Influence of an artificial entrance channel on the ichthyofauna of a large estuary

The opening in April 1994 of a deep artificial entrance channel into the shallow, microtidal and large Peel-Harvey Estuary (136 km²) in south-western Australia has led to major changes in the ichthyofauna of this system. This conclusion is based on statistical comparisons between data derived from samples of fish collected seasonally by seine net in the short, narrow and shallow natural entrance channel and in two large basins of the Peel-Harvey Estuary during 1996 and 1997, i.e. after the opening of the artificial channel, and data previously recorded seasonally using the same sampling regime during 1980 and 1981, i.e. before the construction of that channel. These comparisons show that the marked reduction in macroalgal growths that occurred between these two periods was accompanied by a decline in the abundance of fish, and particularly of macrophyte-associated species such as Pelates sexlineatus and Apogon rueppellii. There were also strong indications that the number of fish species usually present in the estuary declined, which would be consistent with a reduction in habitat heterogeneity. The comparisons also imply that the construction of the artificial channel led to: (1) a decline in the extent of interannual differences in the species richness and abundance of fish, presumably reflecting a reduction in the variability of environmental conditions in different years; (2) a greater penetration of the estuary by marine species and an increased contribution of these species to the fish fauna overall; (3) the influence of "season" on the ichthyofaunal compositions of assemblages within the estuary becoming more important than region. The second and third changes reflect a combination of increased tidal flow, which facilitates a more effective dispersal of fish, the exposure of fish to stronger tidal cues, a far greater proximity of the more distal regions of the estuary to the sea and, in the case of the third change, a far less pronounced difference between environmental conditions in the two basins.