Blue-green algae and fish population changes in a eutrophic estuary

Evidence is presented which indicates that dense blooms of the blue-green alga Nodularia spumigena have affected fish and crab populations in a large estuarine system in south-western Australia. For example, the numbers of fish were generally very low at sites in which the chlorophyll a level, an excellent indicator of Nodularia density during the late spring and summer, was above 100 μgl −1. Moreover, commercial fishermen have recorded greatly reduced catches in Nodularia-affected areas and dead fish and crabs were found in regions where Nodularia was very dense. While the effects of this blue-green alga apparently led to death in the case of some bottom-living species in the most affected parts of the system, more active species moved into regions where Nodularia was virtually absent.