Fish diversity and juvenile habitat use in Indo-Pacific clear-water mangrove habitats

Clear-water mangroves, which thrive in oligotrophic waters with minimal terrestrial input, are connected within a coral reef-seagrass-mangrove ecosystem referred to as the coastal ecosystem mosaic. However, Indo-Pacific clear-water mangrove studies are limited compared to Caribbean research. This study examined spatiotemporal variation in fish assemblages in clear-water mangrove forests on Orpheus Island, Australia, across seasons, tidal stages, depths, and substrates. Our data contribute to understanding how fish use clear-water mangrove forests in tidally dependent ecosystems, and how Australian ecosystems differ from the Caribbean. A total of 5513 individuals were recorded from 70 species (28 families), dominated by reef-associated taxa. Multivariate analyses showed that fish community composition differed among substrates and seasons. Juveniles were more likely to swim into the forest than along the edge, and were observed at shallower depths. Substrate did not predict juvenile occurrences; instead, species-level responses diverged, indicating species-specific rather than substrate-driven habitat use. Life-stage models further showed that fish with a trophic level >3.5 had higher odds of being classified as either juvenile or small adults, though intercepts indicated substantial baseline probabilities among lower-trophic taxa as well. These results support a seascape-nursery perspective: clear-water mangroves in Australia contribute transient foraging and refugia opportunities within a connected coastal mosaic. However, as only 1 island was investigated, these results should be compared with other Indo-Pacific clear-water mangroves to place them in context of local variation and habitat-use patterns. Recognising the seascape role of clear-water mangroves can guide conservation efforts to support their ecosystem function and biodiversity.