Marine and estuarine ecosystems in the Port Davey Bathurst Harbour region: biodiversity, threats and management options: a report

• Main Authors: Edgar, GJ, Barrett, NS, Last, PR, Driessen, MM
• Format: Report
• Language: English
• Published: Aquenal 2007
• Subjects: Davey; Gibba; New Zealand; Schooner Cove; Turnbull; Woody Islands; Crassostrea gigas
• Online Access: https://eprints.utas.edu.au/10549/; https://eprints.utas.edu.au/10549/1/Port_Davey_Compilation_Report_Aquenal.pdf

Physical characteristics of the region The most prominent geomorphological feature along the South West Tasmanian coast is the Port Davey – Bathurst Harbour estuarine system, which includes Bathurst Harbour, Bathurst Channel, Port Davey and associated small embayments. While all estuarine systems possess a unique combination of physical characteristics, the Port Davey – Bathurst Harbour system can justifiably be regarded as Australia’s most unique estuary, and perhaps the most anomalous globally. The unusual combination of factors operating in this system include: 1. negligible human population density in catchment 2. little anthropogenic impact in estuary 3. large water volume and surface area 4. ria formation, but with many fjord characteristics, including narrow deep channels and relatively shallow sill at seaward entrance 5. high levels of riverine input 6. strongly stratified conditions that vary seasonally, with marine salt wedge and low-salinity surface layer 7. low-salinity surface layer deeply tannin-stained, blocking light penetration to depth 8. low levels of nutrients and low aquatic productivity 9. seaward entrance of main channel protected from wave action by island barriers 10. strong barometric influence on tides 11. low residence time of bottom water 12. high stability of bottom water with respect to temperature and chemical signature Biological characteristics of the region The anomalous combination of geomorphological, climatic, hydrodynamic and hydrological conditions operating in the Port Davey region has generated an aquatic environment with a highly unusual mix of flora and fauna. The uniqueness of this biota exceeds that of terrestrial habitats in the Port Davey catchment, which are similar to terrestrial habitats in other catchments of the Tasmanian west coast. Like the adjacent terrestrial environment, the estuarine ecosystem has compositional and phylogenetic relationships with the fjord regions of southwestern New Zealand and Chile. Unique and characteristic features of the Port Davey – Bathurst Harbour ecosystem include: 1. presence of extremely narrow-range endemic species 2. high variability in biotic composition over small spatial scales, with several different zones distributed along the estuary and different bands distributed with depth in each zone 3. relictual Gondwanan elements present (at least 80 mya), notably including the Port Davey skate 4. displaced deepsea species, including many octocorals 5. paucity of introduced taxa 6. predominance of fragile and delicate sessile invertebrates, including slowgrowing fenestrate bryozoans 7. presence of threatened taxa 8. bryozoan-based sediments that are more characteristic of the continental slope than inshore environments 9. relatively few molluscs, echinoderms, or decapods. A total of 46 unicellular algal taxa, 108 seaweeds and seagrasses, 496 invertebrates and 102 fishes have been recorded from the region during recent surveys. Floral and faunal assemblages should be resurveyed to gain a more complete picture of species composition and changes through time. Additional new species, many of which are likely to be endemic to the region, should be discovered with further field investigations. Distribution of biota Plant and animal communities associated with reefs in Port Davey differ greatly from those found in Bathurst Channel, with few species in common. Major differences are evident between plant and sessile invertebrate assemblages associated with eastern and western Port Davey shores. Few studies have investigated planktonic or soft-sediment assemblages in the region. The Bathurst Channel biota varies along the estuarine cline, with foliose algae reaching 5 m depth in the western region but not penetrating below 1 m in the east. By contrast, sessile invertebrates are most abundant below 5 m depth in the west and below 2 m in the east. Multivariate analysis indicated a total of eight major biotic assemblages that characterise different sections and depths of the Channel. For management purposes, the Bathurst Channel-Bathurst Harbour subregion has been subdivided into five zones with different biological characteristics. From west to east, these are: (i) Bramble Cove Zone, extending from Port Davey to western Turnbull Island, (ii) Bathurst Channel Sea Pen Zone, extending eastwards to Forrester Point and Munday Island, (iii) Bathurst Channel Tube Worm Zone, extending to Joan and Farrell Points, (iv) Bathurst Narrows Lace Coral Zone, extending to Bathurst Harbour, and (v) Shallow Embayment Sediment Zone, which includes Bathurst Harbour and the larger embayments off Bathurst Channel including Joe Page Bay and Horseshoe Inlet. The five different zones are subject to different types and levels of threat. Threats Seven major threats to the integrity of the Port Davey – Bathurst Harbour ecosystem are outlined and discussed: boating impacts, fishing impacts, diver impacts, effluent impacts, introduced species, global climate change, and onshore activities. In order to reduce the potential of these impacts to threaten the unique Port Davey ecosystem, 20 management recommendations are outlined: 1. Anchoring should be prohibited within the main channel of Bathurst Channel (defined within a baseline extending between rocky headlands along northern and southern shores) and in adjacent bays for 200 m distance from the main channel. 2. A Code of Conduct for Boat Operators and Vessel-based Visitors should be prepared and distributed to commercial and private vessel operators visiting Port Davey. Port Davey-Bathurst 3. A booklet on boating in Port Davey should be produced that includes the Code of Conduct, legal regulations, background information on the unique biota, threats to biodiversity, quarantine protocols to reduce risk of transfer of introduced species, safety issues, health risk associated with eating local shellfish, actions required in the event of an oil spill, and location of no-anchoring zones. 4. An oil spill response strategy for the region should be finalised, and an oil spill response kit permanently stationed at Port Davey. 5. The boundary of the Bathurst Channel No-take Sanctuary Zone should be extended to include Shanks Island and the western as well as the eastern side of Breaksea Island to a distance of 500 m offshore, and the boundary of the Inner Saddle Bight No-take Sanctuary Zone should be extended to include Outer Saddle Bight and Point Lucy to a distance of 500 m offshore. 6. Diving should be prohibited without special permit to a distance of 200 m offshore from Farrell Point, Munday Island and Little Woody Islands. 7. A Code of Conduct for Divers should be prepared and distributed to divers visiting Bathurst Channel. 8. The Code of Conduct should state that no contact with reef surfaces is permitted while diving in Bathurst Channel. 9. The Code of Conduct should include a warning that diving should not be attempted in Bathurst Channel unless the diver is experienced with dark cold conditions and strong currents, and can achieve a high level of buoyancy control. 10. All commercial dive operations should be required to include a dive guide accredited by Parks and Wildlife Service (PWS) with demonstrated experience in the area, and who is responsible for compliance with the Code of Conduct. 11. Biological monitoring at key dive sites and “no-dive” zones using video and photoquadrats should be undertaken at intervals not exceeding 3 years, and annually if resources allow, to provide information on diver impacts and reef assemblage resilience. 12. If deterioration of habitats at key dive sites is noted during monitoring, then controls should be placed on diver visitation at these sites. 13. Diver visitation rates at key dive sites should be recorded and compiled on a PWS database. 14. Discharge of sewage effluent should be banned from live-aboard vessels operating within Bathurst Channel east of Schooner Cove, Bathurst Harbour and adjacent inlets. 15. A monitoring program should be established to assess the distribution and dynamics of the toxic dinoflagellate Gymnodinium catenatum, and the impact of this species on the natural food web. 16. A health warning should be issued that eating shellfish from Bathurst Harbour or Bathurst Channel could cause 17. A systematic monitoring program to detect the presence of introduced marine pests should be established with monitoring interval of three years or less, with emphasis on the detection and range mapping of target pest species (Crassostrea gigas, Asterias amurensis, Gymnodinium catenatum, Carcinus maenas, Corbula gibba, Undaria pinnatifida and Musculista senhousia) and the New Zealand screw shell Maoricolpus roseus. 18. Baseline surveys of zooplankton species distribution should be conducted, with emphasis on assays for the presence of larval Maoricolpus roseus. 19. Long-term changes in water temperature should be monitored using loggers deployed at multiple sites and depths along the Bathurst Channel estuarine cline. 20. Populations of the hollow-spined sea urchin Centrostephanus rodgersii should be monitored in Bathurst Channel in conjunction with surveys of introduced marine pests, MPA zone ecological reef monitoring, and surveys of native sessile invertebrate assemblages. A total of 41 research recommendations are also outlined and prioritised, as described in Section 6.