Rottnest moorings destroying seagrass

WA boaters are in the spotlight after researchers from Edith Cowan University (ECU) recently discovered that the mooring fields around Rottnest Island have destroyed an area of seagrass that could cover more than 48 Olympic pools.

The ECU researchers claim seagrass covering 48,000sq m has been scoured from the sands around Rottnest Island by almost 900 mooring chains used by recreational and commercial boats.

Seagrass is of immense value as nursery and breeding areas to fish and as a food source to turtles, but it is also the new darling of environmental groups due to its CO2 storing properties.

At the same time, boaters need moorings, and strong ones at that, especially off Perth where the Fremantle Doctor blows from 15-20 knots and beyond most summer days.

Reliable moorings are essential for safety and for meeting the insurance obligations in boating policies that would amount to billions of dollars for moored craft around Australia.

The new research just published in the journal Nature: Scientific Reports surveyed the "scars created by mooring chains in the bays around Rottnest Island" and found there is a pressing need for the new-style seagrass-friendly moorings rather than the old block-and-chain models.

SEAGRASS AS CO2 STORAGE
Dr Oscar Serrano, who led the research with Professor Paul Lavery and Professor Pere Masque from ECU, said the movement of the chains scraped seagrass off the seafloor increasing atmospheric carbon dioxide.

"As moored boats drift with the currents, wind and waves they drag a heavy chain across the seafloor and that chain acts just like a razor across the skin removing the seagrass," he said.

"But unlike a five'o'clock shadow — in this case, the seagrass doesn’t grow back.

"Unfortunately these protected, calm bays favoured for boat moorings are also prime habitats for seagrass.

"Seagrass is an important habitat for many species of fish as well as a food source for dugong and turtles.

"More importantly in a global sense, seagrass absorbs carbon dioxide at more than 40 times faster than tropical rainforests.

"What that also means is that when the seagrass meadows are wiped out, the carbon dioxide which has been absorbed over hundreds of years is released back into the atmosphere," Dr Serrano champions.

Dr Serrano says core samples were taken in the scarred areas and where seagrass still existed. Those samples showed on average more than 75 per cent of carbon absorbed in those seagrass meadows was lost, ergo, increasing atmospheric carbon dioxide.

SEAGRASS-FRIENDLY MOORINGS
Efforts to preserve seagrass meadows by using seagrass-friendly mooring lines is resulting in the recovery of seagrass in some areas of Rottnest, however, overall seagrass cover is decreasing, the research says.

The size of mooring scars in Stark Bay on Rotto's north coast is said to have increased about 500 per cent from 2000sq m in 1980 to 9000sq m today due to erosion of the already scarred areas by wave action.

"Once the mooring chains have started the process of scouring, waves will likely continue spreading those scoured areas," says Dr Serrano.

"In Stark Bay, we’ve seen the scarred areas join up to become large areas devoid of any seagrass."

The destruction of the seagrass meadows has important implications for the ecosystems of Perth’s favourite marine playground, researcher say.

Dr Serrano said it is important that steps are taken to protect the seagrass meadows around Rottnest Island and the rest of Australia.

"These older style moorings have started to be replaced on Rottnest Island, but that needs to continue here and in other popular mooring sites," he said.

OYSTERS AND SEAGRASS
In other enlightening science regarding seagrass, it was recently found that new oyster leases seeded new seagrass beds at an amazing rate. There appears to be some mutually beneficial relationship between the two species, which isn't surprising if you know your estuaries.

Perhaps oyster farmers could grown seagrass rolls that could be transported and transplanted to key areas like Rottnest? Either that or boaters with moorings might have to buy carbon credits or plant new-growth forests or Aussie bush to go carbon neutral?

But all jokes aside, the immediate solution definitely lies with embracing seagrass-friendly moorings, as is happening right around the country in mooring bays with segrass, providing they can withstand Perth's notoriously wild and windy weather.

The new research is available at Nature: Scientific Reports website