The decline of species can be largely attributed to the impacts of disturbances, both directly on the species and indirectly on essential components of their habitat. Disturbances which have negative effects (direct or indirect) on a species are referred to as threatening processes.
A review of the current state of knowledge of aquatic species, and of threatening processes was conducted to provide information to assist in setting priorities for management, research and surveys and the integration component of the Central Highlands CRA. The review covered priority aquatic species (Table 7.9) in the Central Highlands, and was based on existing scientific literature and expert opinion.
A number of processes can, or have the potential to have, serious impacts on aquatic ecosystems, and therefore on aquatic species. A list of broad disturbance categories and their major impact on aquatic ecosystems is shown in Table 7.10.
By far the most common effect of most disturbances is the increase in sediment accession to rivers and streams. While sediment in streams is part of the natural erosion process, and fauna are presumed to be adapted to natural variations in sediment levels (e.g. natural increases during high flows), several activities can lead to additional sediment inputs into streams.
Increased levels of sediment can adversely affect all aspects of freshwater ecosystems by reducing water quality and degrading or destroying habitat. Increased turbidity can have adverse physical, physiological and behavioural effects on stream dwelling plants and animals. Sediment is harmful to gill structure, clogging gill mucus and causing asphyxiation. Fish that feed using vision to locate prey can also be affected by the reduced visibility in the water column caused by increased turbidity. Elevated levels of deposited sediment can smother stream beds, simplifying the habitat, and exposing fish to increased predation and stress. High levels of sediment can fill in deep pools, destroying the entire habitat for some species. Crevices in the substrate between rocks or debris serve as critical habitat for fish, mainly as egg deposition sites and rearing areas for juveniles. Sediment settling out can fill these spaces and subsequently destroy important habitat. Increased sedimentation of rivers is a listed Threatening Process under the Flora and Fauna Guarantee Act 1988.
Other disturbances alter the natural stream chemistry, increasing levels of nutrients (fire, timber harvesting, grazing, waste disposal) or toxic chemicals (pest control, mining, waste disposal). Introduction of toxic material into rivers is a Potentially Threatening Process listed under the Flora and Fauna Guarantee Act 1988.
Dams represent a severe disturbance to aquatic systems. Where low level offtakes are used (many older dams) water temperature can be lowered substantially. Storage and release of water at different times also changes natural flow regimes. Dams can also present a barrier to migratory fish species. Altered temperature regimes, altered flow regimes of rivers, and barriers to fish passage are all listed Potentially Threatening Process under the Flora and Fauna Guarantee Act 1988. Increased sedimentation can also occur either during construction or cleaning of storages.
Introduced species can have serious impacts on stream fauna through increased competition for space and food, or through direct predation on native species.
It should be noted that some species can be affected by disturbances which occur some distance away. Additional sediment in a stream can gradually move downstream during floods. If severe, this can affect areas and species many kilometres from the source of the disturbance. Dams acting as fish barriers can affect large areas upstream. A barrier at the mouth of a river can effectively exclude species from the whole of the catchment upstream.
Table 7.10: Broad disturbance category with potential impacts on aquatic ecosystems
Few data are available directly relating the impact of most disturbance categories to aquatic species. In most cases, the impact on priority species has been predicted from the results of similar studies conducted elsewhere. For example, no data are available on the impact of clearfelling on the four upland Galaxias species (G. fuscus, G. olidus, G. rostratus and G. truttaceus). However, Graynoth (1979) showed that clearfelling without buffers in New Zealand severely reduced numbers of the local species G. divergens in streams, probably through increased sedimentation. While clearfelling without streamside buffers in Victoria is not permitted, it is reasonable to assume that any similar increase in the amount of sedimentation in the Central Highlands (through any of the disturbance categories in Table 7.10) would produce a similar reduction in density of Galaxiids in Central Highlands streams. This is a primary reason why Victoria has implemented a Code of Forest Practices for Timber Production and other regulations designed to minimise or avoid significant sediment inputs to streams.
The one disturbance where good data are available is the impact of dams on aquatic fauna. Changed flow, temperature and sediment have all been implicated in the decline in native fish and invertebrate species in the Mitta Mitta River below Dartmouth Dam (Koehn et al. 1995), and in the Thomson River below the Thomson Dam (Doeg et al. 1987). Sedimentation due to weir cleaning reduced fish and invertebrate densities in Armstrong Creek in the upper Yarra River catchment (Doeg and Koehn 1995).
With these considerations in mind, species affected by each of the disturbance impacts listed in Table 7.10 are presented in Table 7.11. It should be noted that it is mostly of little relevance which disturbance activity creates the disturbance impact. However, the intensity of the effect may differ between the sources, with, for example, weir cleaning producing very high turbidity and deposited sediment levels for a short time, while drainage from roads may produce lower levels but extended over time.
Table 7.11: Species affected by each of the disturbance impacts listed in Table 7.10.