Estuarine fish ecology in three-dimensions: The ecological consequences of terrain and dredging on fish assemblages, ecosystem processes and habitat functions

Hayden Borland

doi:10.25907/00107

The structural complexity of habitats has pervasive ecological effects on the diversity of fish assemblages and the functioning of ecosystems within coastal and marine seascapes. This is because structurally complex habitats support diverse food resources and abundant predator refuges that promote the growth and survival of a diversity of fish species. Many studies have, therefore, recorded more fish species and larger fish populations in, or near, structurally complex habitats in a diversity of ecosystems, including coral reefs, seagrass meadows and mangrove forests. Variation in the structural complexity of these habitats has, however, largely been quantified at small spatial scales, using in-situ measures of complexity (e.g. chain and tape rugosity). Recent advancements in marine habitat mapping technology (e.g. aerial imagery, LiDAR, echo-sounders) have allowed researchers to map the bathymetry (i.e. topography in the sea) of marine seascapes at much higher resolutions, and this means that we can now test for possible effects of habitat complexity, measured as three-dimensional terrain variation (e.g. terrain complexity, relief, morphology), on fish assemblages over much wider spatial scales. This thesis reviewed the global literature linking fish assemblages to terrain variation and has shown that the relief, class, complexity and morphology of terrain features shape the abundance and diversity of fish populations and assemblages, and the health and functioning of ecosystems, in a diversity of marine seascapes. The findings of this research demonstrate that fish respond to terrain variation in distinct ways in different seascapes, and show that the ecological consequences of changes in seafloor terrain have not been examined in estuarine seascapes, until now. Estuarine seascapes contain a diversity of structurally complex ecosystems (e.g. mangrove forests, seagrass meadows) and habitats (e.g. rock-bars, intertidal flats, subtidal channels, rock-walls), that provide important habitat functions (e.g. foraging grounds, reproduction sites, juvenile nurseries) for coastal fishes. This thesis demonstrates that the ecological values of two important nursery habitats, mangrove forests and seagrass meadows, are modified by variation in seafloor terrain, and that the abundance, diversity and assemblage composition of nursery species, are strongly associated with distinct terrain features in these seascapes. Estuaries are, however, also hot-spots for human recreation, commerce and development, and in urban estuaries, the seafloor is dredged to improve the navigability of estuarine shipping channels, and this typically results in the modification and transformation of seafloor terrain features. This thesis shows that the ecological significance of terrain is fundamentally altered by dredging, that fish abundance, composition, and functional diversity (i.e. variation in the physiological, morphological and behavioural traits of species) is associated with contrasting terrain features in natural and dredged estuaries, and that variation in the spatial extent and orientation of dredged channels is linked to functional changes in a diversity of urban seascapes. This thesis highlights the importance of terrain variation for fish assemblages and ecosystem functioning in marine seascapes and details the ecological consequences of seafloor modification by dredging in modified estuaries. The results of this research support the inclusion of terrain into coastal management and spatial conservation plans that aim to improve biodiversity, ecosystem functioning and fisheries productivity, and identify opportunities for optimising dredging operations to reduce impacts in urban seascapes.

Estuarine fish ecology in three-dimensions: The ecological consequences of terrain and dredging on fish assemblages, ecosystem processes and habitat functions

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