the snapper-urchin-kelp myth
marine reserves do not cause habitat changes
by Dr Floor Anthoni (2009)
in-depth logo Floor Anthoni


Scientists noticed how inside the Goat Island marine reserve, urchin barrens disappeared as the kelp forest took their place. They reasoned that this was caused by the large snapper and crayfish who eat the sea urchins, who otherwise would have eaten the kelp. Fewer urchins means more kelp. Thus big snapper leads to more kelp. Urchins bad, kelp good. Marine reserve good.
However, the scientists were dishonest and ignored previous experiments and later experiments that disproved this hypothesis. Seafriends has extensively debunked the research done in a separate, large chapter, but here we'll present the important arguments so that everyone can understand the issue.


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What did scientists notice?
Scientists noticed how urchin barrens disappeared from the Goat Island marine reserve, around 1998, twenty years after the reserve was established. They reasoned that large snapper had been eating sea urchins that normally attack kelp, because inside the marine reserve the snapper were allowed to grow more numerous and bigger, and so did crayfish. Their conclusion: marine reserves can have late effects on their environment, a so-called trophic cascade. Although entirely reasonable, this is entirely false. Regrettably, this myth has spread far through the scientific literature and is now taught at NZ schools! A major scientific scandal! So what is the real story?

What did scientist do?
Scientists observed that the barren urchin habitat at Goat Island and nearby Tawharanui( marked in red) was invaded by the kelp, and they reasoned that this was caused by marine protection. However, when they looked at all other east coast reserves (Mayor I, Hahei, Poor Knights, Long Bay), this was not the case. They then compared Goat Island and Tawharanui with the area outside, and obtained questionable just positive results. However, had they looked at nearby Kawau and Little Barrier Island (marked yellow), they would have seen that there too, the urchin zone had disappeared (in 2007 this extended over most of the east coast). Scientists had conveniently hushed up that in 1993 the whole kelp forest disappeared and in 1994-98 the sea urchins died by eating poisonous plankton slime. We observed that the urchin zone disappeared only where the kelp forest had disappeared in 1993.
But there was more evidence, as a study of a large number of marine reserves, from the very south to the very north, showed that marine reserves had no influence on what grows where. The study showed that degradation (murkiness and deposition) were nearly 100 time more important than marine protection. Yet scientists never proved or even considered that they were observing symptoms of degradation rather than symptoms of protection.
kelp bed death 1993
f016803: the whole kelp forest in the Goat Island marine reserve disappeared overnight in early January 1993 when Cyclone Oli struck. But the kelp had died months before due to very dense plankton blooms, also observed by scientist.
extent of kelpbed death 1993, map
In the winter of 1993 we dived along transects shown by the small dashes. We established without doubt where the kelp had died, and also where it died the year before. Notice the two red areas which are the Goat Island marine reserve and the Tawharanui marine reserve. Scientists should have looked at the yellow areas, Kawau and Little Barrier Island, where the kelp also disappeared in 1993, and where the urchin barrens disappeared later.

Since about 1985 there have been sporadic outbreaks of a poisonous slime, identified as an Ostreopsis benthic dinoflagellate species. Although it does not attach firmly to the substrate, it is nonetheless rather sticky and not easily removed by waves. After the kelpbed death, the sea urchins were no longer 'penned in' their barren habitat zone and wandered off into the deep. As a result, the insufficiently grazed barrens were first infected by this poisonous slime that killed a large percentage of sea urchins. The barren area was now so poorly grazed that the kelp could survive where it previously couldn't, and it invaded the urchin barrens. From an ecological standpoint, this was possible only because the kelp plants were all of same age. We have arial photos (see below) that show that the main invasion happened in 1999, and not as scientists say, gradually from the very beginning of the marine reserve.
It must also be noted that we discovered that the urchin barrens are in fact storm barrens, created by large storms such as tropical cyclones. It must also be noted that we haven't had many such storms in recent times, and Cyclone Oli of 1993 is the last one. It can be expected that the next Tropical Cyclone (hurricane) will clear the storm barrens again. In the meantime, much smaller storms cause huge wash-ups of broken kelp.

Ostreopsis slime infestation
f030305: since 1985 there were occasional outbreaks of this poisonous dinoflagellate slime of an Ostreopsis species. Notice how all grazing animals are missing from this one square metre patch, as they have all died.
sea urchin died from ostreopsis slime
f034409: a sea urchin has cleared (eaten) a swath of poisonous Ostreopsis slime and is now dying with its spines folded.
huge kelp washup Goat Island beach
0901181: 7 March 2009. An insignificant storm caused a huge wash-up of stalked kelp because these plants established themselves in the storm barrens, where they can be removed easily by storms.
Goat Island aerial shows barren zone
f992323: This aerial photo was taken in April 1999, showing the main barren area of the Goat Island marine reserve. Notice how the kelp is beginning to invade, but most of the invasion happened suddenly a year later. Scientists maintain that it was a gradual process that began before 1980, which is proved false by this photo.

effect of marine protection diagramWhat do our children learn?
Outside marine reserves, one finds barren areas dotted with sea urchins. These attack the kelp, thereby creating a barren zone and reducing biodiversity. The fish here stay small because they are caught. Inside marine reserves, however, the snapper and crayfish grow large enough to eat the sea urchins (kina) and this allows the kelp to return, feeding the weed eating fish, while also increasing biodiversity. As fish become more numerous and large, their spawn increases substantially. Currents move their eggs to the areas outside, replenishing fish stocks. Kelp GOOD, urchins BAD.

But what is the reality?
Snapper and crayfish do not have a preference for sea urchins, as has been shown by several studies. Urchins died from poisonous slime. After the kelp forest died, urchins spread out. The new generation of kelp was all of same age, swamping all areas. Large predators eat other animals, and the total spawn mass decreases (sea urchins are copious spawners). The barren habitat is home to large populations of other grazers like snails, limpets, chitons and fishes like triplefins. It is a very productive area because the sunlight reflects from the light-coloured calcareous algae (pink paint). It is also the spawning habitat of many plankton feeders like demoiselles, triplefins and black angelfish. Worst of all, when seaweeds are not eaten, they rot, infecting other species. In March 2009 most life disappeared from the famous Echinoderm Flats. In 2007 we established that the urchin barren zone disappeared from North Cape to East Cape, and also on outlying islands. In all cases there was an infestation of Ostreopsis.

snapper eating sea urchin
f023514: a young snapper speeds away with a small sea urchin presented to it, proving that snapper can eat sea urchins. However, these small ones live a secretive or cryptic existence, hidden under stones, until they are large enough to venture out in the open. Snappers do not bother about the larger ones.
cryptic hidden sea urchin kina
f032321: small sea urchins live a cryptic existence, hiding amongst and under stones. A snapper would not be able to dislodge a baby urchin like this one. But at night the urchin feels safe to forage near its hideout.
f033721: broken urchins found near a crayfish hole. When crayfish eat urchins, they pry them from the rock and take them home where they open them from the side of the mouth. Divers smash urchins from the top, so one can tell the difference.

Why is Ostreopsis such a problem?
We don't know for sure, but Ostreopsis may just be one of the symptoms caused by overnourishment of the sea. An excess of nutrients from the land fertilises the plankton and encourages the growth of bacteria and noxious microbes such as Ostreopsis. Over the past 20 years we have observed and documented the rapid decline of nearly all marine species, either directly from disease or indirectly from lack of food. Seafriends was established precisely because we saw this happening as early as 1987! There exists a new threat in the sea that is growing rapidly and is already larger than the threat from fishing.

What went wrong?
How was it possible that scientists made such a big mistake? 
  • scientists did not understand that urchin barren zones are created by storms; a better name for them is storm barrens.
  • they ignored failed experiments and contradictions and a nation-wide study of habitat change  in marine reserves (done later)
  • they took as control a semi-sheltered coast perpendicular to the ones studied, rather than Kawau Island and Little Barrier where the urchin barrens also disappeared

One would have thought that the first marine reserve in New Zealand would have been monitored extensively, thoroughly and continuously, but this is not so. In the beginning years there was enthusiasm for counting the fishes, in the hope that they might become numerous. The graph shows that this was not the case. It must be remembered that whenever a fished area is closed, the fish stocks immediately begin to increase, as was inded the case with crayfish, shown below. 
The graph shows how snapper first increased as expected, but then declined again. The only real winner here is the red moki, a fish that finds all it needs inside the marine reserve where it is attracted to a sleeping hole. It doesn't migrate out for feeding or for spawning, as most snappers do. Scientists can show that the reserve contains more big snapper than outside, but most of this is caused by displaced fishing to the area outside. In any case, the disappearance of sea urchins cannot be explained from the population of snappers.
fish counts at Goat Island
The crayfish population inside the Goat Island marine reserve can be called a success story, again because crayfish do not migrate a lot and they find all they need inside the marine reserve: food, mates and a sleeping hole.
The graph shows that crayfish populations immediately began to increase and then hovered around amximum of 30 bugs per 500 square metres. However, in 1998 we observed the sudden collapse of the population, as documented in monitoring the Goat Island marine reserve.
In 2000 scientists discovered that 5 out of 6 crayfish had disappeared. Since then the population is slowly recovering, although not very visibly for divers.
The populations outside the marine reserve remain much smaller because of displaced fishing effort, poorer habitat and too aggressive fisheries management.
Goat Island crayfish rocklobster count

How widely spread is the myth?
One need only go to a search engine and search for 'urchin barrens' to see how widely spread the myth has become, even among scientific 'peer reviewed' articles. Indeed, it has become accepted wisdom. The following quotes may illustrate:
As you can see, other myths have crept in as well. One even goes as far as turning the argument around, declaring that every urchin barren was once a kelp forest before the sea was overfished. This amounts to saying that every desert on Earth was once a lush forest, before people came. It is quite worrying that the snapper-urchin-kelp myth is now taught at school and printed in school textbooks.
You may now be interested in what New Zealand storm barrens look like, as these are often not populated by sea urchins. You may also be interested in reading our scientific rebuttal, which is receiving more and more scientific support.

habitat change by fish?

Related chapters on the Seafriends web site:
Storm barrens of northern New Zealand: an overview of storm barrens, with and without sea urchins. Many photographs.
Science exposed: Exposing fallacies in ecological marine research in northern New Zealand (urchin barrens rebuttal)
Introduction to habitats: what makes life underwater so different?
Biodiversity: what is biodiversity and how does it apply to the underwater environment?
Marine conservation: understanding marine protection and how it is overvalued.
Frequently asked questions about marine reserves: your no-nonsense introduction to the myths around marine reserves.
Goat Island: the Goat Island marine reserve. Does monitoring show high abundance and diversity?
Poor Knights: the Poor Knights Islands marine reserve. Does it protect biodiversity?


  • Cole Russell G , Tony M Ayling, Robert G Creese 1990: Effects of marine reserve protection at Goat Island, northern New Zealand. New Zealand Journal of Marine and Freshwater Research, 1990, Vol. 24:197-210.
  • Haggitt T, Kelly S 2004: Cape Rodney to Okakari Point Marine Reserve Lobster Monitoring Programme: 2004 Survey. Dept of Cons.
  • Nick T Shears. Russell C Babcock, March-June 1998. Marine reserves demonstrate top-down control of community structure on temperate reefs . Springer Verlag May 2002.



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