The Eastern Tropical Pacific extends along 5000 km and is home to hundreds of fishing communities that heavily depend on marine resources. The warming of the region over the past 50 years and the coastal development have both threatened the maintenance of important nursery habitats, which are critical during the early-life stages of commercially important fish populations. Given such conditions, how do local environmental conditions and the connectivity of these mangroves sites affect the growth of fish juveniles and their future recruitment into adult stocks? To address this question, we quantified trace element signatures in 265 otoliths of yellow snapper (Lutjanus argentiventris) juveniles inhabiting mangrove forests from two regions of the Eastern Pacific, the Gulf of California and the Galapagos Archipelago. We used Laser Ablation ICP-MS to estimate the concentration of nine trace elements (Mg, Mn, Cu, Zn, Sr, Ba, Pb, Rb and Li). Given that the life stages of yellow snapper can be roughly estimated from total length, and considering the linear relationship between ablated otolith length and fish total length (R = 0.9, p < 0.05), we calculated the mean concentration for each element per life stage (larvae, settlers, post-settlers and migratory immatures).
We found significant differences in the concentration of 6 elements (Mn, Cu, Sr, Ba, Rb and Li) between the otoliths of juveniles collected in the Gulf of California and Galapagos. Ba and Mn concentrations were higher in the Gulf of California, suggesting freshwater and hypoxic conditions inside mangroves. Otoliths from Galapagos showed higher concentrations of Li, presumably associated to hydrothermal activity along mid-oceanic ridges. The otolith fingerprint showed significant differences among the mangroves of the Gulf of California, suggesting high heterogeneity in the temperature, salinity and chemical conditions in these sites, in accordance with the size of the sampled region (~600 km) and the local coastal oceanographic conditions. Conversely, the otolith fingerprint in Galapagos fishes were more homogeneous among the mangroves, probably reflecting the higher mixing rates in the region driven by constant upwelling and the convergence of water masses and oceanographic currents. Fish otoliths from western, central-eastern and southern biogeographical regions could be differentiated. Future research may shed light on how oceanographic processes and coastal human activities affect the traceability of chemical signatures in the otoliths. This information will allow to assess the importance of different mangroves for the sustainability of regional snapper populations.