The effects of shoreline armouring on estuarine fish are contingent upon the broader urbanisation context

Natural ecosystems in estuaries are modified by the effects of runoff from disturbed watersheds and are frequently replaced by armoured estuarine shorelines. Whilst the effects of these 2 stressors are widely recognised, they are typically studied in isolation, and it is not clear how these contrasting types of urbanisation interact to shape faunal assemblages. In this study, fish assemblages were surveyed with underwater videos arranged in a 200 m grid throughout the lower reaches of 3 estuaries in eastern Australia (resulting in ≥63 sites per estuary and 277 sites in total) which differed in their extent of shoreline and watershed urbanisation. Overall, the least urbanised estuary supported more than twice the number of fish species and a significantly greater abundance of fish. The spatial patterns of fish diversity and abundance within estuaries were related to the proximity of urbanised shorelines, with most fish groups aggregating near armoured shorelines. These effects of distance from urbanised shorelines were, however, modified by the degree to which the entire estuary had been modified. We show that the ecological effects of urbanisation can extend across estuaries and suggest that coastal landscapes should therefore be managed as interlinked mosaics of both natural and artificial habitats.