Anchoveta Engraulis ringens along the Chilean coast: Management units, demographic units and water masses: Insights from multiple otolith-based approaches

Three management units (MUs) are used for stock assessment and fishery regulations of anchoveta Engraulis ringens in Chilean waters (>65% of E. ringens latitudinal range): MU-I [18-25)degrees S, MU-II [25-32)degrees S and MU-III [32-42)degrees S. To evaluate whether these three MUs correspond to separate demographic units (DUs), as well as to estimate mixing rates and spatial distribution of these DUs, we combined three types of otolith natural markers: isotopic signatures (delta C-13 and delta O-18), elemental compositions (Na, Mg, Mn, Sr and Ba) and microstructural indexes. All otolith markers were determined in nuclear and marginal regions of juveniles and adults from cohorts 2012 and 2015. Differences in core region markers indicated spatial segregation between three environmentally distinct nursery areas, probably related to Subtropical Water (MU-I), Subantarctic Water (MU-III) and the Subtropical Convergence (MU-II). Comparison of core and marginal regions, from juvenile and/or adult fish, suggested that adult fish remained nearby nursery areas, separated from fish nursed in other MUs. Nonetheless, ontogenetic migrations from warmer (offshore) nursery habitats to cooler (deeper/more coastal) feeding habitats occurred within MUs. In summary, our results support the consideration of the three MUs as three separate DUs. Estimated mixing rates indicated MU-II received contributions of 31% from MU-I and 3% from MU-III, while complete segregation existed between MU-I and MU-III. Such mixing rates between MU-I and MU-III seem large enough to justify its further consideration by stock assessment models and management procedures.