of meta-population structure of the sardine stock in the Atlanto-Iberian waters. However, because there was evidence of some mixing among adult aggregations, and adult aggregations were largely derived from a common northern juvenile recruitment area they cannot be classified as entirely separate stock units for fisheries management purposes. The maximum likelihood analysis suggested for the 2004 cohort that replenishment of adult populations of sardine along the Portuguese coast was mostly derived from a single northern recruitment area.