Multi-decadal change in a tropical seagrass meadow reshapes juvenile fish assemblages

Abstract

Seagrass meadows are essential habitats that support diverse and abundant fish assemblages, yet they are increasingly threatened by disturbance from anthropogenic impacts and extreme climate events. This study quantified changes in fish assemblages associated with seagrass loss and recovery in a tropical estuarine system in Cairns Harbour, Australia, using a multi-decadal beam trawl dataset spanning pre-disturbance, disturbance, and recovery periods.

Seagrass loss from consecutive storms and floods was associated with substantial declines in fish abundance (∼84%) and taxonomic richness (∼73%), consistent with major habitat loss. Following disturbance, both abundance and richness recovered to levels comparable to pre-disturbance conditions, indicating recovery based on fish abundance and richness.
Despite this recovery, assemblage composition differed significantly among pre-disturbance, disturbance and recovery periods, indicating a restructured fish community, with shifts in functional composition and trophic structure rather than a return to pre-disturbance conditions. Indicator species analysis further highlighted this transition, some taxa were associated with disturbance conditions, while seagrass-associated species were absent or slower to recover.

These findings demonstrate that recovery following seagrass disturbance is characterised not by a simple return to pre-disturbance conditions, but by a reorganisation of fish assemblages across both taxonomic and functional dimensions. This highlights the importance of integrating abundance, composition and functional metrics when assessing ecosystem recovery in tropical seagrass habitats.