THE ECOLOGICAL EFFECTS OF STRUCTURAL FLOOD MITIGATION WORKS ON FISH HABITATS AND FISH COMMUNITIES IN THE LOWER CLARENCE RIVER SYSTEM OF SOUTH-EASTERN AUSTRALIA

The effects of flood mitigation structures on the quality of estuarine and freshwater fish habitats in the lower Clarence River system of so uth-eastern Australia were investigated. Surrounding land use, fringin g vegetation, overall level of habitat disturbance, distance from the sea, salinity, and water temperature were examined and compared betwee n four sites on natural tributary streams, four sites on channelized f lood mitigation drains gated at their mouths, and ten paired sites (fi ve below and five above floodgates) on flood mitigation drains in this system, from mid 1988 to late 1990. Quantitative sampling of the fish fauna at each of these sites was conducted quarterly over this 21/2-y r period. Juvenile fishes were sampled using netting enclosures togeth er with application of the ichthyocide rotenone, while subadults and a dults were sampled using multiple-panel gill nets. In general, fish ha bitats in the flood mitigation drains, and especially those above floo d gates, had more intensive surrounding land uses, less natural native fringing vegetation and, overall, were more highly disturbed than tho se in the natural tributaries. Salinity at the various study sites was largely dependent on the pattern of seasonal (mainly summer and autum n) rainfall and distance upstream from the sea. Salinities usually dif fered only slightly between gated and ungated sites at similar distanc es from the sea, indicating that the floodgates were generally ineffec tive in preventing the penetration of saline river water into the drai ns immediately above them. These gates were, however, very effective i n preventing the establishment of fringing mangrove vegetation in the drains above them. The main ecological effects of these flood mitigati on works have thus been to generally degrade the overall quality of av ailable fish habitat, particularly in terms of reductions in natural f ringing vegetation (mangroves in the more estuarine-dominated areas an d overhanging terrestrial trees in the more freshwater-dominated areas often being replaced by grasses and rushes), and to increase the inte nsity of surrounding land use (natural forest often being cleared and wetlands drained for cattle gazing and sugarcane growing), both of the se factors contributing to increases in generalized aquatic habitat di sturbance. Results from the study of the fish assemblages in these nat ural and man-altered habitats revealed the following general patterns. Highest fish species numbers and abundances occurred in the ungated n atural tributaries and in drains downstream of floodgates. These habit ats also contained the largest proportions of both commercial fish spe cies and individuals as well as the majority of species and individual s with marine-estuarine affinities. Both total and commercial fish spe cies numbers generally declined with decreasing salinity and increasin g distance of the sampling sites from the sea. Even though saline wate rs from the main river system penetrated the majority of the floodgate s during most of the study period, fish passage through these gates wa s found to be very restricted. Fish assemblages above such gates were generally dominated by primarily freshwater species, as compared with primarily saltwater (estuarine-marine) species below. The conversion o f the great majority of small mangrove-fringed tributaries in the lowe r reaches of this river system into uniform floodgated drainage channe ls has thus resulted in the destruction of, and impeded fish access to , large areas of previously available estuarine fish nursery and feedi ng habitat. These drainage channels are now dominated by terrestrial-f reshwater vegetation above where they are cut off from the main river channels by the floodgates, and the period of their construction has c oincided with that of reported declines in fish catches in this river system. On the basis of the above findings, it is recommended that the se floodgates be left fully open at all times except immediately prior to and during floods in the river system, thus facilitating the re-es tablishment of fringing mangrove vegetation along the banks of the art ificial drains in the lower reaches, generally improving flushing and thus water quality in these drains, and allowing the establishment of primarily estuarine-marine fish communities, including more species of economic importance, in them.