You might think seaweed is the annoying stuff that washes up on beaches, and gets caught in your boat motors, paddle or fishing gear. But ANU marine scientists have revealed that seaweeds are vital to the world’s second largest fringing coral reef, Ningaloo, on the west coast of Australia.
Most people know about Australia’s iconic Great Barrier Reef, but they might not be aware of another large coral reef on the other side of Australia called Ningaloo, which spans 260 kilometres of coastline. This spectacular reef is a World Heritage Site thanks largely to its significant biodiversity and cultural importance—you can also spot majestic whale sharks during their migration.
The Western Australian and federal governments take shared responsibility for protecting this fragile marine ecosystem. Seaweed has traditionally been seen by scientists and marine park managers as a threat to coral reefs like Ningaloo due to a common phenomenon whereby seaweed inundates reefs. However, Dr Chris Fulton’s team of marine ecologists at ANU believe that seaweed has been unduly vilified. In fact, their research in collaboration with the Australian Institute of Marine Science and the Western Australian Department of Parks and Wildlife shows that seaweed acts as a critical nursery for many of Ningaloo’s baby fish.
“We’re realising now that these seasonal seaweeds are growing up just at the right time to make homes for each new batch of baby fish that are born at Ningaloo each summer,” Fulton explains. “The seaweed becomes a temporary nursery habitat for fish like emperors, cods, snappers, all of which people love fishing for, as well as many other types of fish that do important jobs around the reef. The seaweed provides a complex habitat for these little fish to shelter within.”
The seaweed normally thrives through summer and spring, growing up to a metre in height to form lush meadows that cover hundreds of hectares of the Ningaloo lagoon, before breaking off again each winter to float away as rafts or wash ashore on beaches. However, the effects of climate change—such as increased storms, cyclones, extreme rain events and fluctuating seasonal temperatures—can change the timing of when the seaweed canopy breaks off.
“So if the fish arrive at the reef and the seaweed habitat is not there for some reason, what happens? Do they get munched on mercilessly by their predators, or do they go elsewhere? Either way, this could mean very few fish will grow up to be adults and contribute to the future populations at Ningaloo.”
With the help of Honours and PhD students, Fulton’s team conducts underwater surveys and takes sea temperature readings every six months, alternating between summer and winter. With this data, they can determine whether there is a match between the numbers of baby fish arriving, the size of the seaweed meadow habitats and the abundance of future adult fish populations.
“By making this connection, we’re hoping that surveys of the seaweed nursery habitats can serve as a signal for good or bad years in the future fish populations at Ningaloo.”
Fulton says his research team is working with the marine park managers to help with the planning and development of Ningaloo, and raising public awareness of seaweed’s important role in helping Ningaloo to be this stunning World Heritage ecosystem.
Lessons from the team’s research should also help with the management of fish nursery habitats elsewhere. Seasonal seaweeds like the ones at Ningaloo grow on coral reefs around the world, including the Red Sea, the Caribbean, the Pacific Islands and even the Great Barrier Reef.
“Potentially, if we can prove this concept at Ningaloo, it could have benefits for how people view and manage these nursery habitats and coral reef fisheries around the world.”