The role of catchment land use and tidal exchange in structuring estuarine fish assemblages

Changes in land use often increase nutrient loading to aquatic ecosystems, affecting primary productivity and water quality, with flow-on effects to consumers. We explored whether fish-assemblage composition, species diversity, and the representation and richness of ecological guilds were associated with catchment land use in 31 estuaries in Victoria, Australia. Fish assemblages were surveyed using fyke and gill nets. Species were assigned to ecological guilds based on salinity associations and their use of estuaries, water column position and trophic characteristics. The Shannon diversity index, representation and richness of several marine-associated species and trophic guilds dominated by marine-associated species were positively related to tidal exchange, indicating the widespread influence of marine connectivity on estuarine fish assemblage and trophic structure. These patterns were driven by adult life stages. The richness and the proportion of demersal species in juvenile assemblages were negatively associated with the proportion of the catchment with land uses expected to elevate nutrient loads (e.g. animal production, horticulture, industry and urbanization). This relationship may reflect shifts in vegetated habitat, resource availability or changes in water quality induced by nutrient enrichment. Juvenile demersal fish show promise as an indicator of the effects of catchment land use on the structure of estuarine fish assemblages.