Do the characteristics of the ichthyoplankton in an artificial and a natural entrance channel of a large estuary differ?

Abstract

The Peel-Harvey Estuary in south-western Australia became so eutrophic in the 1970s and 1980s that a radical approach was adopted in order to increase tidal exchange between this estuary and the ocean and thereby flush greater volumes of nutrients out to sea. This involved constructing a deep artificial (Dawesville) channel between the estuary and the ocean, at a location ca. 15 km south of the shallower and natural (Mandurah) channel. Samples of fish larvae were collected monthly throughout 1997 to determine whether the characteristics of the ichthyoplankton on flood and ebb tides differed within and between channels and whether they underwent the same types of intra-annual changes in the two channels. In both channels, the number of species and concentration of larvae were both significantly greater on flood than ebb tides and the species composition differed significantly between tides. The concentrations of larvae were at their greatest, by far, between early spring and early autumn. Flood and ebb tides in both channels were dominated by the goby Favonigobius lateralis, which was transported out on ebb tides as preflexion larvae and subsequently brought back on flood tides as postflexion larvae. The larvae of Hyperlophus vittatus, Pseudorhombus jenynsii and Rhabdosargus sarba, which spawn outside the estuary, were far more abundant on flood than ebb tides, whereas the reverse pertained with Atherinosoma elongata, Apogon rueppellii and Pseudogobius olorum, which spawn within the estuary. In contrast to the situation with the diversity and concentration of larvae, the species composition on both flood and ebb tides differed significantly between channels. However, the latter differences were small and mainly due to relatively greater numbers of larval F. lateralis on ebb tides in the Mandurah Channel and the very occasional occurrence of reef species on flood tides in the Dawesville Channel. During the year, the species composition underwent cyclical changes on both tides and in both channels, due to out of phase sequential monthly changes in the concentrations of certain species.