Functional connectivity in the Yangtze Estuary habitats for an “estuarine opportunist” fish: an otolith chemistry approach for small yellow croaker (Larimichthys polyactis)

Understanding the connectivity in estuarine waters is important for sustainable fisheries management and safeguarding of estuarine ecological environments. Knowledge regarding the functional connectivity for some “estuarine opportunist” fish species between the Yangtze River estuary (YRE) and its adjacent coastal waters is still scarce. We investigated the connectivity for an exploited sciaenid, the small yellow croaker (Larimichthys polyactis), using otolith elemental composition. In 2020, 115 juveniles were collected in four putative habitat patches, and elemental fingerprints (strontium, barium, magnesium, manganese) of the otolith margin were used to determine spatial differentiation. The results indicated that multi-elemental fingerprints had limited efficacy in identifying habitat patches at a fine spatial scale. However, an elevated Ba/Ca ratio proved to be effective in identifying the presence of L. polyactis in the YRE, which was found to be 12.5 ± 0.7 µmol/mol. In 2021, 51 adult fish were collected from the Lvsi fishing grounds, and the core-to-edge Ba/Ca profiles of the adult fish were analyzed to evaluate the connectivity in the YRE and its adjacent waters. Among the specimens examined, 41% exhibited a life history associated with estuarine habitats. In addition, among the fish with estuarine life history, 72% of the estuarine life history occurred during the juvenile stage, and almost none at the larval stage. The functional connectivity of L. polyactis in estuarine habitats is conspicuous and closely linked to ontogeny. This study emphasizes the need to incorporate the concept of functional connectivity for a more comprehensive understanding of estuarine ecosystems. Furthermore, it highlights the significance of estuaries for “estuarine opportunist” fish, warranting increased attention and research.