KINA BARRENS

Around the northeastern coast of the North Island, ‘kina barrens’ have become an unwelcome feature of the underwater landscape.

Kina barrens are characterised by bare rock overrun with sea urchins (kina). The reason kina barrens exist is widely believed to be an explosion in the sea urchin population because predatory snapper and crayfish are no longer present in quantities large enough to control urchin numbers. (Some scientists believe barrens are nothing to do with kina but rather the result of storm damage… Either way, the balance has been upset.)

Kina feed on kelp and when their population density gets high enough, they quickly denude reefs of their kelp cover. The extent of reef stripped bare by rapacious urchins is growing and new kina barrens are forming all the time. Along rocky shorelines in parts of the Hauraki Gulf, it is the dominant underwater feature.

Crayfish and snapper are apex reef predators. Larger examples of both prey on urchins, keeping their numbers in check. But in the Hauraki Gulf most of the large snapper and nearly all the crayfish have been removed by over fishing, allowing kina to proliferate.

Kelp forests provide crayfish with cover from predators, as well as habitat for the animals they feed on. Without kelp forests, crayfish larvae have nowhere safe to settle, negatively affecting recruitment, which further impacts on crayfish numbers.

And in negative feedback loop, kina barrens are not suitable habitat for crayfish, so there are now fewer places for remaining lobsters and any new recruits to live. With so few crayfish (and large snapper) present, kina numbers will continue to grow, along with the extent of kina barrens…