Coping with connectivity between populations of Merluccius merluccius: An elusive topic

The elemental composition of European hake otoliths was analyzed at the core, settlement mark and otolith edge zones corresponding to the periods of pelagic early life stage, settlement to the bottom, and juvenile life, respectively. Barium, Sr, Mg, Mn and Ca were measured using LA-ICPMS on the sagitta otoliths of hake sampled in 6 different Western Mediterranean areas, separated by tens to hundreds of kilometers. The otolith edge, which should have incorporated the water mass signal of the sampling area, showed a high degree of overlap between geographical areas. This precluded the discrimination of geographically distinct groups. However, there was a clear ontogenetic signal in all the fish, with higher Sr:Ca in the otolith core. Ba:Ca was higher at the settlement mark, while Mn:Ca was overlapped with the core values. The otolith edge had lower ratios for all elements except for Mg. The conflicting effects of the ontogenic patterns and the inconclusive allocation of a geographical signal at the edge, precluded to assign individual hake to a natal origin based on the composition of the core and settlement mark zones. Our results may be explained by high mobility of the juvenile hake in this area of the Mediterranean, or by the relative homogeneity in the water masses for the four elements considered in this study. While this subset is useful for distinguishing major water masses or regional differences between, for example, the Atlantic and the Mediterranean, a larger set of elements could be needed for smaller-scale studies.