Abstract

Executive summary:

• Experts from consulting, government and research including individuals with up to 50 years’ experience in Perth coastal waters discussed potential drivers of seagrass decline based on trends from the long-term seagrass monitoring program in Cockburn and Warnbro Sound and Owen Anchorage (27 sites from 2003-2020) and their personal knowledge and expertise.

• The collation of seagrass monitoring data demonstrated that of the 27 sites monitored, only six had a significant decline over time, and these sites were both within and outside of Cockburn Sound (Warnbro Sound, Southern Flats, Jervoise Bay, Garden Island and Woodman Point).

• For the sites that were declining, consistent drivers of decline across these sites were not identified based on expert opinion, indicating the importance of local conditions for influencing seagrass condition.

• Seagrass sites where there was high confidence in the drivers that were impacting condition included Woodman Point, Mangles Bay, Jervoise Bay, Warnbro Sound 2.0 m and Warnbro Sound 3.2 m.

• The drivers of decline that had the most confidence based on expert opinion were erosion and sediment burial in shallow sites, anchor damage at Mangles Bay and sediment plumes from sand mining wash plants at Woodman Point. Other drivers of decline or poor condition were increased temperature, storms and altered hydrodynamics.

• Actions to halt decline and build resilience in seagrass meadows were brainstormed based on the drivers of decline and considering future dredging pressure. Participants were asked to think outside of the box to collate a list that contained innovative and untried actions as well as more well-known or tried and tested actions.

• The drivers from which relevant actions were identified included dredging, shipping and boating, fishing, coastal infrastructure, pollution, biological interaction including grazing and climate change.

• The actions were categorised into the themes of management, nature-based solutions, engineering solutions, industry, knowledge and education.

• Seventy-five different actions linked to the specific drivers were identified that could be implemented to improve seagrass condition and/or build resilience in seagrass ecosystems within Perth coastal waters but also be applied more widely. Before these actions are implemented, an extensive risk benefit analysis should be conducted.

Keywords

seagrass decline, seagrass monitoring, coastal ecosystems, Cockburn Sound, Warnbro Sound, Owen Anchorage, seagrass condition, environmental drivers, sediment burial, erosion, anchor damage, sand mining, dredging impacts, coastal management, ecosystem resilience, marine conservation, climate change, coastal waters, nature-based solutions, environmental management

Document Type

Report

Date of Publication

4-2025

Type of File

PDF

Publisher

Western Australian Marine Science Institution (WAMSI)

School

School of Science

RAS ID

100636

Funding Information

The $13.5 million WAMSI Westport Marine Science Program was funded by the Western Australian Government, Department of Transport. WAMSI partners provided significant in-kind funding to the program to increase the value to >$22 million.

Creative Commons License

Creative Commons Attribution 4.0 License
This work is licensed under a Creative Commons Attribution 4.0 License.

Copyright

Unless otherwise noted, any intellectual property rights in this publication are owned by the State of Western Australia

Content Type

Full object

Recommended Citation

McMahon, K., Strydom, S., Dunham, N., Whale, J., Webster, C., & Said, N. (2025). Drivers of seagrass decline in Perth waters and actions to halt decline. Prepared for the WAMSI Westport Marine Science Program. Western Australian Marine Science Institution, Perth, Western Australia. https://wamsi.org.au/app/uploads/2022/06/Drivers-of-seagrass-decline-Appendices.pdf

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