Fish in Artificial Urban Waterways: Ecology, Feeding and Contamination
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To maximise opportunities for coastal land with waterfrontage, property developers have claimed natural wetlands (saltmarsh, mangroves) for construction of artificial urban waterway developments in many places. These created habitats differ from nearby shallow estuarine habitats; they lack the macrophytes common in natural estuaries, they receive untreated urban stormwater runoff, and typically comprise a highly ramified network of narrow and deep channels. Few studies of these habitats exist and there is no clear understanding of their ecological value and role as coastal fish habitat. Managers are therefore faced with the challenge of managing existing systems, and of selecting coastal wetland habitats for protection from new waterway developments. Artificial urban waterways are an obvious feature of the coastal landscape in southeast Queensland, Australia. Because of the extensive network of these systems, many suffer hydraulic problems, and in response, legislation forced property developers to shift waterway design to estuarine lakes with restricted tidal exchange. This hydraulic restriction seemed to solve increases to the tidal compartment imposed with further artificial urban housing waterway developments, however, no consideration has been given to connectivity with downstream waterways for fish. My research demonstrates that these lakes, like open flow through canals, support many of the same fish species of economic importance that occur in natural wetlands and that there is no apparent trapping of fish in lakes. Salinity is lower in lakes because of their tidal restrictions, and while this is only weakly correlated with fish abundance, even in massive lake developments (280 ha surface area), it is the environmental factor that best explains fish assemblages. Recruitment of young fish is also influenced by lake design, with their arrival in lakes slightly delayed behind that in open canals. Few studies have tested whether the ecological processes supporting fisheries production in artificial urban waterways are different to those in natural habitat. I used stomach content analysis and stable isotopes (carbon and nitrogen) of snub-nosed garfish (Arrhamphus sclerolepis) to examine their nutrition in artificial and natural wetlands. A. sclerolepis in natural wetlands have enriched carbon isotope values (-13.9‰) because they consume large amounts of seagrass during the day and night, and at night also ingest small quantities of crustacean prey. A. sclerolepis in artificial urban waterways have depleted values (-19.1‰) because they consume macroalgae during the night, and switch in the day to terrestrial insects washed from gardens lining the waterways. This means that fish show remarkable plasticity in the new wetland habitat, retaining the same feeding strategy from natural wetlands of bulk herbivory with the inclusion of smaller amounts of animal prey. It also suggests minimal exchange of fish populations between natural and artificial habitats over the period of weeks to months, Mathematical modelling of the carbon and nitrogen isotope signatures of A. sclerolepis and all feasible source mixtures confirmed that this diet switching is part of a feeding strategy requiring multiple food sources in each habitat.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
Griffith School of Environment
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Artificial urban waterways
Ecology of artificial urban canals