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Sea weeding: how gardening in Port Phillip Bay helps native kelp survive

Dead-heading underwater kelp is all in a day’s work for divers helping Port Phillip Bay to thrive after invasion by a seaweed pest. Ricky Sproule continues our special series on invasive species, Gone Feral, co-published with Cosmos online.

Sea weeding: how gardening in Port Phillip Bay helps native kelp survive

Parks Victoria divers removing the reproductive parts of the invasive kelp Undaria pinnatifida, which arrived in Port Phillip Bay in the late 1990s, likely on the hull of a container ship. Photo: Parks Victoria

Story by Ricky Sproule
 

South of the city of Melbourne, beneath the surface of Port Phillip Bay, lie vast forests of kelp.

Just like trees in terrestrial ecosystems, kelp provides shelter and food for a complex living community. And just like our forests on land, the kelp of Port Phillip Bay is struggling with assaults on multiple fronts – global heating, human encroachment and invasive species.

Human interventions are needed to mitigate human impacts, and one underwater intervention is surprisingly similar to managing invasive species on land.

A few times a year, when conditions are right, Parks Victoria Marine Ranger Monique Bregman leads a team of divers out into Port Phillip Heads Marine National Park to deadhead weeds.

The native golden kelp Ecklonia radiata was once the dominant large kelp in Port Phillip Bay. Photo: Parks Victoria.

The native golden kelp Ecklonia radiata was once the dominant large kelp in Port Phillip Bay. Photo: Parks Victoria.

The vigorous invasive kelp Undaria pinnatifida arrived in Port Phillip Bay in the late 1990s, likely on the hull of a container ship. Since then, it has spread across the bay, competing with native kelps like golden kelp, Ecklonia radiata.

Today, you can see undaria beneath most jetties around the city, swaying gently in the current. You also might have seen it swaying gently in a bowl of miso soup. Undaria is a traditional food in east Asia, best known by its Japanese name, “wakame”.

“You’d never eradicate it, you’d never get rid of it,” says Bregman who leads marine programs for Parks Victoria.

“If we can let certain small spaces be undaria free, it’s helping the biodiversity of that marine space cope with all the other pressures it’s having to face.”

Like most invasive species, undaria grows quickly and reproduces prolifically. It quickly takes over places where native kelp has died back due mainly to increasing sea surface temperatures, together with pollution, disturbance or overgrazing by sea urchins.

One location where Parks Victoria is intent on suppressing undaria is Popes Eye, a popular, protected dive spot with a huge variety of marine creatures, cool-water corals and native kelp occupying the footings of a 19th century fort.

Undaria was first noticed there in 2017 and, soon after, Parks Victoria removed over two tonnes of the seaweed. These days rangers only remove the reproductive part, the sporophyll, in a process similar to deadheading flowers.

Undaria pinnatifida, also known as wakame, has quickly taken over vast swathes of the bay. Photo: Parks Victoria

Undaria pinnatifida, also known as wakame, has quickly taken over vast swathes of the bay. Photo: Parks Victoria

Bregman’s team dives on calm days during the brief window of so-called slack tide. Rangers move along the horseshoe of Popes Eye cutting juvenile plants off at the base with scissors. These float away and decay. Wherever they find the reproductive part of the plant, they cut it off and put it in a ziplock bag to contain any spores.

Bregman has been weeding Popes Eye for seven years and says it has definitely had a positive impact. The biodiversity of the kelp forest has not declined in the same way as other reefs where undaria has been allowed to dominate.

One key aim of weeding is to maintain safe havens of native habitat while marine animals adapt to the new conditions. Recently, some herbivorous fishes have been observed eating undaria, but Bregman says it will take a long time for the whole ecosystem to transition.

Meanwhile, the humans living around Port Phillip Bay have been far quicker to realise the value of undaria as a food source, thanks to the efforts of people like Mike Houghton and Ric De Vries.

Soon after undaria arrived, they obtained a license to  commercially harvest seaweed in Port Phillip Bay. They promote and sell locally harvested wakame as a sustainable “superfood”.

Despite fields of undaria spreading around our coasts, most wakame consumed in Australia is imported. Haughton says the wakame industry in Australia can expand but he is adamant that good regulation is required to maintain high standards.

“It really worries me that you get idiots out there who think ‘oh beauty, this is a good way to make a quick dollar’, they’ll harvest wakame and dry it in their backyard … but it’s not that simple”.

Haughton’s business, The Australian Seaweed Company, cannot harvest within marine protected areas such as Popes Eye.

Citizens can’t remove anything from within the no-take zone of the Marine Park, even invasive species. According to Bregman, Parks Victoria wants to keep that boundary firmly in place, given concerns around illegal fishing.

Bregman says we could never harvest enough wakame to eliminate it from Port Phillip Bay. For good or ill, it’s here to stay, but she hopes that maintaining some refuges free from undaria will give underwater organisms the best chance of adapting to the many changes they face.

The invasive underwater weed is often eaten in soups and salads. Photo: Benoît Prieur.

The invasive underwater weed is often eaten in soups and salads. Photo: Benoît Prieur.

This story is co-published with Cosmos online – ‘The Science of Everything’. 

About The Citizen

THE CITIZEN is a publication of the Centre for Advancing Journalism. It has several aims. Foremost, it is a teaching tool that showcases the work of the students in the University of Melbourne’s Master of Journalism and Master of International Journalism programs, giving them real-world experience in working for publication and to deadline. Find out more →

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