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Dive Otago tertiary training manager Jen Clent, of Dunedin, said during the "Project Aware" clean-up day divers recovered more rubbish and weed than she expected.
"I was a [dive] student here in 2003 and undaria was not really an issue then. But now it's everywhere," she said.
While Dunedin Dive Club and Dive Otago members scoured the seabed, pupils from Romahapa School, near Balclutha, eight members of the Green Island Girl Guides, and several volunteers collected 10 bags of foreshore refuse.
Romahapa conservation group president Hayley Witt (10) said pupils and parents travelled from South Otago for the clean-up day "because they really liked helping out with conservation".
"Project Aware" clean-up days are run in conjunction with the Professional Association of Diving Instructors.
The Project Aware Foundation is a nonprofit organisation dedicated to conserving underwater environments through education, advocacy and action in 175 countries, with support from divers and water enthusiasts.
The clean-up day around the mole was first held in 1993.
• The Aramoana mole is a 1km-long rock wall built in 1885. It is reinforced by several scuttled hulks lying on the south side at depths to 22m. The shallowest, Mokoia, lies in 7m of water and is used for dive training.