The Ichthyofauna of an Intermittently Open Estuary: Implications of Bar Breaching and Low Salinities on Faunal Composition

Abstract

Fish in the shallows of the intermittently open Moore River Estuary in South-western Australia were sampled monthly between February 1994 and February 1995. The bar at the estuary mouth became sufficiently scoured out by freshwater discharge on two occasions, totallingc. 41 days, to allow seawater to penetrate the system. Although saltwater intrusion on one of these occasions resulted in salinities in the mouth rising to 25 for a short period, the mean monthly salinities never exceeded 7·4 in the lower reaches and 4·5 in the middle and upper reaches. Despite the protracted periods when this estuary was closed, some marine species, which typically use estuaries as nursery areas (marine estuarine opportunists), such as the mulletsMugil cephalusandAldrichetta forsteriand the whiting speciesSillago schomburgkiiandSillago burrus, were relatively abundant in this system. However, other marine species, such as the atherinidAtherinomorus ogilbyi, the gerreidGerres subfasciatusand the clupeidHyperlophus vittatus, which are common in the lower reaches of nearby permanently open estuaries, were rare in the Moore River Estuary, possibly reflecting a preference of these species for high salinities. Furthermore, the estuarine speciesAtherinosoma elongataandCraterocephalus mugiloides, which typically occupy the basins of permanently open estuaries where salinities are usually >20, were not caught in the Moore River Estuary. The composition of the ichthyofauna changed progressively in an upstream direction, reflecting mainly a decline in the number of marine species and an increase in the density of estuarine species along this axis. The mean number of species and the densities of six of the eight most abundant species were significantly greater during the night than the day. Since the degree of variation among samples was also far less at night than during the day, there appears to be a marked tendency for a particular suite of species to congregate in the shallows at night, which would reduce the chances of predation by avian piscivores which are locally abundant. The composition of the ichthyofauna of the Moore River Estuary is compared with those of permanently and seasonally open estuaries elsewhere in temperate Western Australia and Southern Africa. These comparisons indicate that the faunal composition of these estuaries changes markedly when salinities decline to very low levels. The faunal compositions of seasonally and intermittently closed estuaries are also influenced by the duration and size of the opening in the estuary mouth.