Subtidal seagrass surveys at Slipper Island Whakahau and Great Mercury Island Ahuahu 2025

In 2019, Cawthron Institute (Cawthron) conducted a survey of the Great Mercury and Slipper Island seagrass meadows for Waikato Regional Council to map the extent and assess the ecological condition of subtidal seagrass meadows in Huruhi Harbour and South Bay. Seagrass extent in nearby Parapara and Stingray Bays was also recorded.

Following recommendations by Clark and Crossett (2019) to resurvey the meadows every 3–5 years, Cawthron were commissioned to repeat the survey in 2025 (the focus of this report), aiming to determine whether the extent or ecological condition of the meadows had changed since 2019.

This report presents a comprehensive resurvey of the seagrass meadows at South Bay and Huruhi Harbour, as well as qualitative surveys of seagrass meadows at Home and Parapara Bays. 

Results were compared with those from the 2019 survey, and where appropriate, with earlier surveys from 2004 and 1975–76, to understand how these subtidal meadows have changed over time. The report provides valuable information to support the protection and management of subtidal seagrass in the Waikato Region

Report: TR 2025/35

Authors: Dan Crossett, Dana Clark, Lisa Floerl, Courtney Moir (Cawthron Institute)