|In the spring of 1992, very dense plankton blooms reduced the light reaching the kelp forest, such that the entire forest died inside the Goat Island marine reserve and elsewhere. This research was undertaken from private funds, assisted by a grant from the Ministry of Fisheries (then MAF), to discover the extent of the kelpbed death. At the same time, other observations were made. This document is the result of this investigation, adapted for the Internet.|
This research was undertaken purely out of interest for the sea. The author makes between 100 and 250 dives a year, most of which on the North Island's East coast.
Between October 1992 and December 1992 we have seen plankton blooms as never before in the area around Leigh. Not many divers would have dived in those months but we needed to set up the SEAFRIENDS saltwater aquariums and made many dives to collect specimens. In November 1992 the blooms were so dense that the amount of light at 20m depth was barely enough to distinguish between the white sandy bottom and the dark, kelp covered, rock; even after 30 minutes of adjustment. In addition the water stayed unusually cool for the year. Having had experience in transplanting seaweeds to our aquariums and being familiar with the light levels they require, it was feared that major damage would result to under water plants and for that reason to the whole underwater community.
In the first week of January, cyclone Oli sheared the tops off most of the kelp plants, which had died weeks earlier. Almost literally the kelp forest disappeared overnight. No trace was found of the missing canopy. Although at that time the 100% death boundary was around 14m, kelp plants kept dying during successive months, pushing this boundary towards the 10m.
Immediately in January we started our preliminary survey, the results of which form part of this report. During this survey we also discovered the disappearance of other species, ranging over all phyla from sponges to fish. The fish disappearance was discovered in February 1992 but it was reported to the diving fraternity in the Dive Log magazine of December/January 1993 .
After having reported our findings extensively to the Leigh Marine Laboratory,
we had to give up further investigations through lack of finance. Letters
to various research institutions did not engender either response or enthusiasm.
In June 1993, however, Maurice Miles from the North-Harbour Public Health
Office brought us in touch with the right people at MAF, which resulted
in a small grant of $1000 to pay for fuel, film and fills. We were then
able to complete this survey. The above map shows all the survey sites.
Click on the map for an enlarged version.
I wish to thank Dr Bill Ballantine for his inspiring views over the years that have contributed to this research. His help and proof reading have improved this report considerably.
Looking back, this survey has taken:
this report we have brought together the data collected from 39 dives but
also observations and anecdotal evidence relating to the plankton blooms.
When reading this report, it is useful to know that the rocky shore in
this part of New Zealand is zoned, mainly because of wave exposure and
the availability of light:
This chart lines up the events around the plankton blooms of 91/92 and 92/93.
By June 1993 it became clear that we were looking at major effects affecting a very wide range of organisms: seaweeds, sponges, hydrozoa, bryozoa, anemones, seasquirts, molluscs and fish. Also the plankton blooms were estimated to extend over most of the northern Hauraki Gulf. There was obviously not enough time to do accurate measurements, as the traces of dead kelp were disappearing rapidly. It was decided to do many dives, covering a vast geographical area but to measure quantities roughly in terms like 'many' or 'few'. It was hoped that the many sites would cross-verify one another geographically.
In this report all our observations have been brought together, in order to stimulate thought and discussion.
Our main objective was to find the extent of the plankton blooms through
the kill it had caused to the seabed. Because plankton blooms occur unexpectedly
and develop and disappear quickly, they are very hard to quantify. Furthermore
a very high number of expensive samples is needed to measure both its intensity
and expanse accurately. Fortunately however, the damage caused can be used
as a measure of the bloom's intensity.
Consider the blooms a 'photographic negative' that reduces the light. The kelpbed damage would appear as a 'photographic print'. By measuring the maximum depth of the kelp, the bloom's intensity could be measured. The photographic print also left clear traces in the colour of the pink paint, which covers most of our rocky shore. Palm-sized inprints of kelp could be distinguished, separate from those left behind by sponges, anemones and other sessile organisms. Also the pink paint is dark in colour where it occurred underneath a covering kelp canopy.
We could even measure the 91/92 blooms which we had seen devastate the kelpbeds around Little Barrier Island. These sites would be recognisable by 1-1.5 year juvenile Ecklonia and perhaps the remnants of holdfasts, and their prints.
The 'photographic print' could also give us an indication of how the currents flush the Hauraki Gulf.
Another important objective was just to look around to see what's there and to observe other phenomena. For instance, by looking at many sites, it was hoped to obtain a better insight into how Ecklonia reacts to light starvation. Overall, the exercise has given us a much better understanding of the rocky shore ecology in this area. This report contains a number of exciting and unique results, which have not been published before. It also corrects some misunderstandings scientists have about the rocky shore ecology and the cause and extent of kelpbed deaths. Contrary to traditional scientific research with a before and an after situation, our research became like detective work, relying on traces left behind by the past.
Why each criterion was chosen, will be discussed before its results in the next chapter. The preliminary survey of January/February 93 had shown that more things needed to be looked at than just kelp. Apart from obvious physical factors such as temperature and visibility, we measured accurately the depths of the various ecological zones. We also qualitatively measured the species abundance for various sponges, hydrozoa, bryozoa, anemones and fish.
Immediately after each dive an extensive report was recorded on a tape recorder, following a checklist of items to look at. The same day, the dives were transcribed onto paper with as much detail as possible, including a sketch of the shape of the transect. The picture shows the results of a typical transect. For this report the salient information was extracted and tabled, as shown in the appendix.
Ideally, a survey site should have the following properties:
The seascape changes very rapidly in response to wave exposure. Measuring two sites, only 50m apart, can give very different results. For this reason we were interested in the relationship between biological zoning and exposure. Dive sites have to be chosen with much care in order to minimise unwanted variation and to maximise meaningful information. For instance, an exposed rockface would have an enormous barren zone which yields very little information and it would lack overhangs, cracks and caves which sustain a rich variety of life. By choosing sampling sites strictly at random, it is very likely that the studied effect will be drowned in unwanted side effects. When setting out a transect, it is often better to sample over a wider area in order to record what is typical rather than what the transect line actually yields.
For optimal kelp growth, a north facing slope is wanted but very often these slopes are also very exposed. For studying sponges, a site with a deep reef habitat is required but it must also be sheltered. But wherever shelter is found, the bottom is shallow too, leaving no room for a deep reef habitat. Places with deep bottoms are also often very steep, thereby shading out the sunlight for part of the day and making life difficult for sea urchins. Places with drop-offs, cracks and gullies invite a richer diversity of life but they also form barriers to sea urchins, changing their 'natural' range. Several dives had to be aborted because the chosen spot proved to be unsuitable. In practice we spent much time studying (three-dimensional) depth sounder recordings before deciding where to dive.
|Measuring what is missing
It is scientifically impossible to measure what is missing unless a baseline from a previous study is available. For this reason alone, inaccurate measurements through quantifications like 'few' and 'many' are sufficient. As the results show, they were indeed entirely adequate for the purpose. Babcock  uses ratios of dead over living things to measure the kelp deaths. But this method is useful only immediately after a disastrous event and only if a small proportion died. For instance where stipes and holdfasts have disappeared altogether, the obtained measure would be incorrect. It would also be inappropriate where no living things remain, or no traces. But this is exactly what happened to the kelp forest.
|The influence of time
This research spans a period of 8 months, during which nature has not been idle. The kelp didn't die as if run over by a grassmower; an instantaneous event. But it died back gradually after the initial sudden 'early' deaths. For that reason, all of the January dives needed to be redone, which wasn't altogether possible. For instance, dives 28-32 have not been duplicated. It was hoped that the remnants of Ecklonia holdfasts could still be found on the rocks as proof that the old forest had once been there, but alas after 7 months, most traces of early kelp death had been erased. Babcock  has measured how long it takes for all traces to rot away (about 6 months). Fortunately, however, their 'photographic imprints' remained.
In the meantime the environment started to recover. Kelp recruits were found where once the kelp forest stood. But this opened the opportunity to single out those places with poor recruitment and to use the presence of kelp recruits as an indication of where the old forest once had been.
|Results and discussion
The lower boundary of the kelp forest is caused by lack of light. The kelp bed thins out and the deep reef habitat, dominated by sponges, begins. Along this boundary, the kelp plants can just survive and recruitment is possibly marginal too. Where the water is clear, this boundary is deep, whereas it is shallow in murky waters. Since kelp plants live for about a decade, the depth of the kelp's lower boundary is related to the average clarity of the water over the life of the kelp. Sites 31,32,35 of the inner Hauraki Gulf had to be excluded because of meaningless data. For site 33 an estimate was made, knowing that 50% mortality happened at 10m. It must be noted here that it was the worst time to do these measurements because the whole kelp bed, including its lower boundary had been thoroughly upset.
The results, plotted here, suggest an ingress of clear water through the Craddock Ch. A rather sharp boundary exists between the Chickens Is and the Hen Is, and also between Simpson's Rock and Great Barrier. The water around the north of Great Barrier is unexpectedly murky. This observation also correlates with the extents of the plankton blooms. It suggests that this area suffers under plankton blooms quite regularly.
This survey gave us a better understanding of how Ecklonia reacts to light starvation. The diagram shows the story pictorially.
(A) shows a healthy plant with its main areas of growth: the crown that
sprouts the fronds, the stipe that grows thicker and the base that sprouts
the holdfast. The crown grows out from a growth centre, the merristem,
located just above the stipe. When Ecklonia is eaten short (B), as was
observed at the Rainbow Warrior, it will regrow completely within a year.
But what was observed was a successive loss of fronds, then the entire
crown including the crown's growth centre (F). This was followed by rapid
decay of the stipe (F and G) and ultimately the holdfast (H and I). The
last traces that may remain are a cluster of brachiopods and small seasquirts.
Eventually only a lighter spot on the pink 'paint' (Lithothamnion sp)
may be all that is left.
Assessing the damage was very simple. A simple scan of the area quickly
showed how much of the canopy was still left. The top boundary could easily
be established from either the presence of sea urchins or from the lighter
colour of the pink 'paint'. (In the darkness underneath the kelp, the pink
'paint' takes on a deeper pink, almost purple colour). The lower boundary
was also easy to find. It was as deep as recruitment could be found, and
old marks of kelp holdfasts. In the transect data the density of the kelp
canopy is drawn to scale.
The results have been plotted below for early kelp death and late kelp death. As can be seen, the two correlate. It shows that initially a narrow band between Mangawhai and The Needles caused a short sharp death. But later this band widened to cause late death over a considerably larger area. Note how this area curves around the top of Great Barrier, suggesting that during the blooms, clean water flushed in from the south while pushing the plankton bloom over the top of Great Barrier. But most importantly perhaps, that during an El Niño season, the flushing of the Hauraki Gulf may stagnate.
|The 91/92 blooms
It was possible to assess the magnitude of the 91/92 blooms in the following way: If the 91/92 blooms caused substantial death of the kelp bed, then the recruitment afterwards would now exist exclusively of 1 to 1.5 year juveniles, which are easily distinguishable from other year classes. Juvenile Ecklonia can easily be distinguished from the mature plants by its much smaller size and thinner stipe. It also looks very clean and fresh. So places with very little old kelp but a lot of juveniles were almost certainly hit by plankton blooms in 91/92. We had seen it occur at Little Barrier and could test our case there.
In this map we plotted the sites that were definitely positive and those that were definitely negative. Again, a band results that looks remarkably like a smaller version of the 92/93 blooms in the maps above. It also shows that the 1991/92 kelp death has largely remained unnoticed, since the places where it occurred, are not frequently dived on.
Considering the complicated dual phase reproductory cycle of Ecklonia radiata, its quick recruitment leads one to think that 'spore banks' are present under the old canopy. What these 'spores' are doesn't really matter, but central to the idea is the concept that such spore banks, once activated (perhaps by light), cannot be activated again. Thus a second knock-back should result in subsequent poorer recruitment. We could test this hypothesis by plotting those sites that had a very much poorer recruitment in 1993/94. The map above shows that these sites concur with the 91/92 blooms shown next to it. Thus support has been found for the 'spore bank' hypothesis.
A decline or loss or damage can be shown if:
The common white anemone (Actinothoe albocincta) was chosen because it is common and rather sturdy. Although it has been knocked back it was found to be still common. It has been found to explode through recruitment of vast numbers of small anemones in all the sites that have been hit by the plankton blooms twice.
The hydroid tree (Solanderia sp) has become uncommon. The few 'shrubs' still alive, look scruffy with much 'deadwood'. Only outside the Hauraki Gulf do they still occur in their former abundance.The stippled area in the map connects sites with unusually low occurrences. These appear to correspond to the 1991/92 and 1993/94 plankton blooms.
The big black Ancorina sponge (Ancorina alata) was chosen because it was so common in most habitats. It occurs from as little as a few metres below the surface all the way down to the deep reef. After January we had seen it die and rot away at an alarming rate, e.g. one in every 30 was rotting away. However, this study revealed that, although their numbers and sizes have been affected, Ancorina fared reasonably well.
effect of the plankton blooms on both Ancorina and Solanderia
was not clear-cut. The cluster diagram shows the abundance of both plotted
against the severity of the plankton bloom, as measured by the amount of
kelp death. The black dots are the places around the Colville Channel that
are affected by sedimentation (mud) and some current, whereas the blue
dots are where the kelpbed died twice. The graph appears to infer that
is a robust survivor, but dies when stressed by both mud and overly dense
The Solanderia cluster diagram shows the Colville Channel cluster as distinctly separate, suggesting that Solanderia dies from mud but survives where currents are strong (top two black dots). The blue dots in this cluster suggest that the plankton blooms had a notable effect. The sites with many Solanderia were all located near currents. Where the plankton blooms happened twice, fewer Solanderia were encountered than where it happened only once.
The yellow Polymastia sponge was chosen because it is common and tolerates sand. Like Ancorina it is normally found at a wide range of depths. It used to be common before March 1993 but its numbers have been decimated everywhere. Although some of the sites studied do not provide typical Polymastia 'territory', the overall picture looks pretty grim. However, again sites 4,25,26 looked 'normal'. In other places many very small Polymastia have been seen.
cluster diagrams depict possible effects of the dense plankton blooms
on other species. On left 0% kelp mortality and on right 100%. For the
white anemone (Actinothoe) the situation looks confusing until the
cluster coloured in blue at the top left is isolated, because these belong
to the sites that lost their kelpbed the year before, followed by population
explosions of these anemones. The yellow Polymastia sponge shows
a similar sensitivity but Tethya sponges appear to be more sensitive.
However, the situation with the bryozoa is not at all clear, whereas observations
have shown them to be quite sensitive to land-based pollution. Locations
bordering the Colville Channel (black dots) appear to have lost their bryozoa
before the event, whereas Arid Island (top left) retained some of its previously
very many bryozoa.
It is surprising that the dose sufficient to kill the kelp, was also enough to kill all Tethya sponges. The pink Tethya ingalli appeared more sensitive than Tethya aurantium, and would be a good indicator species.
Reader please note that the above cluster diagrams were
inserted in July 2003 after completing the Plankton
Balance hypothesis. I then realised that data not previously analysed
could support it. The cluster diagrams indeed provide support if the organisms
died because of the same dense plankton blooms that killed the kelp, which
cannot be proved. However, maximum kill occurred in the cleanest of waters,
making it more probable. It is pleasing that the results did not
disprove the hypothesis (after dense plankton blooms filter feeders are
still healthy). Note that the kelp died from lack of light, whereas these
filter feeders who thrive in darkness, died from other causes.
The purple fingersponge (Callyspongia ramosa) lives along steep cliff faces and around the lower kelp boundary where it forms big fans of curved fingers. It likes currents and dislikes dust. It died very early on. Around Leigh we could not find a single live one. Chris Battershill investigated this further [personal communication] and confirmed our findings. He also mentioned that this had happened before, in the 81/82/83 El Niño season. Site 9 (East of the Chicken Islands) was the only place where five very small individuals were found.
The author posing by a deceased Callyspongia
When touched, this sponge fell apart like soaked parchment.
at Floors Reef, Goat Island, 18 January 1993
Yellow boring sponge
The yellow boring sponge (Cliona celata) has also disappeared almost completely. Those patches still found were only barely alive.
Encrusting compound ascidians, the orange and pink blobs and pads found almost everywhere, have also disappeared from all sites visited. (Botryllus sp and Clavelina sp). We have excluded it from our data summaries because it was nowhere to be found. Compound ascidians are thought to be fast growing and quick to re-establish. They were amongst the first organisms to colonise the Rainbow Warrior.
In an article for Dive Log (Appendix 9.3) we alerted the diving community to what we thought was a massive disappearance of pelagic and many reef fishes. All the dives done for this survey furthermore support those findings. In the meantime the common spottie (Pseudolabrus celidotus) must be added to the list (see also Appendix). The fishes least affected are red moki (Cheilodactylus spectabilis) and goatfish (Upeneichthys lineatus). But it has been observed that the entire cohort of 91/92 disappeared in Feb/Mar93. The recruitment of 92/93 is smaller than usual.
Crayfish has become one of the rarest organisms in the Gulf. Very few were encountered. We found more boat anchors than crayfish and also more of the rare giant boarfish! Compared to the densities in the Leigh Marine Reserve, it would have been 0 to 2 percent. Whenever a craypot was encountered near the survey site, its content was inspected and the area up to 40m around, searched for crayfish. In all 6 cases where the craypot contained crayfish, no crayfish could be found in the surrounding area. It suggests that craypots are able to attract crayfish from further away but most important of all, that the days of crayfishing in the Hauraki Gulf may be numbered. But we found increasing numbers of young crayfish from Tryphena going south.
Plankton bloom densities
Because we were diving actively during the plankton blooms (Sep 92 to Dec 92), it pays perhaps to include some anecdotal evidence. The blooms would often concentrate 1-2 metres below the surface, thinning out as one went deeper. In the figure, observed underwater visibilities have been plotted. Curve A was for the early blooms from October 92 to November 92. Curve B shows how in December the bloom mixed into all depths. But even then clear water could often be had underneath the thermocline at 18m. When the blooms were at their height, it would be as dark as a moonlit night at 20m, despite bright sunlight at the surface. Another observation was that now and then a patch of clear water came in with visibility about twice what was common at the time.
After the blooms the rocks were covered with a sticky kind of 'dust' that was difficult to remove. It was oserved that sponges covered by it, died. The 'dust' grew into a kind of 'brown fluff' that covered just about everything. Particularly affected is the deep reef habitat. Our survey has shown that the deep reef habitat is rare inside the Hauraki Gulf. Even outside the Gulf it is often difficult to find deep rocky shores that are not too much exposed. Where suitable places were found, we descended to well over 40m to observe the deep reef habitat. In all cases we found major damage to this habitat. Everything was covered in brown fluff that was hard to remove. It is estimated that over 80% died.
Already during our exploratory survey of Jan 93, sponges were seen rotting
at the rate of 1 in 30. Similar events in our aquariums, showed that
a sponge such as Polymastia could disappear in one week. The sponge
rot has been observed for 3 consecutive months, occurring at approximately
the same rate. It suggests that close to 40% could have disappeared in
in relation to exposure.
Unlike the landscape above water, the seascape changes very drastically with wave exposure, because exposure from waves has such an enormous range, and is influenced remarkably by changes in topography (shape of the shore). In order to find suitable dive sites for a possible follow-up, perhaps serving for base line studies, we wanted to know the effect of exposure on habitat zoning. In order to 'sort' the shore profiles by wave exposure, we made the rough assumption that the depth of the sandy bottom is related to wave exposure (See Harris ). But we had to be careful in selecting representative sites. The strip diagram below shows the result, and the colourful diagram on right its final version. Sites were disqualified for one of the following reasons:
zones without urchins
We found 8 sites with barren zones but no urchins. Grazing was done by the Cook's Turban snail (Cookia sulcata), occasionally assisted by Paua (Haliotis iris). Because we were interested in the natural upper boundary of Ecklonia, caused by wave exposure alone, we plotted these sites against sand bottom depth, which is assumed here to correlate with exposure (as it does in the exposure-zoning chart). Sites 24 and 22 are positioned too far right because they are located in strong currents of the colville Channel. Note that the boundary is caused by the last big storm event, and that a successively larger event may shift it further downward (steeper curve).
The giant heart urchin (Brissus gigas) was discovered because of plankton blooms. In 82/83 the excessive plankton blooms rained down and formed a sticky, thready mass on the bottom. Under 15m depth many organisms died, including scallops, sea urchins, sponges, seasquirts and demoiselles. During a dive at Leigh Reef, many giant heart urchin tests were found on the sand. These were bagged and studied. Up to that time, little was known of this species. (See also NZ Herald article of 19 Feb 1983)
The diagram shows how a giant heart urchin burrows in course shelly sand. As it removes the sand in front, while pushing it up behind, a sink hole appears and a mound of sand behind. The sea urchin consumes the sand grains whole, and digests whatever lives in between. The sink hole collects detritus from the bottom's surface, and this is perhaps the main ingredient of the urchin's diet.
In 1983 also a dying specimen was found by Chris Battershill, near Leigh. Although discoloured already, it showed what the live animal looked like. In 1989, after studying the common heart urchin (Echinocardium australe), we linked 'sink holes' found in the sand to the giant heart urchin. It lives 20-40cm below the sand surface, rather than 3-5cm for the common heart urchin (Echinocardium australe), and has therefore escaped the attention of scientists. Digging a hole that deep (in order to catch it), is rather cumbersome underwater because of the dust it causes and because the hole keeps caving in. At the end one has to dislodge a large, very prickly object, without damaging it. It was shown that the giant heart urchin had lived in their thousands right under the watchful eye of the scientific community of the Leigh Laboratory! What is more, their diggings are rather obtrusive and often half a metre across, and nobody had wondered what they were!
From that point in time, giant heart urchins could be sampled without
disturbing them. In this survey, the sites with these big urchins were
marked. The map shows that they are common in the northern part of the
Hauraki Gulf and further north (including the Three Kings Islands). It
also suggests that they are unlikely to be found inside estuaries, as their
boundary stops half-way the Hauraki Gulf. It must be noted here that giant
heart urchins may also be found in wave-exposed sandy areas but here their
diggings are erased easily by successive storms, and they cannot be sampled
easily. However, my impression is that these heart urchins prefer clear
water and coarse, shelly sand.
In December 1992 a mass stranding of blue penguins (Eudyptula minor, fairy penguin) was observed, but such strandings are not uncommon. When boating from Leigh to Little Barrier and back, in slight seas, usually a dozen or more penguins are seen in a very narrow swath of about 10m wide. But for some time now, only a few (3-4) were seen each time. It was decided to include blue penguins in our survey. However, the only way to count them properly is during very smooth, glassy seas. Then they can clearly be distuinguished from other sea birds up to 50m around. The diagram below gives a transect made under such conditions, between Leigh and Little Barrier island. The histogram suggests that blue penguins like to feed between 2 and 5 nautical miles offshore. A total of 31 birds were counted over a swath of 100m wide, which equals to about 3 birds over a swath of 10m wide - consistent with our initial observations. It is feared that Blue Penguins have been reduced by 60 - 70 %.
[Note, this transect was repeated in February and June 2003, under similar conditions. Only 2 birds were seen!]
Concentrations of blue penguin were also found south of the Chicken Islands and in front of Omaha Bay. In 2003, we found another concentration between Kawau Island and Martins Bay (Feb 2003). We had the opportunity to re-do the penguin transect between Little Barrier and Leigh, in February 2003, and found only 3 penguins over a swath of 100 metres wide. No penguins were found close to Little Barrier Island. It suggests that their numbers have declined further in the decade since.
f015332: a fledgling baby blue penguin of the white-flippered variety is an endearing creature to see, but it does not have the attributes that make for a good pet. At the best of times, the sea is a hard environment for them to survive in, but humans have made this much worse, as our surveys suggest.
This research was based on the assumption, that with some care, the depth of the sand can be thought of as a measure of wave action. Results using this assumption, have proved to be consistent. The method allowed us to 'sort' profiles in a meaningful way, as those shown in the diagram. To our surprise, three platforms appeared below the intertidal platform which falls dry at low tide. It suggests that platforms occur on the boundaries of the major zoned habitats. More, and more careful work needs to be done to substantiate this.
The intertidal platforms (not shown here) are surprising, because it is exactly where they occur, that wave action is highest. Yet the 'dry' coast above it, appears to erode faster. A valid explanation for this is that where the rock is wetted by sea water, a community of living organisms, covers all parts of the rock, thus cushioning the abrasive work of waves. Damage gets repaired, resulting in a very slow overall erosion. The pink paint (Lithothamnion sp.) is a champion in this process, since it is so hardy, while occurring from above high tide to over 70m deep.
Pink paint covers rocks so thoroughly, that we have observed steel anchors
and chains not rusting underneath. It may well be that this stoneweed prevents
oxygen and acids from reaching the rock, thereby slowing down its natural
rate of weathering.
Why pink paint would slow down erosion on the boundaries of the urchin barrens, we fail to explain. Remember that the above subtidal contours may have taken a millennium to form. To find a continuum of platforms surrounding the urchin barrens, suggests that such barrens have been there long before the arrival of mankind in New Zealand. It may well be possible to carbon-date the underlayers of the pink paint, in order to arrive at new insights.
|Conclusions and discussion
This survey has shown that following the severe algal blooms of 91/92 and 92/93, the underwater environment has suffered considerably. A wide range of organisms was implied. Had this survey not been done, the loss of many common organisms might not have been detected. We are also perfectly aware that many organisms may have escaped our attention (note the late discovery of the loss of spotties). There is no simple explanation for the damage caused. It must probably be found in a complex combination of low temperatures, photo starvation, kelp leachates, plankton toxicity and sedimentation.
Because gradients in our data are unusually steep towards Whangarei, the influence of the Whangarei Harbour and its industries (cement, glass, oil refining) must not be ruled out. These industries may provide the nutrients most needed by the plankton community.
The Hauraki Gulf Marine Survey 1993 has provided a set of data that is geographically consistent and that was adequate to answer a number of pressing and interesting questions. However, it has also raised new questions that require a more solid scientific approach including more sampling points, while quantifying tthem more precisely. The study has shown that New Zealand needs a network of sampling sites in order to quantify what is happening to our seas. It won't be easy to choose the best and least number of sites. But the experience gained from this survey could be used to advantage.
The processes that occurred during the blooms were all very sudden and amazing. Likewise the road to recovery will be unexpected. If we are to learn from the biological oscillations that are bound to follow, we need to have a policy for monitoring the recovery process. This survey could serve as the 'base line' for the recovery process. We also need to look at long-living organisms that may store a record of year-to-year progress, like growth rings. Such growth rings would give us an idea of how the organisms 'rated' their living conditions for each year. Some shellfish and corals would be suitable and perhaps the most amazing and hardy organism of all, the pink 'paint' (Lithothamnion sp.).
 Anthoni, J F. "The disappearing fish act". Dive Log No 13. Dec/Jan 1993.
 Babcock, R C and R G Cole. "The extent of die-back of the kelp Ecklonia radiata in the Cape Rodney to Okakari Pt Marine Reserve". Advice to the Department of Conservation. June 1993. Leigh Marine Laboratory.
 Ballantine, W J. "The algal bloom and climate anomalies of 1992". Leigh Laboratory seminar. May 28th, 1993.
 Greig, M J and Proctor, R. 1988."A numerical model of the Hauraki Gulf, New Zealand". NZ Jnl of Marine and Freshwater Research. 22.391--400. 1988.
 Harris, T F W. "North Cape to East Cape".University of Auckland. Leigh Marine Laboratory Bulletin 28. 1985.
 Harris, T F W. "Hauraki gulf tideways. Elements of their natural sciences". University of Auckland. Leigh Laboratory Bulletin No 29. 1993.
|OBSERVATIONS ON THE DECLINE OF SPOTTIES
IN MAHURANGI HARBOUR
On 14 March 1993, Mrs Margareth Jarius from Snells Beach, related the following observations to me:
Margareth is a keen fishing lady and fishes regularly on several places
along the Mahurangi estuary. She has noticed a considerable reduction in
the numbers of spotties caught as a by-catch while fishing for snapper.
The inner Mahurangi, Scotts Landing and Mullet Point are places where she
fishes regularly. Previous to 2 years ago, the spotties were a nuisance
because in an hour's fishing, one would catch 3-4 of about 20-25 cm. At
Mullet Point she would catch 2-3 per hour. But since 2 years, the size
she catches has disappeared. In fact, she hasn't caught a single one in
about 30-40 trips of 2-2.5 hours of fishing
The Snappers caught seem to be bigger (3.5 pounds, 40 cm) but with a noticeable size gap between them and the undersized ones (less than 20cm). Her partner agrees with her that the snapper are harder to catch ("The fishing has dropped away"). It is felt that the Snapper haven't changed suddenly but reduced in numbers gradually.
At Mullet Point the trevallys caught appear to be bigger. Kahawai suddenly decreased about 2 years ago.
Margareth has also seen big fish in schools of 6-10, visit the upper estuary along the oyster tiles. Amongst these tiles they would stand head down, working the bottom, their yellow tails sticking out above the surface. Since the water was about 1m deep, these fish must have measured between 80 and 120cm. When these fish swim horizontally, they show a big yellow dorsal fin above the water. Apart from that, the fish are very shiny/silvery, unlike kingfish. Their tails also appear broader than those of Kingfish. For several years now she has observed them returning to the upper Mahurangi in the month of February.
I have suggested that these fish may have been samsonfish. These are sideways compressed (unlike kingfish), up to 150cm long, silvery with yellow fins and tail and they occur in small groups. Although they are uncommon in our waters, they are occasionally seen.
Margareth Jarius; 295 Mahurangi East Rd; Snells Beach