Despite its catchment being subjected to substantial anthropogenic modification to support agriculture and urban development, Toby Inlet is highly valued by the community and considered to provide important habitat for fish and waterbirds. This study aimed to quantify these perceived biological values by determining the fish and benthic macroinvertebrate species present in the system and elucidate whether the number of species, abundance, diversity and faunal composition differed among regions of the estuary and/or changed when the bar at the mouth of Toby Inlet was closed (November 2017) and open (March 2018).
A total of 12,438 fish from 17 species were recorded in the shallow, nearshore waters of Toby Inlet across the two sampling occasion. Most of these are short-lived, small-bodied (< 50 mm total length), belong to the atherinids and gobys and complete their life cycle within the estuary. While the mean number of species, density and diversity remained similar among regions and seasons, faunal composition changed following the opening of the bar, but this was mainly due to the timing of reproductive cycles of these typically short-lived (1 year life cycle) species. It also reflects the fact that these species are generally highly euryhaline and able to tolerate even the hypersaline conditions that occurred in the upper region in March.
In contrast to nearshore waters, the 620 fish caught and released in the offshore waters comprised mainly marine species. The composition of this fauna changed markedly following the opening of the bar as only four species were recorded in November, increasing to 12 after the bar had been open for several months. All of the species recorded only in March spawn in marine waters and where able to recruit to the estuary as the bar was open and salinities had increased markedly, facilitating their survival in the estuary. The changes in faunal composition were mirrored by changes in mean number of species and catch rates, both of which were greater in March. Regional differences in fauna were also detected, due to the declining influence of these marine species in the upper estuary and Deadwater. This latter region contained an impoverished fauna due to its shallow depths and limited connectivity to the ocean (the Station Gully mouth was closed throughout the study) and the lower region of Toby Inlet.
The benthic macroinvertebrate fauna, which was only sampled in November, comprised a range of insect, crustacean, annelid and mollusc species typically found in either wetland or estuarine environments. This reflects the low salinities at the time of sampling (2-8) and the connectivity of the system to wetlands upstream of Caves Road Bridge. While some, annelids and molluscs were found in each of the four regions, some wetland insects and amphipods were more common and abundant in the upstream regions (upper estuary and the Deadwater), whereas the reverse was true for several estuarine species of annelids and a bivalve.
The opening of the bar, allowed the immigration and emigration of fish species from the estuary and increased salinity. This influenced the fish fauna in offshore more than nearshore waters, as most of these species breed in marine waters and thus require the bar to be open to be able to recruit to the estuary. As invertebrates were only sampled in November, when salinities, were very low, there would be value in repeating the sampling when salinities were greater to elucidate what effect bar opening may have on that fauna, particularly as many species of fish and birds use this fauna as a critical food source. It should be noted that sampling was only conducted in areas of downstream of Caves Road Bridge, but that an option to construct a second mouth in the upper region of Toby Inlet would influence the fauna in that region and thus there is a need to collect some baseline data from that area to allow the potential impacts to be explored. However, the data in this report provide a robust baseline against which future changes in the fauna of Toby Inlet could be detected and also may help in the development of management strategies to improve the health of this highly-valued estuary
