Cold-water food chain in trouble

Life on Earth is a miracle of interconnectedness. In its simplest form, a plant gathers energy from the sun and grows leaves. A small creature eats the leaves, and then a larger creature eats the herbivore. This sequence of connections is called a food chain.

We know that if you damage the bottom of a food chain (by taking away the plants), the rest will starve. What is less obvious, however, is that the top and middle ends are also important. A clear example is the waves off the coast of Otago.

Along the Otago coast, just offshore from the rocky platforms, there are rimurimu (kelp) forests. These fabulous ecosystems are home to numerous kelp feeders, including pāua (abalone), kina (sea urchins), pātangaroa (starfish) and pupu (snails). Some of these nibble on the kelp itself, others harvest kelp flakes from the water. The kelp forest acts like a large net, catching larvae of fish and koura (crayfish) that settle and develop in the protective fronds. Blue cod, terakihi, greentails and crayfish grow up here and then feed on the kelp feeders as adults.

All over the world, this is a typical cold-water food chain, and it is under threat. The predators – big fish, crayfish, seals and seabirds – are being wiped out by the planet's largest predator, humans. If we catch too many of these predators, the kelp eaters will take over. In many places you can see where kelp forests once were – it looks like a logged forest full of tree stumps. These places are underwater “wastelands” where grazers have mowed down every last kelp and all that is left is bare rock.

So is the solution to kill some of the grazers? In California, where sea otter hunting caused the barren land to grow, people have come up with a solution to restore the kelp forest. Divers have been equipped with hammers and instructed to kill as many sea urchins as possible. In nine hours, two divers can smash more than 10,000 sea urchins, and they still do. But can you fix a broken food chain with hammers?

Like a metal chain or a necklace with links, if one part breaks, the whole thing is broken. And it's not that easy to fix a food chain. Restoring ecosystems requires a comprehensive approach where we clean up the seas by addressing our land-based activities so that light can shine on our marine plants while also allowing marine grazers and predators to thrive.

Kelp forests are arguably New Zealand's most diverse ecosystem, so it is worrying that our kelp forests are also under threat. “The Kelp”, a vast forest that stretched from Brighton to Taieri Mouth, has largely disappeared. Current kelp forests, such as those in Otago Harbour and off Karitane, are at risk and are shrinking.

A combination of factors is responsible: sedimentation and pollution from land dim light and reduce growth, while warming waters and heat waves cause weakness and stress, allowing increasingly strong and frequent storms to destroy kelp. And kelp eaters are probably doing their work too, as we overfish the predators.

When kelp forests disappear, the coastal ecosystem changes. Fish that once lived there, such as ling and hāpuku, are driven further out to sea, as are the sea lions that hunt them. At the same time, the bare patches where kelp once grew are vulnerable to invasion. While invasive marine plants can take over, they do not provide shelter or food like kelp does.

In Karitāne, north of Dunedin, local East Otago Taiāpure are working to keep the kelp forest-kelp food chain intact. They are planting on land to keep sediment out of the rivers. They have imposed restrictions on fishing and are re-seeding kelp where it used to be. They hope that heatwaves and ocean storms won't undo their good work. If they can balance the ecosystem with thriving kelp, grazers and fish, we may once again be able to experience the miracle of a functioning food chain without having to use a hammer.

Eating Kina

• Evechinus chloroticus is the green sea urchin or kina, which only occurs in New Zealand. They are often the size of a fist, but can grow to the size of a salad plate (approx. 17 cm in diameter).

People have long enjoyed the salty, foamy taste of kina gonads. It is said that the roe tastes best when the kōwhai are in bloom. We have been trying to catch and farm them for export to Asia since 1986, but these efforts have not been very successful. Nevertheless, kina are an important part of kai moana in Aotearoa.

Abby Smith is Professor of Marine Science at the University of Otago.