Commercial fishermen’s fears that herbicide run-off is killing seagrass and their livelihood in the Great Sandy Strait are being investigated for the first time by the Queensland government.
Seagrass in the UNESCO-recognised marine habitat — a 70-kilometre stretch separating the mainland from K’gari (Fraser Island) — has dramatically declined since it was first mapped by James Cook University (JCU) in 1998.
But preliminary results show herbicides do not appear to be the cause.
Commercial fisherman Mark Alexander had claimed herbicide run-off from pine forests was contributing to the loss of thick seagrass meadows in the southern section of the strait where he targeted whiting, bream, flathead, and mullet.
“I’m not exaggerating when I say there have been thousands of acres of seagrass lost, gone, died,” the third-generation fisherman said.
“It just makes you sad. You know seagrass is the main driver in marine productivity and there’s just none left and it is affecting fish stocks dramatically.
“In the good old days, there were thousands and thousands of sand crabs and you hardly ever see a sand crab anymore. It’s just shocking.”
Herbicides ‘probably not’ killing seagrass
The Department of Environment, Science and Innovation (DESI) began testing for up to 50 herbicides and insecticides at 21 freshwater and saltwater sites in the Great Sandy Strait last August.
DESI scientist Andrew Moss returned to those sites after heavy rain in January and March this year.
“To allay these fears on the part of the local fishermen and the local population, we need to eliminate or at least determine what particular issues are the main cause of it and, obviously, herbicides are one potential cause,” he said.
Mr Moss said preliminary results showed no significant concentrations of herbicides or insecticides in the Great Sandy Strait. All results were below maximum accepted Australian standards for marine environments.
“It looks like we can probably eliminate [herbicides] as a cause, given the results so far but, obviously, we’re going to keep the study going,” he said.
“It’s not finished yet.”
Floods impact seagrass density
JCU researchers went to the Great Sandy Strait and Hervey Bay in 2022 after two floods battered seagrass habitat.
Professor Michael Rasheed said sediment in the floodwater cut crucial light to aquatic plants and caused freshwater stress and algal growth.
Dr Rasheed added that researchers found a significant reduction in seagrass density on the banks when they returned last year. But, encouragingly, some seagrass had come back to most areas.
“The cover is still very low, typically around 2 per cent of the banks within those footprints,” Dr Rasheed said.
“So it is very light cover, but that’s nearly double what was there in 2022, so even though it’s small it is headed in the right direction.”
Dr Rasheed said the principal cause of seagrass decline was likely associated with flood plumes and sediment, which reduced water quality and available light for seagrass to photosynthesise.
He commended the work of the Mary River Catchment Coordinating Committee that is working with landholders to tackle the worst areas of riverbank erosion.
“Anything we can do to reduce erosion and runoff and reduce turbidity and the impact of those [flood] events is going to be beneficial,” he said.
“Catchment management is going to be of great benefit to helping seagrasses cope with these kinds of flood events, which will happen again.”
Risks of herbicides ‘taken seriously’
HQPlantations general manager of stewardship, Michele McAndrew, said herbicides were part of the company’s integrated weed management approach.
But Ms McAndrew said that levels were kept to an absolute minimum and alternate techniques such as slashing, mulching, cattle grazing and prescribed burning were also used.
“Typically, we’ll only apply herbicide to each planted area three to six times over its 30-year growth cycle,” Ms McAndrew said.
She said HQPlantations had undertaken regular monitoring for herbicides at six permanent sites within the Toolara and Tuan Forests since 2019 and had not detected herbicides in any samples.
“In addition, we have had a long association with the Mary River Catchment Committee and have been participating in their Water Watch programs in the region for over 10 years,” she said.
“We take our responsibility as a good land manager seriously. We understand risks of using herbicides and put in place control measures such applying buffers around sensitive areas.”
Dr Rasheed said he was optimistic about the future, adding that deep-water seagrasses in the middle of Hervey Bay appeared to have been better able to recover from the impacts of flood plumes than intertidal seagrass.
“Seagrasses have the capacity to grow really quickly, which is great, so given the right conditions they can bounce back.
“It’s just tough when you have lots of flood events happening.”
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