Reports suggesting that the wreck of the Rena has produced a large oil slick are incorrect according to a Maritime NZ spokesman, with the brown stain seen on the water turning out to be a swarm of krill.

Friday brought eye-witness reports of big blotches over a large expanse of water a few kilometres west of Motiti Island.

A reported oil spill near the wreck of the Rena was nothing more than a swarm of krill.

But Maritime NZ's Steve Rendle says the substance has since been confirmed as krill - millions of small crustaceans that are found in all of the world's oceans.

Krill are near the bottom of the food chain, feeding on phytoplankton to provide food for whales, seals, penguins, squid and fish each year, and is replaced by growth and reproduction.

Most krill species display large daily vertical migrations, thus providing food for predators near the surface at night and in deeper waters during the day.

The movement also explains why the oil spills being reported are also quickly dissipating.

Some small pockets of oil are believed to remain trapped in the wreck and some of its machinery.

Spokesman for the Rena project Hugo Shanahan says the krill was reported and confirmed as such to Maritime New Zealand and the Bay of Plenty Regional Council.

'The patrol vessel Kaimai (used by Resolve Salage & Fire) noticed what appeared from a distance to be a brown bloom in the water stretching along a tideline east of Astrolabe Reef,” says Hugo.

'On several close inspections the brown bloom was confirmed as krill - a small shrimp-type specimen massed in large numbers.

'This has been confirmed to Maritime New Zealand and the Bay of Plenty Regional Council, in conjunction with confirmation of the recent oil blooms that surfaced during ongoing clean-up operations.”