Fish ecology in urban estuaries is poorly understood. As coastal landscapes are transformed, recognizing the impact that urban structures, such as marinas, seawalls and wharfs, have on local fish populations is becoming increasingly important. The extent to which fish are able to maintain natural ecological assemblages can be measured, to a certain extent, by how closely they mimic natural habitats. In Sydney Harbour, assemblages of fish associated with artificial structures were compared with those associated with natural rocky reefs. Sampling was carried out in five locations, each with a marina, swimming enclosure and natural reef. In each location, different habitats supported different assemblages, but differences between habitats were not consistent among locations. Subsequent sampling compared artificial and natural sites in three different areas in each of three different estuaries. Results indicated that differences in fish assemblages between artificial and natural sites were greater than differences between sites within each habitat, but there were no patterns among different positions in an estuary or from estuary to estuary. This study provides initial evidence that, although artificial habitats generally support the same species as found on natural reefs, assemblages usually differed between natural and artificial habitats. In addition, without knowing if these habitats do, in fact, sustain viable populations of fish, it would be premature to label artificial structures as effective habitat for fish.

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Assemblages of fish associated with exposed and sheltered sides of breakwaters and adjacent rocky reefs were sampled at coastal marinas on the north-west coast of Italy. Contrary to expectations, assemblages associated with the exposed side of artificial breakwaters did not differ from those at adjacent natural rocky reefs. Assemblages of fish associated with the sheltered side of breakwaters did, however, differ from adjacent rocky reefs and, to a lesser extent from the exposed side of the breakwaters. The internal sheltered side of marinas supported fewer species than did the external breakwaters and adjacent reefs and lacked common reef-associated species, such as Chromis chromis and Oblada melanura. In contrast, they supported greater numbers of new recruits of the sparid, Diplodus vulgaris.

In the marine environment, artificial structures are not, in general, managed for their value as habitat and are often built with no a priori expectation as to the assemblages which 10 may colonise them. It may, however, become increasingly important to consider the value of such structures as habitat when decisions are made with respect to the management of artificial structures. This study investigates the role marinas play as habitat for fish by examining the distribution of fish associated with these urban developments and how this distribution relates to