F)Fledg1f\Fg PROPOSAL at PELICAN ROCKS, SWAN RIVER ESTUARY
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• f)flEDG1f\fG PROPOSAL AT PELICAN ROCKS, SWAN RIVER ESTUARY REPORT BY THE ENVIRONMENT AL PROTECTION AUTHORITY SEPTEMBER 1979 DEPARTMENT OF CONSERVATION & ENVIRONMENT WESTERN AUSTRALIA BULLETIN ND 77 REPORT BY THE ENVIRONMENTAL PROTECTION AUTHORITY ON A DREDGING PROPOSAL AT PELICAN ROCKS, SWAN RIVER ESTUARY SEPTEMBER 1979 Department of Conservation and Environment Western Australia Bulletin No. 77 ENVIRONMENTAL PROTECTION AUTHORITY BP HOUSE. I MOUNT STREET, PERTH, WESTERN AUSTRALIA fiOOO Telephone 322 1477 r 7 HON. MINISTER FOR WORKS ;,;-.· Your Ref L _J Our Ref 28/73 PELICAN ROCKS DREDGING PROPOSAL I wish to advise that the Authority has given further con sideration to a proposal promoted by the Yachting Association of W.A., to dredge an area known as Pelican Rocks in the Swan River Estuary. Following discussions with University researchers, and advice from other people, there can be no doubt that dredging of Pelican Rocks will affect the fishery of the Swan River Estuary, and Melville Water in particular. This will have serious short term implications on the existing users of the Estuary, and may result in long term changes in the use of the river by fish and other fauna. In view of the significant adverse effects the proposal would have on the estuarine fauna and fishermen, both amateur and professional, the Authority recommends that there should be no dredging of the Pelican Rocks area. The Authority has further resolved that, as a consequence of its considerations of the likely effects of dredging it was opposed to any dredging of the shallows of the Swan and Canning Estuary unless there were compelling reasons for such a pro- · ject to be undertaken. As stated in previous correspondence on this subject, the dredging of this area would provide additional yachting space on the Swan River. However, the proposal is seen only as a short term solution to boating congestion of Melville Water. A report describing the background and details of the proposal, the evaluation of the various study reports and the effects dredging .would have on the biological resources is attached for your consideration. /) ,,;7 .-',//. I f _ •• - "f" , / 1'/,. I , . I I k/./ / f I · ' ' l? / .· .,l [L,,� C. F. PORTER CHAIRMAN 13 SEPTEMBER, 1979. CONTENTS 1. SUMMARY 1 2. BACKGROUND 2 3. DETAILS OF PROPOSAL 3 4. STUDY REPORTS 4 4.1 Engineering Investigations 4 4.2 Macrobenthic fauna 5 4.3 Fish Studies 7 4.4 Utilisation of the Pelican Rocks Area 8 Yachting Fish resources Other activities 5. ANTICIPATED EFFECTS OF DREDGING PELICAN ROCKS 10 ON FISH AND CRUSTACEAN POPULATIONS REPORT BY THE ENVIRONMENTAL PROTECTION AUTHORITY ON A DREDGING PROPOSAL AT PELICAN ROCKS, SWAN RIVER ESTUARY 1. SUMMARY Since 1971, representations have been made to Government Departments to increase the area of navigable water in Melville Water, and particularly at Pelican Rocks, for recreational purposes. Various studies have been undertaken since 1977, at the request of the Environmental Protection Authority, to determine the likely impact that dredging the shallow bank at Pelican Rocks would have on the estuary, from both physical and biological aspects. Dredging of the Pelican Rocks shallows will seriously affect the resident fauna of the area and fauna visiting the area. The resident benthic invertebrate fauna at the dredging site will be destroyed and fauna resident in the deeps receiving the spoil will be buried. As a consequence, the resultant reduction in animal numbers will affect a number of fish species, such as black bream, cobbler, Perth herring, mullet, whiting and tailor, all of which use the Swan River Estuary for breeding, nursery and feeding purposes. The dredging operation will result in a reduction in the fish population in the estuary, during the dredging period, and probably for some time after, depending on recovery of the invertebrate fauna. Any decline in the availability of these fish species will affect catches by commercial and amateur fishermen. The Swan River Estuary is the second most important estuarine fishing area for commercial fishermen in Western Australia. Only Peel-Harvey Inlet has a larger commercial catch. As an amateur fishery, however, the Swan and Canning Rivers are the most important, with over 1,000 fishermen having been surveyed on one evening in 1977. As currently envisaged, the dredged material will be discharged into deep water adjacent to Pelican Rocks. This will result in substantial amounts of silt and clay being suspended in the water column. Not only will the increased level of turbidity result in visual discolouration of the water for the duration of the dredging operation, but the suspended solids will affect the estuarine fauna. Filter feeding invertebrates, such as mussels and barnacles, will be seriously affected, while certain important commercial fish species, such as tailor and yelloweye mullet will avoid areas of high turbidity. Deposition of the spoil in deep water will not only des�roy the resident invertebrate fauna, but it will also reduce the area of deep water available to the various fish species which require saline conditions at certain times of the year and during specific stages of development. -2- There are extensive areas of shallow water (less than 2 m) in the estuary, similar to those of the Pelican Rocks area. These are biologically the most productive part of the estuary because they lie within the photic zone and because invertebrate populations are greatest there. The shallows are essential to the well being of the estuarine ecosystem and any reduction in them will reduce the food and available feeding grounds for both fish and waterbirds. 2. BACKGROUND In 1971, the Yachting Association of Western Australia began making representations to various government bodies concerning the desirability of dredging certain shallows in the middle and lower sections of the Swan River Estuary to increase the area of navigable waters and hence reduce yacht and boat congestion. The Association considered that that portion of the shallow bank extending from Pelican Rocks in a westerly direction towards Knott, Concrete and Foam Spits should receive the highest priority for dredging. The Yachting Association requested the Minister for Works to investigate the possibility of dredging the Pelican Rocks area. This preliminary evaluation, which was conducted by the Public Works Department in 1972, indicated that the project would have to be spread over three years at an estimated total cost of $800,000. At this time, it was proposed to dispose of all material into deep portion; of the river. An alternative suggestion for disposal by stockpiling and carting away was expected to double the cost of the project. A further possibility of creating two islands at Pelican Rocks with the spoil was proposed by the Swan River Conservation Board. Following this investigation, the proposal was referred in April 1973 by the Under Secretary for Works, to the Environmental Protection Authority, (EPA). The Authority decided that the Pelican Rocks proposal fell within an area in which its Estuarine and Marine Advisory Committee (EMAC) could investigate and report. In September 1975, the EPA received and considered the report from EMAC and concluded that the spoil could be distributed into areas where shell had previously been excavated. However, in the absence of detailed data the Authority could not agree to this disposal method. The island alternative was totally opposed by the EPA on the grounds that it would alter the aesthetic appeal of the river and affect river flow patterns. The Authority suggested that : "instead of committing substantial funds to a dredging project that will only provide short term relief to river boating and yachting congestion, the Government should consider the merits of expending this amount of money towards the provision of a facility outside the river, or facilities such as well-designed marinas." -3- As a result of these conclusions, the Minister for Works requested the EPA to outline the data required to alleviate the Authority's concern regarding deep water disposal of spoil. On l June 1976, the EPA determined four questions that would need to be answered: i. Would the three-year dredging programme cause unacceptable visual levels of turbidity? ii. Would material settle on the Como foreshore with detrimental effects on either the recreation usage or the wading bird habitat? iii. Does the bottom fauna represent a particularly significant food source for fish? iv. What impact would dredging of the area have on amateur and professional fishermen? The Authority suggested that five areas of study could form a basi s upon which a judgement on these questions could be made : a. Composition of material proposed for dredging in relation to its potential for remaining in suspen sion, and measurement of surface and bottom currents pertaining to the proposed dump areas. b. Nutrient levels of material in relation to its potential for releasing nutrients during summer periods thus causing plankton and other algal blooms. c. Importance of the benthic fauna as a food source for fish and crustaceans, and of these fish and crustaceans. d. Utilisation of the area by amateur and professional fishermen. e. General utilisation of the river by the community at large. 3. DETAILS OF PROPOSAL A report has been prepared by the Public Works Department f on the basis o 1 deep water disposal of the spoil from Pelican Rocks. The current proposal being examined by the PWD is to dredge approximately 107 hectares of shallows at Pelican Rocks to depths of 2 metres and 2.74 metres below mean sea level and to dispose of the spoil in adjacent deep channels. (See Fig. 1). The estimated total project cost is approximately $2.5 to $3 million at December 1978 values. This estimate assumes that no undue constraints are placed on normal dredging practice.