Guest speaker Marnie Giroud, from the Swan River Trust, presented a talk entitled the ‘Natural history of the Swan/Avon and its management’ to our DRB naturalists group.
Marnie brought along information sheets and booklets, that relate to fish life, species identification and water quality in the Swan River and Swan-Canning estuary as well as pamphlets concerned with water conservation, the responsible use of garden fertilizer and weeds potential. The information brochures are all issued by Government Departments, some combining research with Universities.
The Swan River is one of the major attractions of Perth and is highly valued by the community. It has a total length of ~67 km and together with the Canning River (length ~100 km) has a catchment covering 2126 km2.
The rivers and estuary have a range of important environmental values in their riparian vegetation and diverse aquatic ecosystems. Notable fauna in the estuary include bottlenose dolphins, sea horse colonies, sea stars, jellyfish (25 species), fish (130 species, including Perth Herring and Mullaway), prawns (two species), blue crabs and freshwater crayfish. Bird life includes our iconic black swans and cormorants (four species), and reptiles include frogs and the endangered long-necked Western Swamp Tortoise. One species of native water rat (not Rattus rattus) and quenda are among hairy mammal species encountered along the rivers.
The dolphins of the Swan River (Indian Ocean bottlenose dolphin, Tursiops aduncus) comprise a pod of ~25 resident members, identified individually from characteristics of their dorsal fins. Sadly, numbers dwindle through accidents (seven deaths in recent times), especially due to fishing equipment. There are also rare heart-warming stories. We were shown a short film relating the successful release of a young dolphin that had been entangled in fishing gear. After several hours attempting to capture and hold the dolphin, skilled police officers managed to cut it free although whilst the efforts were ongoing the dolphin mother encircled them continually and also attempted to grab the youngster.
The rivers are managed by several agencies with the support of 50 DEC Statutory Authority members including scientists with different specialisations. The many ‘Friends’ groups and volunteers are all honorary guardians from the Community and Industry and their contributions are inestimable. In 2006 the State Government passed the Swan and Canning River Management Act, demonstrating the importance of the rivers and the need for a structured and co-ordinated management plan.
The Swan and Canning Rivers are beset with ongoing problems, some natural, but most originate from the high density of residential and farmed lands and their proximity to the river. Water quality and the ecological health of the rivers are under constant pressure; some of the changes to the water include periodic low oxygen conditions, algal blooms causing fish deaths, storm water outflow and reduced rainfall in the catchments. One of the most important factors is the need to reduce nutrients to the rivers, especially in the zone between Chittering and Armadale (hence the literature on responsible gardening and plant management).
The issues that relate directly to the use of fertilizers was demonstrated when we were shown two jars of water; one, a sample of river water and another that contained river water with just two drops of added fertilizer. The first was clear and contrasted to the other that was luridly green-coloured by algal infestation. Change in chemistry (nutrients) to the waterways results in dramatic algal development and to one of the major problems that occurs along the course of the rivers.
Important water quality improvement projects aim to reduce nutrients to the rivers by circulating water through special wetland zones (such as the site along Riverside Drive). Wetland sites also improve the ecological diversity along the river and add habitats for various animal species. There are also oxygenation points along the river, such as one at Guildford, which add oxygen to the water in low-oxygen times that occur periodically. This treatment affects about a 3 km stretch of river following treatment but does save particular fauna in times of stress.
The impact of rapid population growth, increased urban development and increased use of boats on the rivers in the Swan Canning catchment, has led to a decline in water quality, dwindling water resources and native vegetation and erosion along river banks (especially noted where boat users are active). Awareness of the complex issues affecting the river systems is an important part of its management beginning with education projects involving school children.
Postscript
Within the week following the talk a press release reported that State Government funding of $1.5 million over three years will go to ten new projects to help reduce nutrient loads from the catchment entering the rivers. The projects range from re-engineering storm water drains and improving fertilizer management to re-vegetating foreshores to help filter and block nutrients.
Susan Stocklmayer