Marine reserves don't just benefit sea creatures living within them, but those beyond their borders as well, Kiwi researchers have suggested.
University of Auckland scientists have observed a higher proportion of young snapper in fishing areas north of Auckland were related to adult snapper from the Goat Island marine reserve - tens of kilometres away.
While it's been hotly debated whether marine reserves have benefits outside their bounds, the new evidence appeared to confirm these ocean havens can serve as local fish nurseries.
The study was the first time scientists have found evidence outside the tropics of a direct parental link between adults in a marine protected area to juveniles outside.
Led by Professor John Montgomery, Dr Shane Lavery and former postdoctoral fellow Dr Agnes Le Port, the research team used a combination of genetic testing and hydrodynamic modelling of snapper larvae.
They found at least 11 per cent of juvenile snapper up to 40km away were the offspring of spawning adults from the reserve at Leigh north of Auckland, whereas no offspring matches were found to adult snapper sampled from non-reserve areas.
"The contribution from the reserve is about 10 times higher than would be expected if snapper larval contribution was simply proportional to geographic area," Montgomery said.
An area of 400sq km was included in the study, from Mangawhai in the north to Mahurangi in the south.
Goat Island marine reserve - which has "no take" rules strictly banning fishing - made up just 1.3 per cent of the area studied.
"This is the first estimate of the larval contribution of adult snapper from the reserve into the surrounding fishery and shows that even though the reserve is a tiny percentage of the area studied, it is more than pulling its weight in contributing to snapper populations outside," Montgomery said.
Previous research had shown that while snapper moved over a wide area, resident snapper within the reserve tended to stay put for some time, forming a significant breeding population of large individuals.
"Enough of them stick around within the reserve for our data to show a direct and significant link between the adults in the reserve and the juveniles many kilometres away."
For the research, adult snapper within the reserve were caught, tagged and a fin clip sample taken before being returned to the water.
The same technique was then used in non-reserve areas.
Montgomery, of the university's Institute of Marine Science, said the hydrodynamic modelling work done for the study was the first time scientists had used the method alongside multiple genetic techniques.
"Hydrodynamic computer modelling uses our understanding of tidal currents and wind patterns to predict where snapper larvae end up.
"The agreement between predicted dispersal and the genetic matches helps validate the modelling and its potential use in the design of future marine protected areas."
Stretching from Cape Rodney to Okakari Pt, and established in 1977, Goat Island was the first-ever "no-take" reserve created in New Zealand, and today remains one of the most popular diving spots in the country, renowned for the variety and abundance of its marine life.
Within its territorial sea, New Zealand now has 44 marine reserves, eight marine mammal sanctuaries and four benthic protection areas, most of them subject to no-take rules.
The Government, proposing to update the 46-year-old Marine Reserves Act with a new Marine Protected Areas Act, has moved to create more marine protected areas, along with proposed new recreational fishing parks in the inner Hauraki Gulf and Marlborough Sounds.
With a bid to create a massive new sanctuary in the Kermadecs presently stalled, New Zealand protects 6.9 per cent of its marine environment through some form of protection but less than 1 per cent of our Exclusive Economic Zone - the world's fourth largest - is protected through no-take reserves.
Scientists know that more than 15,000 marine species inhabit our territorial seas and our EEZ, but there may be a further 50,000 yet to be found in our waters.