For many commercial and recreational fishers, marine protected areas (where fishing is excluded) are viewed with scepticism.
Critics have questioned the legitimacy of what is referred to as the spillover effect — where excluding fishing is hypothesised to produce ecological and/or commercial fishing benefits beyond the boundaries of the protected area.
But research published today in Science suggests fishing exclusion zones can benefit both fishers and the marine environment, and that fish yields for some species can be boosted up to several hundred kilometres away from the protected habitat.
Lead researcher John Lynham from the University of Hawaii says marine protected areas allow the entire food web within them to recover.
That in turn boosts the numbers of some of the wider-roaming pelagic fish that move out into the waters beyond.
"Removing a lot of the human influence, it's kind of like the [COVID-19] lockdowns on a giant scale — everything has a chance to recover," Professor Lynham said.
The researchers looked at commercial fishing catch data from the waters surrounding the world's largest marine protected area, the Papahānaumokuākea Marine National Monument, off north-west Hawaii.
This marine park, which excludes all extractive industries including mining, and commercial and recreational fishing, originally covered 360,000 square kilometres.
However, its size was more than quadrupled in 2016 to 1.5 million square kilometres.
To see whether there was any change in catch rate before and after the 2016 expansion, the researchers compared current and historical catch data, from fishing conducted in the waters outside the expanded 2016 border.
Because changes in fishing technology and human behaviour could skew the results, they restricted their data to deep-set longline fishing, and for boats that used only this method across the study period, between 2010 and 2019.
Deep-set longline fishing involves a long mainline with numerous baited hooks attached, in this case set at a depth of about 250 metres.
The researchers also focused on what is called catch per unit effort or CPUE. In this case, the CPUE was defined as the average number of fish caught per thousand hooks.
They collected data for two of the most commercially significant species caught off Hawaii — yellowfin tuna and bigeye tuna — plus a third category of "other" species, and used catch data for up to 600 nautical miles from the present-day protected-zone boundary.
The increase in catch following the expansion of the monument was most significant for yellowfin tuna, with an increase in catch rate of 60 per cent in waters within 100 nautical miles of the boundary.
For bigeye tuna, the increase was over 11 per cent, and for all other species combined there was an 8 per cent increase.
As expected, the benefits weaken as you move further from the protected area boundary, but increases in yellowfin tuna and bigeye tuna catch rates were found all the way out to 300 nautical miles.
Benefits likely to keep growing
Bigeye tuna and yellowfin have a life span of about seven years and can reproduce at about 2 or 3 years old.
Given the expansion of the marine protected area only happened six years ago, it's likely this spillover effect could get stronger as fish populations within the monument continue to grow, Dr Lynham said.
"Some of the experts on bigeye tuna think we need to leave this for a couple more years before we see [stronger] effects on them.
"[And] if we wait a couple of years, I think we're going to see greater effects on yellowfin."
Fisheries and coral reef ecosystems researcher Naomi Gardiner from James Cook University said genetic analysis had previously shown spillover effects for some reef-dwelling species on the Great Barrier Reef.
However, Dr Gardiner, who wasn't involved with this research, said it was especially encouraging to see the benefits also translating to these large, ocean-going species.
"I think it's particularly exciting because it's about tuna — these large, pelagic species living in open water and moving around a lot."
As well as being ecologically important, she said the research potentially had big economic implications.
"To be able to demonstrate that a large [marine protected area] is having this effect on tuna, it's the most dominant fishery in the world. It's very important to the global economy."
Australia has a mixed recent track record on marine protected areas.
Management plans were approved in 2013 for several large marine parks, including the Coral Sea Marine Reserve — Australia's largest, which lies to the east of the Great Barrier Reef in northern Queensland.
However, those plans were suspended under the Coalition, and a systematic downgrade to protections in at least 26 marine park areas was enacted in 2018.
According to a 2021 Marine Ecology research paper: "The downgrade to the Coral Sea Marine Park affected 75 per cent of the total area of the [marine protected area] and constitutes the largest downgrade to a single (marine or terrestrial) protected area in history worldwide."
At a Press Club speech earlier this year following the release of the damning State of the Environment report, Minister for the Environment and Water Tanya Plibersek pledged to protect 30 per cent of Australia's land and waters by 2030.
The minister claimed that we'd already exceeded that target in marine areas.
But critics blasted the claim, saying the minister was using the Coalition's watered-down marine protected areas to come up with those figures.
Dr Gardiner said today's research shows the value of large-scale marine protected areas, not just for reef-dwelling species but for larger, keystone and commercially important fish.
"The fishing pressure on these tuna has been enormous. You've stopped that pressure in the reserve, you've let the adult populations grow up, and the breeding population in the reserve is bigger, so you're getting more output.
"It adds weight to these large [marine protected areas]. The Coral Sea Marine Park in time should hopefully show the same benefits."
