Scientists have reached back centuries into past to reveal how crucial nurseries for the Hauraki Gulf's snapper have vanished.
Successive reports have highlighted the plight of snapper and other species living in Auckland's blue backyard, with the most recent State of our Gulf stocktake highlighting how snapper levels had plummeted by 83 per cent since human arrival.
Now, a new study, drawing on prehistoric records, has suggested snapper have also lost what were once critical nursery areas.
In their project, Auckland University of Technology marine biologist Dr Armagan Sabetian and colleagues recovered snapper bones from two ancient middens - sites where food waste was buried by Māori - near Omaha and Long Bay.
The samples from these two sites, carbon-dated back to the 16th and 15th centuries respectively, were then compared with modern snapper samples from fish caught nearby today.
By using a process called laser ablation, the team were able to build a chemical profile from the centuries-old bones.
They then applied satellite tracking codes to the chemical profiles, to essentially recreate snapper movement through the gulf's prehistoric riverine, estuarine and marine environments.
"The analogy I'd like to use is your fitbit watch, which has a series of algorithms and codes that takes into account various parameters, such as speed, elevation, temperature, and pulse, to calculate your caloric use," Sabetian said.
"Our methodology was somewhat similar to that."
The analysis found that, six centuries ago, the gulf was a pristine environment with plenty of freshwater rivers emptying into it, creating vast nearshore "brackish" environments.
"Snapper in the Hauraki gulf used to spend a year in these nearshore nursery areas, all coming in synchronously after hatching and enjoying the protection of nursery environments before venturing back to the sea as young adults," Sabetian explained.
"Modern snapper are not doing this. Not only some of them are refusing to come into nearshore areas after hatching, those that do come are chaotically going back and forth between nursery and adult habitats."
Sabetian said the observed lack of time spent in brackish nurseries ultimately pointed to a disrupted habitat-use pattern during the species' early growth.
"We blame the loss of nearshore nursery habitats such as sea grass meadows and healthy estuaries over time for this change."
The study also noted that, although this change had been dramatic over time, there appeared to have been little impact from pre-colonial settlement.
Sabetian said environmental monitoring reports had already told us how the gulf's near-shore benthic habitats were currently in a bad state.
"They paint a grim picture from excessive nutrient input and sedimentation arising from coastal and ocean sprawl," he said.
"But now we have high resolution data that show the implications of these anthropogenic factors."
Snapper depended on nearshore environments during their critical early growth, and previous research had shown that depriving them of this resulted in lower growth rate and condition.
"The narrative around the snapper fishery in the gulf has primarily focused on reducing fishing effort, but I believe we should also be having discussions around managing or rehabilitating their critical nursery habitats," he said.
"Our research can also help assess the effectiveness of conservation and management efforts."
Earlier this year, the Government launched a new strategy to revive the gulf's damaged ecosystems.
That included 18 new marine protection areas, a new fisheries plan with a range of changes to fishing practices and catch settings, and better monitoring.
Snapper-boosting reserve's benefits worth millions
Meanwhile, another new study shows "no take" marine reserves don't just have environmental benefits for the gulf's snapper – but also economic ones, to the tune of millions of dollars.
The just-published research, led by University of Auckland scientists, built off a recent assessment which looked at the economic boon of rejuvenating depleted snapper within the Cape Rodney-Okakari Point Marine Reserve.
That found that just over 10 per cent of newly-settled juvenile snapper, sampled up to 55km outside of the reserve - or an area spanning about a third of gulf - were the offspring of adult snapper from within the reserve.
In the latest study, marine scientist Professor Simon Thrush and colleagues converted that boost in numbers into the dollar-value of catchable fish.
"Calculating the price of fish from the commercial fishery is easy as there are numbers from landed value," he said.
"For the recreational fishery, this is more difficult because there is no market - but we can use data from fishing surveys to estimate what a fish caught recreationally adds to the economy, via gear purchase, fuel, boat purchases, or charter fees."
When scaling those calculations down to the area they analysed, the researchers estimated the boost to the commercial fishery to be around nearly $1.5m annually, while the economic injection from associated recreational fishing was higher still, at around $3.2m.
"We were interested in exploring what the blue economy might really mean, in the same way as we think of the green economy have a stronger focus on sustainability, ethics and equality," Thrush said.
"Specifically, the reserve at Leigh is the first in Aotearoa New Zealand and we have a reasonable amount of data to underpin economic analysis in an ecologically grounded way," he said.
"If we can reveal that the reserve is of benefit to those whose value system is only based on dollars, then this might open up some new discussions."
Thrush described the study, published in the journal Marine Policy, as a "small but important step" in quantifying the economic benefits of marine reserves.
"This is only based on one fish species, but an important species in the Gulf. This does not undermine their massive biodiversity, ecological, cultural and ethical and tourist values."