Impact of dams on fisheries

River regulation in Qld has resulted in many dams, weirs and barrages constructed on waterways to provide water for electric power, agriculture, water supply and flood control. One of the most serious environmental consequences of river regulation is the obstruction to fish passage.

Fish migrate between different environments depending upon which phase of their life cycle they are in. These phases are reproduction, production of juveniles, growth and sexual maturation (Larinier, 2000). Australian fish are predominantly catadromous, which means they spend most of their life in fresh water and migrate to the sea to breed, and potomodromous, which means they migrate wholly within fresh water (Kerby, 1992, Thorncraft and Harris, 2000).

Obstructions to migration can have dire consequences. If migration is impeded, spawning will not occur. Fish stocks decline and this threatens the long term survival of migratory fish species. With 125 native freshwater fish species recognized within Qld, the obstruction of fish migration poses a serious threat. This degree of threat can be significantly lowered through the incorporation of effective fishways. A fishway is defined as a passage of water around, through or over an obstruction to facilitate the movement of fish (Kerby, 1992, Thorncraft and Harris, 2000).

The objective of this position paper is to provide a summary of literature on the ecological and economical impacts of impoundments on fisheries in Qld. This comes as a response to the Department of Natural Resources, Mines and Energy discussion paper Water Resource Charges. This paper outlines three main areas; water management costs, externalities and scarcity.

Externalities are defined as an activity that creates a cost or benefit for another party for which that party is not compensated. Impoundments can be considered an externality as they are created for water extraction, storage and delivery but do not convey anywhere near the full costs involved in actually supplying this water.

Status of fishways in Qld

At present there are only 37 fishways within Qld and only 15 are considered to be effective or likely to be effective and 4 are partially effective or likely to be partially effective (Qld Fisheries Service, 2004). The fishways of Qld consist of three different types: the vertical-slot fishway, rock ramps and fishlocks. Many fishways used in Australia are based on designs used for salmon and trout in the Northern Hemisphere. These fishways are ineffective for native fish species as they are too steep, too turbulent and cease to function during Qld's extended periods of low flow.

Impact of impoundments

Impoundments affect Qld’s fisheries resources ecologically and commercially. Ecological impacts include the loss of habitat, resulting from degradation and barriers to fish migration. Alteration to fish migration results in a decline in abundance, species distribution truncation, localised extinction and reduced species diversity (McGill, 2001). Another impact of impoundments are the consequences altered environments have on the commercial viability of fisheries resources. The decline of migratory fish species due to barriers imposed on fish migration results in a loss of economically valuable species.

Ecological impacts of impoundments

Impoundments create two degrees of habitat loss: complete and partial. Complete habitat loss is due to complete obstruction to passage due to both the absence of fishways and the presence of ineffective fishways. Partial habitat loss exists when habitat is compromised and modified/degraded due to processes such as sedimentation.

Secondary to loss of habitat, is modification to communities due to obstruction to fish passage. Obstruction to fish passage results in modified communities due to population and species decline and even extinction.

Complete loss of habitat

Barriers lacking effective fishways can impose complete obstructions to fish passage, thus locking up areas of vital habitat for migratory fish species. These forms of barriers are most evident in the form of large dams.

Dams have generally resulted in negative impacts to native riverine fish species while encouraging exotic species. This has partly been attributed to the disruption of seasonal flood cycles, and to ineffective fishways on dams acting as barriers to fish movement. Heidenreich and Lupton (1999) state that declines in the distribution and abundance of native fish in Australia are predominantly caused by barriers to their migration (Heidenreich and Lupton, 1999).

Barriers without effective fishways isolate areas of available habitat within a catchment, the cumulative effect of several of these barriers resulting in a significant impact on fish communities (Thorncraft and Harris, 2000). Complete barriers to fish migration also include smaller structures like road crossings and low weirs. These forms of barriers have less immediately noticeable impacts on fish communities. However, if conditions allowing fish passage occur only infrequently, they may not correspond with the natural timing of fish migration, or suitable conditions may not extend over a long enough period to allow movement by a sufficient proportion of the population (Thorncraft and Harris, 2000). It is in these situations that recruitment to upstream areas is reduced and mortalities increase (Heidenreich and Lupton, 1999, Thorncraft and Harris, 2000).

Partial Loss of Habitat

Impoundments ultimately modify riverine habitats with the most common alterations being sedimentation and changes to water quality. Reservoirs act as sediment traps and can degrade habitat above the dam as well as below the dam, and reduce storage capacity (McAllister et al., 2001). Channel deepening often occurs downstream of storages as available sediment is removed from the flow and trapped in the dam, ultimately giving downstream flow more energy to scour (Markham, n.d.). Deposition of this scoured sediment often occurs further downstream. Fish species are affected by increased sedimentation through changes to turbidity, infilling of pool habitats and smothering of food and spawning areas (Waters, 1995).

Predation

The spatial distribution of species is significantly modified after construction of impoundments due to factors such as predation. Barriers that enable small species to pass but block larger fish from migration can alter the ecology of rivers by reducing the biomass of predators at or near the top of the food web (Gehrke et al., 2001). Fish migration is delayed for all class sizes at barriers with ineffective or no fishways and results in a concentration of migratory species in habitat favoured by predatory species (Larinier, 2000). This results in modified communities and altered biodiversity.

Extinction

The most serious ecological impact of impoundments with ineffective or no fishways is species extinction. If some species are prevented from free migration between feeding and breeding areas, the likely outcome is extinction as no spawning grounds would be present downstream of dams (Larinier, 2000). As outlined in Barry (1990), obstructed fish passage has led to many instances of declining populations or extinctions of species in Qld.

Commercial impacts of impoundments

The commercial impact of impoundments on Qld river systems has not been explored. This gives rise to concern as the freshwater fisheries resources of Qld are valued at over $500m annually in recreational fishing to the Qld economy (QLD DPI, 2004).

Barramundi (Lates calcarifer) is a $6m commercial and $8m recreational industry (ABFA, 2004). Barramundi are catadromous and migration between freshwater and saltwater is imperative for different stages of their lifecycle. Walker (1985) found that dams, weirs and tidal barrages have resulted in declining catches of barramundi (Lates calcarifer) in Qld’s northeastern rivers. Ineffective fishways that hinder these vital cycles exert pressure on fisheries with Ribeiro et al. (1995) indicating that downstream fisheries can be reduced by up to 77% as a result of recruitment failure.

The abundance of golden perch (Macquaria ambigua) has dramatically decreased in the Murray-Darling system due to migration obstruction (NSW DPI, 2004). Bay prawns comprised 36% of the total economic catch of prawns in the Lower Burnett (Heidenreich and Lupton, 1999). Bay prawns use brackish areas to spawn and the loss of this habitat area as a result of the tidal barrage built in 1976 is severe and has implications not only for the regional economy but also for the availability of food for fish species in this zone (Heidenreich and Lupton, 1999).

Summary

Impoundments bring about significant modifications to aquatic ecosystems. One of the greatest problems for these systems is barriers to fish passage. A preliminary assessment of 66 case studies conducted by McAllister et al. (2000) found that 55% of impacts were related to obstruction to fish migration. Riverine environments provide important habitats to economic and non-economic species which must have unregulated access to fresh and marine habitats to allow migration within throughout the catchment so that the viability of fisheries resources are not compromised. – Qld Conservation Council

This is a condensed version of the original paper. The complete version, with references, is available at www.qccqld.org.au (go to the Newsletters page and select November 2004). You can also get more information by contacting the Qld Conservation Council on (07) 3221 0188.

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