Feeding habitats of juvenile reef fishes in a tropical mangrove–seagrass continuum along a Malaysian shallow-water coastal lagoon

We conducted stable isotope (δ¹³C and δ¹⁵N) and gut content analyses to understand habitat use of juvenile reef fishes (Lethrinus lentjan, Lutjanus russellii, and Epinephelus coioides), particularly the influence of spatial habitat structure along seagrass–mangrove continua. Sampling was conducted in Setiu Lagoon, located in the southern waters of the South China Sea (Malaysia). Gut content analysis indicated that the focal fishes preyed mainly on benthic invertebrates and other smaller fishes; these were used as potential prey items for isotopic tracers to track fish habitat use. Stable isotope analyses highlighted that the juvenile reef fishes were highly associated with seagrass beds as feeding grounds; however, site-specific analyses suggest differences in fish feeding habitats. The juvenile fishes, particularly large juveniles, preyed in both mangrove and seagrass areas from the central lagoon, whereas all juveniles showed preferential foraging within seagrass meadows in the nearby lagoon mouth. Furthermore, the mean stable isotope values of muscle tissue differed among fish size classes, indicating their ontogenetic habitat/diet shifts, especially for L. lentjan and E. coioides. This study provides important information to support current efforts in protecting and preserving mangroves and seagrasses as crucial nursery habitats for juvenile reef fishes from the anthropogenic activities influencing Setiu Lagoon.