Records of fish kills in inland waters of NSW and Queensland in relation to cotton pesticides

Abstract

Fish kills are highly visible events and media publicity of them is powerful in developing the perception that our rivers are highly degraded through the impact of pesticides or other human insults. Ecotoxicology data derived from laboratory experiments suggests that fish, compared with other aquatic species, are relatively sensitive to pesticides used extensively in cotton cultivation. For example, the organochlorine endosulfan yielded 96-hrLC50s ranging from about 0.1µg/L for European carp to 0.2-2.4µg/L for native species. However, there is anecdotal evidence that fish and other aquatic life are healthy in cotton tailwaters likely to be contaminated. We review database of fish kills in northern NSW and southern Queensland kept by state governments, and report of fish kill frequencies in different river basins, their presumed or suspected causes, the species involved, and whether and which, pesticides were detected in each incident. These databases showed that fish kills were reported more often from cotton-growing areas and during cotton-growing season. More than half the 98 recorded fish kills were associated with pesticides by the investigators. However, many of the records gave only circumstantial evidence as to their cause, and only a few were actually investigated in detail. This makes it difficult to make a proper assessment of how pesticides may be implicated in fish kills or of the impact, more generally, that cotton pesticides are having on fish populations. We also present data on residues of endosulfan found in live fish near a lagoon near Wee Waa before and shortly after a fish kill. We conclude that simply measuring tissue concentrations of pesticide residues does not provide direct proof that a fish died from any pesticides detected. Overall we suggest that more careful scientific attention needs to be paid to and fish kills occurring in irrigation areas before we can know the real extent of damage from pesticides and therefore what management is needed