Habitat separation of Craterocephalus (Atherinidae) species and populations in off‐channel areas of the lower River Murray, Australia

Comparative studies of related species may reveal the determinants of distribution and abundance, and elucidate the causes of fragmentation. The intensively regulated River Murray in south‐eastern Australia contains fragmented populations of several small fish species that have more common, widespread congeners. The Murray hardyhead Craterocephalus fluviatilis (McCulloch) (Atherinidae) has a wide but patchy distribution, whereas the unspecked hardyhead Craterocephalus stercusmuscarum fulvus (Ivantsoff, Crowley & Allen) has a similar range but is generally abundant. The two species rarely cohabit. Salinity and other aspects of ‘habitat complexity’ appear to be key factors separating the species and associated fish assemblages, although this requires confirmation. The findings are discussed in the context of other studies of closely related species.