Seru can remember a time when the water was crystal clear. But nitrates accelerate the growth of seaweed which clouds the water 20 feet out from the beach.
Nitrate run-off also stimulates phytoplankton, the food supply of crown of thorns starfish, which destroy tropical reefs by excreting corrosive digestive juices that dissolve coral. These starfish have proliferated at Mana and have to be regularly removed.
Tourists come here to enjoy a pristine environment but their activities have a significant impact, too. Motorised boats and jet skis stir up sediments, Seru tells me, clogging up the young coral and killing it.
Snorkellers and divers stand on coral and break pieces off accidentally - or sometimes for souvenirs. More mouths to feed means bigger fish catches, putting a strain on stocks.
These effects have been so gradual that locals, perhaps surprisingly, were not the first to notice them. Resort guests, returning every few years, started to comment on the changes.
So, in a somewhat canny move, the resort began its E-programme a decade ago. Initial efforts focused on convincing staff of the need for it.
"Their attitude was 'there are plenty of fish, why are we doing this?'," Seru tells me.
Frustrating early experiments to combat soil erosion finally resulted in the planting of vetiver grass, native to India, to stabilise the shoreline.
Labour-intensive coral replanting was thwarted on the south side of the island where the coral has largely died out. Now the north beach has been made a marine sanctuary; motorised boats have been banned.
Swimming is also prohibited except at high tide, and a day is designated every month where teams of staff and volunteer guests work at coral regeneration and tree planting. A new organic farm on the island provides half the fruit and vegetables consumed.
Fiji's Mana Island Resort & Spa. Photo / Greg Bowker
Fiji's waters boast four of the seven living species of sea turtle. Two of them - the green and hawksbill - are endangered. Both nest on Mana. Turtles drown when they become entangled in marine fishing gear and can choke on plastic bags that resemble their jellyfish prey.
The staff are committed to a programme of raising then tagging and releasing turtles. We visit the turtle enclosure just as the water is being changed. Eleven hatchlings are in buckets - they've just been rescued from the staff beach.
My kids have run up and want to hold one. Seru discourages them. I suspect he's learned the hard way from youngsters squeezing their soft shells too enthusiastically.
These are all significant initiatives. But there's not much Project E can do about rising sea levels or nitrate pollution.
Another irritation is the rubbish that washes up on South Beach. Seru suspects it comes courtesy of the more relaxed regime at the backpacker accommodation to the east of the island - an accusation the owners are quick to deny.
The backpackers' merges into resort worker accommodation and a Fijian village. There I meet Jolame, who farms kava (the hangover-free Fijian national drink). While his wife braids my daughter's hair, Jolame prepares fresh fish.
In his dirt floor bure the dining table doubles as a bed for his family of five; the walls are drop-down palm blinds. This traditional lifestyle treads about as lightly upon the earth as it's possible to in the 21st century.
Fiji's economy depends on tourism, but it comes at a cost - an outsize eco footprint that remains long after the visitors are gone.
CHECKLIST
Getting there: Mana Island Resort & Spa lies in the Mamanuca group, 32km west of Nadi. Fiji Airways flies from Auckland to Nadi daily and weekly from Christchurch. Fiji Airways is the only airline flying to Fiji that offers child fares.
Further information: See fiji.travel.
