More than 500 common species of fish, seaweed, coral and invertebrates that live on reefs around Australia have declined in the past decade, a study has found, as experts warn “not all is well in the ocean”.
Global heating was likely the main driver of the falls, with marine heatwaves and a rise in ocean temperatures hitting species that live on rocky and coral reefs.
The study, published in the journal Nature, monitored 1,057 species and found 57% of them had declined, and almost 300 were declining at a rate that could qualify them as threatened species.
About 28% of the species analysed had suffered drops of 30% or more in just a decade, with species that live in cooler waters particularly hard-hit.
Prof Graham Edgar, a marine ecologist at the University of Tasmania and the study’s lead author, said the declines were most marked in the rocky kelp-dominated reefs in Australia’s cooler southern waters, known collectively as the Great Southern Reef.
“These declines are happening out of sight and with very little public attention,” he said.
Edgar said there were many more species in the waters that were not being monitored and were also very likely to be declining.
“We’re really only looking at the tip of the iceberg here. Species could be going extinct now,” he said.
“This is very concerning to me. I’ve been swimming up and down counting fish and seaweed for more than 30 years and I’ve seen first-hand the effect of warming on the system.
“With the direction this is going, it’s a huge worry.”
The loss of kelp was particularly important, Edgar said, because they were the cornerstone around which many habitats existed in the continent’s cooler waters.
Larger fish were declining faster than smaller ones, the study found, probably because of pressure from fishing compounding the rising temperatures.
About 35 researchers from multiple institutions came together for the study, which drew on existing data from the Australian Institute of Marine Science as well as monitoring from an army of volunteer divers.
Only species that had been observed enough times to generate analysis were included.
“Without the volunteer efforts of Reef Live Survey divers, we couldn’t have done this work,” Edgar said.
For many reef species, increasing ocean temperatures were presenting an “existential threat” with knock-on effects for ecosystems and commercial fisheries, the authors wrote.
Species in waters in Australia’s south that were closer to big urban centres such as Melbourne, Adelaide and Sydney, were being affected by not only warming oceans but also pollution, coastal development, fishing, aquaculture and land run-off.
Although the study focused on species living on reefs, the authors said marine wildlife was probably also declining in other rapidly warming cool temperate waters.
Dr John Turnbull, a marine ecologist at the University of Sydney and a study co-author, has witnessed declines while diving as a volunteer.
He saw cool water corals near Sydney bleaching for the first time, and visible declines in the numbers of weedy sea dragons and urchins.
“We’re seeing these declines first-hand. The losses in the south of Australia are not really known,” he said.
The loss of urchins had a knock-on effect, he said, as they were food for larger fish, including blue gropers that can grow to a metre in length.
There was evidence some species were moving towards the cool end of their ranges, Turnbull said, which was a problem in southern waters because species “run out of runway” with no available habitat farther south.
Associate Prof Zoe Richards, a marine invertebrate expert at the Western Australian Museum and Curtin University, said the study “sends a clear message that not all is well in the ocean”.
“This new study provides much-needed empirical evidence that population declines are occurring even among the most common marine taxa,” said Richards, who was not involved in the study.
“These are common species and so are major players in the way these ecosystems function. It’s quite ominous if they are declining.
“You have to ask what on earth is happening to everything else.” (The Guardian)
PHOTO: Graham and Edgar courtesy Great Southern Reef Foundation 2023.