Mapping critical habitat in Yanyuwa Sea Country

Yanyuwa’s comprehensive understanding of maraman, encompasses meadow distribution ecology, and species. Yunyuwa know that walya nyiki-naanji ki-maramanngku (the dugong and sea turtle are kin to the seagrass) and nganji-kin (co-dependent); dugongs and turtles rely on maraman for food, and maraman needs grazing to stay healthy. Na-wurndarnda (mangroves and associated seagrass fringing islands) is valued for nurturing juvenile fish and prawns.

Yanyuwa people strive to best manage activities that are adversely affecting maraman, including boat traffic, extreme weather events, sediment outflows from upstream human activities, sea-level rise, and warming waters. However, structured scientific data describing maraman meadow distribution and health, has been lacking. There was a pressing need for a comprehensive survey and assessment of the intertidal and subtidal benthic habitats adjacent to their mainland and islands, to serve as a temporal baseline, and to increase understanding of the preferred conditions of different species.

Project scientists and Li-Anthawirriyarra Sea Rangers collaborated to undertake this work.

Approach and findings

Approach

Benthic habitat assessments were conducted at 3,248 intertidal and subtidal sites along 130 km of mainland coastline, and around adjacent islands. These were undertaken during the peak seagrass growth period of October, in 2021 and 2022. Ten metre diameter plots were visually assessed by observers from a low-hovering helicopter, to estimate the areas of open substrate, or percentage-cover by seagrass, benthic macro-invertebrates, or algae. As well, seagrass biomass and species composition were estimated within three tethered replicate quadrats, dropped down to the substrate.

All survey data were entered into a computerised Geographic Information System (GIS) using ArcGIS software. Three GIS layers (collections of related data) were created to delineate habitats in the region: a habitat site layer, a seagrass meadow layer, and a seagrass biomass ‘interpolation’ layer (for predictive modelling of unsurveyed locations with similar characteristics). All spatial layers were made publicly available on the internet, at eAtlas.

Findings

Maraman was present at 44% of the surveyed sites; and 16,000 ha of seagrass were mapped, comprising 180 defined meadows. In most instances mainland coastal meadows extended from the mangroves to the edge of the intertidal zone, from several hundred meters to several kilometres; there were ‘hotspots’ with high biomass at some places. Around the islands meadows were narrower and patchier.

Seven seagrass species were recorded, with Halodule uninervis dominant; this is a disturbance adapted ‘pioneer’ species preferred by dugongs and turtles. Algae were present at 33% of the survey sites.

Outcomes

• Expanded and integrated seagrass data to support decision-making and management.
• A comprehensive and progressive approach to integrating and accessing historical data sets.
• A Country management resource for Yanyuwa Traditional Owners.

This project has created baseline records for Yanyuwa ma-Ihanngu coastal benthic habitats. These will allow detection of change over time, and provide the foundation for building a comprehensive understanding of why particular seagrass species grow in certain places, and the associated conditions and pressures that require management.

As well as supporting Yanyuwa’s management goals, publicly available data is now accessible to inform environmental impact assessments, conservation priorities, and decision-making by governments and others.

The project has also benefited all partners through information exchange, and increased li-Anthawirriyarra rangers’ capacity to undertake coastal planning, implement surveys, design research, and engage with GIS technologies. A strong foundation is in place for future monitoring work.

Ongoing work is now integrating pre-existing data on significant species such as turtles and dugongs, and cultural information, with the collected spatial information to improve the depth and breadth of seagrass-related knowledge.

Project location

Yanyuwa/Limmen coastal area (NT)