Importance of seagrass-mangrove continuum as feeding grounds for juvenile pink ear emperor Lethrinus lentjan in Setiu Lagoon, Malaysia: Stable isotope approach

The commercially important pink ear emperor fish, Lethrinus lentjan, often occurs as a juvenile in subtropical and tropical interlinked mangrove and seagrass ecosystems, but little is known about its feeding habits and habitat use. Here, we used gut contents and stable isotopic (delta C-13 and delta N-15) ratios to determine temporal changes in food sources and foraging habits of juvenile and sub-adult fish collected in mangrove forests and seagrass beds in the Setiu Lagoon. Gut content examination identified the main food sources as crustaceans, gastropods, bivalves, and annelids. Stable isotope analysis of food sources showed marked differences between the mangroves (delta C-13 = -26.8 +/- 2.0%(0); delta N-15 = 4.3 +/- 1.7%(0)) and the seagrasses (mean +/- S.D. delta C-13 = -20.5 +/- 5.5%(0); delta N-15 = 5.8 +/- 1.2%(0)). The isotopic composition of L. lentjan revealed that it mainly utilized seagrass-based food sources. Rainfall and the semi-diurnal tidal regimes may affect the foraging habitats of fish in the lagoon. A significant depletion of C-13 related to body size was observed, suggesting that mangroves provided some benefits to sub-adult fish. In contrast, trophic position increased with fish growth, although this increase was negligible. A stable Isotope mixing model confirmed that the seagrass bed constituted the main carbon source for the fish, but with an increasing contribution of mangrove prey, such as sesarmid crabs, related to fish growth. This study provided novel information on seasonal variations in feeding areas and flexible habitat use in L. lentjan in the Setiu Lagoon, which will help optimize management strategies for sustainable use and wildlife conservation.