Scruffy Kelp Nothing to Worry About

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This summer snorkellers at Goat Island Marine Reserve might notice some sick-looking kelp stalks out near Shag Rock, the little reef out in the middle of the bay that many people snorkel to.  This is a natural phenomenon and nothing to worry about.  It is simply the kelp plants dying of old age, and there are many young plants growing to replace them.

The common kelp Ecklonia radiata is abundant in the marine reserve below about a metre deep, with the more strap-like Carpophyllum at and just below low water mark.

Ecklonia kelp is the favourite food of the sea urchin or kina, and outside the marine reserve most of our shallow reefs are devoid of dense kelp and are dominated by hundreds of kina.  We call these bare areas “kina barrens”, because the kina behave like sheep grazing at the rock surface and preventing the kelp forest from returning.

The kina barrens are there because there are not enough snapper and crayfish left in the sea to keep the kina numbers down to a natural levels.  Kina is a favourite food of snapper and crayfish.  Over the past 40 years or so kina have multiplied and eaten most of the kelp forest!  This is a serious ecological change and has caused a huge loss of biodiversity, brought about by too much fishing for snapper and crayfish!

In the Marine Reserve where fishing is not allowed, the snapper and crayfish have slowly built up to natural numbers and have reduced the kina numbers back down to where they should be.  This has allowed the kelp forest to recover to its former glory.  So the reefs in the marine reserve are in a healthy state, but outside the reserve they are seriously degraded in many areas.

Individual kelp plants live around 6 years and can grow to a metre or so tall.  When they die naturally, the fluffy top often drops off, leaving a scruffy rotting stalk.  The loss of the kelp canopy opens up the rock below to bright light, which allows new kelp plants to grow in their place.

It is not quite as simple as that, however.  Kelp has a complicated life history, with two styles or “generations” of plants.  What you see as the kelp plant is the sporophyte or asexual generation, which reproduces by releasing spores to the water, which settle to the bottom and grow into the gametophyte or sexual generation.  This is a tiny plant you would not notice, only a few millimetres tall.  It releases eggs and larvae to the water, which join together then settle to the bottom and grow into the large very obvious kelp plant!

So next time you go snorkeling over shallow rocky reefs you may like to think a little more about kelp, kina, and the effects of fishing on the health of our reefs.

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