An increase in the number of kina sucking the life out of the seabed is causing growing concern among Northland marine environmentalists.

Conservationist Wade Doak said the destruction of the coastal biodiversity is so severe iwi should consider the implications under the terms of the Treaty of Waitangi.

Also called sea eggs and sea urchins, an over abundance of kina in places causes what marine biologists call barrens - areas stripped of other seabed life.

Overfishing of predator species with jaws able to break into kina, such as snapper, red moki, porae and crayfish, is thought to be the major contributor to the breakdown in the ecosystem.

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As kina proliferate they destroy the "forests" of kelp they live on, in turn destroying the shelter for juvenile fish and other species that feed on pea-sized young kina.

Mr Doak said the problem has led to parts of the coast becoming "a marine desert, except for bristling kina spines".

"The only answer is a network of protected areas where normal predation is restored, replenishment zones and 'seed trees'," Mr Doak said. He said Northland's tourism as well as marine ecosystem would suffer the effects of the underwater deforestation.

"The Tutukaka Coast has been rated the second best coastal destination in the world. How do we maintain that reputation?"

He hopes to mount a national campaign to raise awareness about the kina-devastated reefs and wants to see more protected marine areas.

In promoting the Fish Forever group's marine reserve concept for parts of the Bay of Islands, marine ecologist Vince Kerr said the inner Bay of Islands had been in a state of decline for decades. The loss of big specimens had led to kina barrens where kelp forests had been destroyed, leading in turn to the winding down of the whole ecosystem.

The problem is not only in New Zealand waters. Over-exploitation of inshore waters by modern fishing techniques has seen sea urchin (kina) barrens develop world-wide. Dense populations of kina graze every inch of a reef so heavily they do not get enough sustenance to mature and develop roe, becoming inedible themselves as well as incapable of breeding.