The Value of Mangroves in Whangateau Harbour

Whangateau Harbour in flood January 29, 2011

Whangateau Harbour in flood January 29, 2011

This is the submission by Dr Roger Grace to the Independent Hearings Panel in the matter of the Proposed Auckland Unitary Plan on behalf of the Environmental Defense Society and the Royal Forest and Bird Protection Society of New Zealand

Summary of Key Opinions
  1. Mangroves are indigenous plants, and contribute to a range of ecological and other values.  Mangroves, and in particular large mangroves, contribute to the very special character of Whangateau Harbour:
    1. By providing ecosystem services (including erosion prevention, and enhancement of water clarity by providing a substrate for filter-feeders to attach to)
    2. As an important component of coastal ecosystems (for example, as habitat for fish and birds)
    3. By providing recreational and educational opportunities (snorkelling, kayaking etc, including by school groups)
    4. They also contribute to the scientific value of Waikokopu Creek as part of the vegetation sequence that connects the foreshore with the adjacent Scientific Reserve.
  2. The adverse effects of removing mangroves from Whangateau harbour include:
    1. Loss of recreational opportunities
    2. Loss of habitat (shelter and feeding opportunities) for juvenile parore and mature parore (who shelter in individual mature mangroves within seagrass beds).
    3. Loss of the value of Waikokopu Creek for scientific study (as part of a relatively intact vegetation sequence together with the neighbouring Scientific Reserve).
  3. In Waikokopu Creek, the very slow expansion of mangroves is not associated with increased sedimentation, as in northern parts of the Whangateau. This very slow mangrove expansion is unlikely to be causing harm to amenity values, recreational opportunities or ecological values.
  4. Previous (unconsented) mangrove removal has not been effective at restoring sandy substrate.
  5. Hand-clearance of mangroves is very time and labour intensive, which has implications for the adequacy of long-term maintenance of cleared areas.
  6. In my opinion, it is not clear that the Whangateau Harbour was historically “free of mangroves”. Historical evidence shows mature mangroves with the canopy above the grazing height of cattle, which commonly grazed in the intertidal zone in former times.  Mangrove removal is not “restoration” of natural habitat.
  7. Rules providing for clearance of mangroves back to 1948 levels are inappropriate, and will result in significant adverse effects on the values that I have identified. The pristine nature of intact natural habitats in the Waikokopu Creek suggest that the inevitable disturbance caused by mangrove removal is unwarranted.  Furthermore the potential value of the site as a study area for natural changes related to climate change and sea level rise is an opportunity which would be squandered by unnecessary interference of mangrove removal.
  8. Large scale removal of mature mangroves would have serious adverse effects on recreation in some areas, and elsewhere in the harbour would have adverse effects on ecological values. Only controlled removal of mangrove seedlings is, in my opinion, justified in some areas on ecological grounds.

Qualifications and experience

My full name is Roger Vernon Grace.

I have a B.Sc., M.Sc. (Hons.) and Ph.D. in Zoology (marine biology) from the University of Auckland (1972), and have carried out marine ecological studies for about 50 years. In 1965 I was one of the first marine research students at the University of Auckland’s Leigh Marine Laboratory.  I was the recipient of a Mobil Environmental Award in 1973, am a life member of the New Zealand Marine Sciences Society, and was awarded a Queen’s Service Medal (QSM) for public service in 2005. I am currently a committee member of the Mid-North Branch of Forest & Bird, and a life member of the NZ Underwater Association.  I have been a committee member of the Whangateau HarbourCare Group for ten years and am regarded as their “chief advisor” on marine matters.

My qualifications and experience are primarily in the field of marine ecology, however I consider that I am also qualified to comment on the recreational and educational values of the Whangateau Harbour as a result of my extensive involvement with the Whangateau HarbourCare Group.

Annexure A is a summary of my relevant experience in both areas.

Code of Conduct

I confirm that I have read and agree to comply with the Code of Conduct for Expert Witnesses in the Environment Court Practice Note 2014. This evidence is within my area of expertise, except where I state that I am relying on what I have been told by another person.  I have not omitted to consider material facts known to me that might alter or detract from the opinions that I express.

Scope of evidence

I have been asked to prepare evidence for the Environmental Defence Society and the Royal Forest and Bird Society in support of their submissions on topic 33.

In preparing my evidence I have relied on a number of relevant publications and reports and refer to these where relevant in my evidence. In particular, I  have considered the following documents for Topic 33:

  • Evidence of Carolyn Lundquist
  • Evidence of Robert Scott
  • Submission of Omaha Beach Community

This brief of evidence will cover the following matters:

  • Status of mangroves in New Zealand.
  • Values of mangroves in Whangateau harbour.
  • Drivers and effects of mangrove expansion in Waikokopu Creek.
  • Ineffectiveness of previous mangrove removal.
  • Mangrove seedling removal trial south of the causeway.
  • Comments on OBC submission.
  • Comments on Dr Lundquist’s evidence.
  • Comments on proposed 1948 Rule for Whangateau.

Status of Mangroves in New Zealand

New Zealand has a single mangrove species (Avicennia marina subspecies australasica).  New Zealand mangroves are a native species, and were present in New Zealand for thousands of years prior to human colonization.  They cannot be regarded as an “invasive species” in the usual sense of the term in regard to species from overseas that have reached NZ and spread to become a nuisance.

Mangroves are important in northern New Zealand contributing to ecosystem health in estuaries, and carrying out a wide variety of ecosystem services including primary production, shoreline protection, sediment trapping, carbon sequestration, and habitat for birds, fish and invertebrates, including both marine and terrestrial species.

The values of mangroves in coastal ecosystems in New Zealand are well documented, particularly in Morrisey et. al. (2007), a report commissioned by the Auckland Regional Council in response to increasing public concerns about mangrove expansion in the region. Dr Lundquist has further noted positive mangrove attributes in her evidence.

Values of Mangroves in Whangateau Harbour

The Whangateau Harbour (shown below) is the most unspoilt estuary on the mainland of the Auckland Region.  It has extremely high water quality, high quality shellfish beds, clean sandy sediments, and highly diverse birdlife and marine life. It has by far the largest shellfish resource in any estuary in the Auckland Region.

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Because of the high quality of the Whangateau Harbour, it is also extremely popular for water sports and recreation, particularly kayaking, windsurfing, kite boarding, swimming and stand-up paddleboarding. Its educational value is also being increasingly recognized, and more and more school groups are using the harbour to further their ecological studies.

The Whangateau HarbourCare Group began shellfish monitoring in the Harbour about 12 years ago, and has provided valuable information on the health of the shellfish beds.  In particular they documented a serious cockle die-off in 2009, and have monitored recovery since then.  Various school groups have taken over the monitoring as part of their ecological studies.

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Since 2009 the Whangateau HarbourCare Group has been sponsoring annual community guided snorkeling days at Horseshoe Island near the Whangateau Domain. On a good spring high tide when there has been little wind and rain for a few days the water clarity is exceptional around the island.  This is largely because of the ecological health of the harbour and its small relatively undeveloped catchment.  Huge numbers of shellfish and other filter-feeders remove plankton from the harbour water, which generally returns to the ocean cleaner than when it came in.  Mangroves are an important component of the filter-feeder ecosystem, as discussed below.

Organised groups such as Experiencing Marine Reserves (EMR), Ecoquest Foundation, and Saltwater Eco, and independent school groups regularly take snorkellers to Horseshoe Island, where they can swim through and amongst the mangroves and into the harbour channel with water clarity up to 10 metres and more.  Such a high quality experience in the mangroves, with its huge educational value, is not achievable anywhere else in the Auckland Region.

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In my opinion mangroves in the Whangateau enhance the range of recreational experiences available, and provide unique opportunities to explore a very different habitat amongst the mangroves, by snorkeling, kayaking, or stand-up paddleboarding.

Some of the mangroves in the Whangateau are old enough and large enough to be able to actually snorkel right through individual trees.  Such a tree is an important feature on a snorkel trail near Horseshoe Island.  Inside the tree snorkelers can see the attached marine life such as oysters, barnacles, top shells and tiny mussels.  They may also see schools of juvenile fish such as yellow eye mullet and parore, and generally enjoy the amazing turquoise vista through the tree to the open water beyond.

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Large mangroves provide a hard substrate for attachment, otherwise lacking in the harbour sand or mudflat environment. Huge numbers of Pacific oysters and acorn barnacles live attached to the branches and sometimes the leaves of mature mangroves.  These animals supplement the filter-feeding activity of the usual burrowing shellfish like cockles, pipi and wedge shells, and thus contribute significantly to the water clarity and general ecological health of the Harbour.

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Mature mangroves also provide shelter and a feeding environment for large fish such as parore and grey mullet now frequently seen in the mangroves, and short-finned eels are a common sight in the mangroves if snorkeling there at night.

Mangroves in parts of the Harbour also provide some protection from wave erosion of the shore. This aspect of mangroves providing erosion protection will become more important as sea level inevitably rises.  We may eventually see people actually planting mangroves to help protect their properties.

Provision of organic matter to the food chain through dropping of leaves, and the carbon-sequestering values of mature mangroves are well known. Also in some parts of the Harbour the forests of mature mangroves support populations of banded rail.

Waikokopu Creek is the area of the Whangateau harbour south of the causeway (shown below to the right of the causeway). .  The area contains an extremely diverse mosaic of intertidal sand-flat habitats, with a quality unequalled in the Auckland Region.  The sand flats are firm and easy to walk on right to the southern end of the inlet, a feature not found in other Auckland estuaries.

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Excellent examples are present of salt marsh, smaller and some larger sparse mangroves, seagrass beds, open sand flats with small shellfish, tidal channel, sand stone reef with necklace weed bed, patches of Pacific oysters, and a unique bed of coralline turf balls akin to the rhodolith beds found in some places offshore.

The Waikokopu Creek is used by a wide variety of wading birds for feeding, including South Island pied and variable oystercatchers, bar-tailed godwits, NZ dotterel, white faced heron and pied stilts. The waters and channels are used by pied shags, Caspian terns and gulls.  The mangroves and salt marsh is home to banded rails and fernbirds.

The sandstone shelf with large quantities of necklace weed about 250 metres south of the causeway bridge is one of two important nursery sites in the Harbour for juvenile parore. According to Mark Morrison from NIWA, who did his Masters thesis on parore, these two reef systems and the Whangateau Harbour are the source of all the parore inhabiting the coast from Pakiri in the north to Kawau Island.

At a certain time of the year, usually in January or early February, large numbers of juvenile parore, barely 20mm long, occupy the mature mangroves in Waikokopu Creek when the tide is in, seeking shelter and feeding opportunities among the branches of the mangroves.   These are probably juveniles derived from the breeding area on the sandstone reef 250 metres south of the causeway.  These mangroves would appear to be important at a critical stage in the life of the parore, and may contribute significantly to the survival of juvenile parore in the Harbour.  Removal of these mangroves would be likely to have a significant adverse effect on juvenile parore.

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On the western side of the main Waikokopu Creek channel, many isolated mature mangroves occur amongst a large area of seagrass.  These mangroves are home to large numbers of adult parore when the tide is in.  They shelter in the trees and make feeding forays out on to the surrounding seagrass beds where they eat quantities of seagrass, leaving a “halo” of grazed seagrass around each mangrove tree.  The pale “halos” are clearly seen in aerial photos of the area.  These mangroves are an important habitat for mature parore and may support the main breeding stock in the harbour. Again, removal would be likely to have significant adverse effects.

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An existing Scientific Reserve (Omaha Taniko Wetlands scientific reserve) adjoins Waikokopu Creek on its eastern side. This Scientific reserve includes the kahikatea forest and seaward vegetation to the line of mean high water of spring tides.  The natural sequence is an internationally recognised fine example of a sere or natural succession which has been protected in the Scientific Reserve for good reason.

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The Whangateau HarbourCare Group is seeking to extend the Scientific Reserve down through the intertidal parts of the sequence to low water. The full sequence is dynamic in the long term from kahikatea forest, manuka and flax wetlands, oioi salt marsh, a few chenier ridges with appropriate vegetation (except invasive casuarina trees), mangroves, sand flats, seagrass to low tidal channel.  Mangroves are an important component of this succession.

By leaving the Waikokopu Creek area to its own devices we can learn what changes will come about through future impacts of sea level rise and climate change. As many other estuaries are manipulated, Waikokopu Creek provides a useful “control” for assessing those changes.  A Scientific Reserve leaves open future options for manipulations for scientific purposes, and is a very sensible option providing adequate protection for the rich mosaic of intertidal habitats of the Waikokopu Creek, yet retaining future flexibility.  Clearance of mangroves will disrupt the vegetation sequence and adversely impact on the scientific value of Waikokopu Creek.

Drivers and effects of mangrove expansion in Waikokopu Creek

In the Waikokopu Creek the very slow expansion of mangroves is not associated with increased sedimentation, as in northern parts of the Whangateau. The sediments in this arm of the harbour, after 45 years since the causeway was constructed, remain firm sands right to its southern extremity.

More fine sediment reaches the Waikokopu via the causeway bridge after heavy rain in the northern catchment and subsequent tidal movement, than comes from the catchment surrounding the Waikokopu Creek (confirmed by recent Auckland Council and Watercare Services studies).

In my opinion the main drivers of mangrove expansion in the Waikokopu Creek are:

  • shelter provided by the causeway,
  • climate warming providing more favourable conditions for mangroves,
  • an abundant supply of propagules from existing mature mangroves most of which cannot escape from the Waikokopu Creek because of the causeway and its narrow channel to the rest of the estuary.

Particularly in the Waikokopu Creek, very slow mangrove expansion is unlikely to be causing harm to amenity values, recreational opportunities or ecological values. Mangroves will not extend further down the shore than 0.4m above low water, so there will always be room for shellfish, worms and other life as food for birds and fish on sand flats and seagrass beds.  And there are abundant and more extensive open sand flats north of the causeway which the birds and fish already use.

Ineffectiveness of previous mangrove removal

In recent years there have been several incidents of removal or killing of mangroves in the Whangateau Harbour, mostly on the Point Wells side.  Near the Waimanu Reserve to the north several mature mangroves were killed by drilling a hole in the trunk and injecting a poison.  Two years later the trees were white skeletons with hundreds of new seedlings growing around the base

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South of the causeway at least three sessions of illegal cutting of mangroves occurred, totaling about two hectares.  These were documented and marked on a Google Earth image and colour coded by year (shown below) and provided to Auckland Regional Council following the latest incident.  Most incidents were out in front of the first few properties south of the causeway.  One event was adjacent to the causeway on the Omaha side, with several mangroves cut off about half a metre from the ground, which subsequently re-sprouted.

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The picture below shows an area cleared in 2010 and photographed on 3 January 2011.

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I revisited the site on 26 February 2015, approximately 4.5 years after these  mangroves were cut and the stumps are still prominent and showing no sign of decomposition.  They are surrounded by numerous Pacific oysters and a few mangrove seedlings, and some necklace weed plants.  Some areas have patches of greenish algal mats visible in the photo below.  There may be slightly sandier sediment than before the mangroves were cut but without proper study it would be hard to tell.  The areas are certainly not what could be called a sandy beach and are not pleasant to walk on.

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On an earlier visit I photographed one of the stumps which showed nearly 60 growth rings. I am not sure if these rings represent annual rings or not, but certainly it was a fairly old tree.

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Inshore of the areas where mangroves have been cleared there is considerable shore erosion of the soft peaty soils which underly the Point Wells mainland.  The response of some of the land owners has been to dump rock and concrete along the shore to attempt to prevent further erosion (example shown below).

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Much of the shore flats within 50 to 100 metres or so of the land edge has an exposure of subfossil tree trunks, roots and branches, which I understand is about 40,000 years old (example shown below).  This represents an old forest and its remains have been sheared off at a particular level then some finer peats deposited on top.  The remains on the flats form a firm substrate for oysters and necklace weed to attach to, and unless there is a considerable amount of sand deposited in this area it is unlikely to revert to a sandy shore.  Additional sand is unlikely to reach this area naturally due to the presence of the causeway.

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Mangrove Seedling removal trial south of the causeway

In October 2012 the Whangateau HarbourCare Group carried out a scientific trial, with Council approval, to determine the effectiveness and time/cost of removing by hand mangrove seedlings within the criteria defined by the Council rules at that time (up to 60 cm, no side branches). An area was chosen south of the causeway on the Omaha side.  The report is attached as Annexure 2.

A group of eight volunteers either pulled or snipped off mangrove seedlings in an area of approximately two hectares, bagged them and dragged them back to the causeway for proper disposal offsite. The rate of clearance was approximately one hectare per 10 people per hour.

Based on the time taken and the area cleared, it was calculated that to clear mangrove seedlings from the entire Waikokopu Creek using those methods would take approximately 500 to 700 volunteer hours annually to “hold the line” and prevent further mangrove spread. On this basis, in my opinion it is unlikely that areas where mangroves have been removed will be adequately maintained in the long-term.

A few weeks following this trial, a group from the OBC organized a second “seedling” removal session. I arrived on the scene half an hour after they had commenced.  They were cutting larger mangroves as well as seedlings and leaving the debris scattered over the clearing area.  I told them what they were doing was illegal and they stopped.  In my opinion, mangrove clearance should not be a permitted activity as if community groups are to be trusted with legitimate mangrove clearance, detailed instructions and appropriate supervision are required.

Omaha Beach Community Submission

The Omaha Beach Community submission seeks that the PAUP provide “every encouragement” assist in the “long term elimination” of mangroves. In my opinion, such an objective is entirely inappropriate, and demonstrates a lack of understanding of the significant and positive benefits of mangroves to the ecology of the Harbour.

I do not agree with the statements in the OBC submission that “…..mangroves present in this coastal ecosystem are not a “natural” part of that ecosystem but are there as a result of invasive spread from elsewhere in the region or country” and “…..mangroves are not a valuable part of the coastal ecosystem and have adverse, not positive, effects.”

The OBC submission does not demonstrate the need for mangrove clearance, especially in the Waikokopu Creek area which is clearly in extremely good ecological health.

Comments on Dr Lundquist’s evidence

Dr Lundquist outlines numerous examples where clearance of mangroves has failed to result in a satisfactory development of a clean sandy environment following clearance. She also outlines a few circumstances in which such clearance has led to successful restoration of a sandy shore.

Dr Lundquist noted in her evidence that “All but a handful of mangrove removal areas in a 40 site survey in 2013 – 2014 were easily distinguished from surrounding areas after dates as long as 13 years after removal. Intact mangrove stumps were still present at the few sites at which sediment characteristics trended toward sandflat characteristics.”  That observation is consistent with my own (shorter-term) observations at Waikokopu Creek.

In paragraph 7.12 Dr Lundquist suggests that “…..the physical and biological characteristics imply that this area [Waikokopu Creek] is likely to have only minor impacts from mangrove removal, if restricted to the procedures suggested in the PAUP.” I do not agree that the ecological effects of mangrove removal in Waikokopu Creek will be minor, for the reasons set out above.

Expert Conferencing Statement.

Dr Lundquist and Dr Dumbell in their expert statement (paragraph 8) indicate “…..continuing mangrove expansion and encroachment places the saltmarsh and seagrass habitats at risk.” Mangroves and seagrass are not mutually exclusive or incompatible.  Drs Lundquist and Dumbell do not appear to have noticed an area to the southwest (not visible from the causeway) where large mangroves and seagrass occur together and compatibly occupy many hectares.  Other areas in the south contain a continuous and healthy seagrass mat amongst small sparse mangroves.

Seagrass is also expanding in the Waikokopu Creek, and it is a moot point whether mangroves are encroaching on the sea grass or the other way around. In recent years small patches of seagrass have developed and are expanding immediately north of the causeway.  Seagrass is not “at risk” from mangrove expansion in the Whangateau Harbour.

Dr Lundquist and Dr Dumbell in their expert statement (paragraph 10) indicate “…..Evidence from two historic mangrove removal sites in the Whangateau Harbour indicate that sites from where mangroves are removed will show trends of returning to sandy intertidal flats characteristic of harbour flats without mangroves.” This statement is contrary to what I observed and photographed on 26 February 2015 at the unconsented clearance sites (see my paragraphs 39 and 42).  The caucusing statement records (site minutes) that Dr Lundquist and Dr Dumbell observed these sites from the causeway which is some 250 metres from the cleared sites.    This may explain the difference in our observations. At that distance, the sites may appear to be returning to sand, but as set out above I did not observe that trend viewing the sites from near-by.

Comments on proposed 1948 rule for Whangateau

Although the OBC may have satisfied the Auckland Council that the Whangateau Harbour was substantially free of mangroves as far back as 1948 (evidence of Robert Scott), implying that the mangrove-free status was “natural”, there is another explanation for the extent of distribution at that time.  Neither party may be aware of a 1901 photograph of Darroch’s boat yard in the lower Ashton Road creek.  The photo, taken from the headland above the Big Omaha wharf, shows a vessel under construction in the lower creek.  In the background is an open sand or mud flat (can’t tell which from the photo) with a few large old mangrove trees in the middle of the open flats.

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These mangroves have tall bare trunks with the canopy above potential grazing height of cattle, which commonly in the early days were allowed to graze on mangroves in the intertidal. These large isolated mangroves could be relict from earlier days when a lot more mangroves may have been growing on the flats, but a few taller ones were out of reach of the cattle.  The size of these trees in the 1901 photo suggest they would be at least 60 years old, probably older, and may have been part of a more extensive mangrove forest in the late 1800’s.There are some smaller mangroves to the left of the tall ones also visible in the photo.

There is also an 1887 survey map, part reproduced below, showing mangroves in three inlets in the northern Whangateau – Ashton Road inlet (called Young’s Creek on the old map), an inlet behind the Whangateau Domain now reclaimed, and Coxhead Creek.

Another photo, taken in 1901, shows Davey Darroch with his workmen at his yard at Whangateau. Mangroves can be seen growing in the background (Auckland Museum photo).[1]

[1] From Jade River, a history of the Mahurangi. R.H. Locker (2001).

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I have read a brief of evidence provided by Dr Mark Bellingham to the Environment Court hearing regarding proposed mangrove removal at Mangawhai (Bellingham, 2012). This suggests that grazing by stock, especially cattle, in northern estuaries was likely responsible for much of the open expanses of sand or mud seen in early photographs.  Mark Bellingham indicated that 5 to 10 years of grazing is sufficient to eliminate mangroves from tidal flats.  A copy of the relevant pages of Dr Bellingham’s evidence are attached as Annexure C.

In the Waikokopu Creek there are several old fencelines through present salt marsh and mangrove areas, a highly visible one through the salt marsh adjacent to the kahikatea forest and the causeway. These fences indicate that stock grazing was common practice at least on some of the tidal flats.

This evidence suggests that the natural state prior to extensive grazing may in fact have been mangrove forest rather than bare sand or mud flats as suggested by OBC and agreed through Council and OBC caucusing. So the 1948 rule may not represent a natural state to attempt to return to, but possibly a degraded state following extensive grazing of pre-existing mangroves.  The historic photographs, and the 1887 map, clearly indicate that the Whangateau Harbour was NOT historically “free of mangroves”.

Removal of mangroves back to the 1948 line would effectively allow removal of all mangroves from the Whangateau Harbour, which in my opinion would have significant adverse effects on marine ecology, ecosystem services, recreational and educational values, and scientific values of the harbour.


Morrisey, D., Beard, C., Morrison, M. Craggs, R., Lowe, M.  (2007)  The New Zealand Mangrove: a review of the current state of knowledge.  Auckland Regional Council Technical Report, No. 325, Auckland, New Zealand, 156 p.

Bellingham, M.  (2012)  Statement of Evidence of Dr Mark Bellingham on behalf of the Royal Forest and Bird Protection Society of New Zealand Incorporated.  ENV-2011-AKL-000110.  Appeal against Mangawhai Harbour Restoration Society Incorporated.

Annexure A

My undergraduate and graduate research concentrated on the ecology of the entrance to the Whangateau Harbour.  My Ph.D. studies at Whangateau were multidisciplinary and investigated the patterns of hydrology and tidal currents; detailed sediment analyses and distribution and an understanding of the dynamics of sediment movement and coastal processes; the distribution of benthic marine life and the associations or communities of animals, their occurrence in different parts of the study area, and their relationship to the hydrology and sediments in the area; a statistical approach to community composition and distribution; and an analysis of the distribution, abundance and condition of dead shells of animals from the point of view of a theoretical future fossil deposit from which we could understand the living communities represented.

My first job out of University was as biologist on a commercial eel farm in Auckland.  I carried out studies on the ecology and upstream movement of glass eels (early juveniles are transparent) in the lower Waikato River, where they were captured and transferred to large tanks and on-grown to various sizes for eventual export to Japanese eel farms.  My duties included disease and parasite investigation and control, and biological management of the eel stocks, as well as disease control in a sideline of the business importing and quarantining tropical freshwater aquarium fish for wholesale to retail shops.

For a few years in the late 1970’s I was employed part time by a biological consulting firm in Auckland, during which time I gained experience in many estuaries and coastal areas mainly in the Auckland Region.  This work included benthic ecology and studies of bird use of intertidal areas and high tide counts of waders on natural and artificial roosts in relation to impacts of proposed large infrastructure projects.

I established my own consultancy in the early 1970’s with clients in Government Departments, Local Government Authorities, and the private sector, and various NGO’s involved in environmental matters, in New Zealand and overseas.

In the mid 1970’s I established a long-term monitoring project at Tawharanui following trends in fish, crayfish and other marine life inside and outside what became the Tawharanui Marine Park which more recently morphed into the Tawharanui Marine Reserve.  This programme is on-going and has provided particularly valuable data on crayfish and habitat recovery in MPA’s.

Major projects of relevance to coastal processes and ecology I have been involved with over the years include sand extraction at Parengarenga, Mangawhai and Pakiri, and several locations in the Kaipara Harbour.  The latter has involved long-term studies since 1990.

I have also been involved with long-term studies at the Port of Tauranga, for their various channel deepening and widening, and spoil disposal operations offshore, since 1988, including Council and Environment Court hearings, the most recent being an Environment Court hearing in 2011..  Over the years these studies involved working closely with coastal processes experts including the late Professor Terry Healy and Dr Willem de Lange.  I also studied long-term changes in shellfish and sand-flat ecology within Tauranga Harbour.

I prepared and presented evidence in support of an appeal to the Environment Court by the Minister of Conservation against the then proposed Whangamata marina, and for another appeal by a local Tangata Whenua group.  Recently I presented evidence on the potential ecological impacts of the marina at Sandspit which is currently under construction.

I am familiar with the coastlines of northeastern Rodney District and have lived in the Warkworth area for approximately 21 years. Prior to living here I regularly visited the local coasts for recreation for many years.  I have had a strong interest in the Whangateau Harbour since the early 1960’s and currently live within 50 metres of its shore.

About ten years ago I helped establish a community shellfish monitoring programme with the Whangateau HarbourCare Group.  This programme is supported by the Auckland Council and the Hauraki Gulf Forum as part of their Community Shellfish Monitoring Programme, and the Whangateau monitoring is one of their longest-running programmes.

I also established on-going recreational and educational walking and snorkel tours to Horse Shoe Island and its mangroves, through both the Whangateau HarbourCare Group and Experiencing Marine Reserves, guiding many groups through the area over the years.  This area is now very popular as a snorkelling site as it is the best area in the Auckland Region to provide the right conditions for enjoyable snorkelling in mangroves.

I have been instrumental in the Whangateau HarbourCare Group’s move to establish a Scientific Reserve in the Waikokopu Creek arm of the Whangateau Harbour, south of the causeway to Omaha.  This would extend the existing Omaha Taniko Wetlands Scientific Reserve (the kahikatea forest) down to low water mark, effectively protecting over 90% of the southern arm of the Harbour and its ecologically valuable mosaic of pristine intertidal habitats recognized as the best example in the Auckland Region.

Annexure B

Mangrove Seedling Removal Trial at Whangateau Harbour

Roger Grace


A trial removal of mangrove seedlings was carried out at Whangateau Harbour on 12th October 2012.  An area of approximately two hectares was cleared in the southern arm of the Harbour (Waikokopu Creek) south of the causeway to Omaha and on the eastern side of the Harbour.  Details of the trial area and actual area cleared are shown in the maps below.  The rate of clearance appeared to be approximately one hectare per 10 people per hour.  To effectively clear, and maintain clear, the appropriate area of mangrove seedlings south of the causeway by these methods would take approximately 500 to 700 volunteer hours annually.


The primary reason for mangrove seedling removal in the southern Whangateau Harbour is to maintain the sandy flats in an open state as a foraging area for wading shore birds, particularly NZ dotterels, godwits, pied stilts and both variable and south island pied oystercatchers.  There is a major breeding colony of the threatened NZ dotterel on the northern end of Maungatawhiri (Omaha) Spit, and the intertidal flats of Whangateau are an important feeding area particularly while the birds are breeding, or when the godwits are preparing to fly back to Alaska.

The area of open sand flats suitable for the birds to feed on has been slowly decreasing since the Omaha causeway was built around 1970 and mangroves accelerated their spreading in the Harbour.  Further spread of mangroves could eventually have some impact on the birds feeding area.  This mangrove seedling removal trial is to determine the feasibility of “holding the line” of mangroves where they are now, and to investigate whether their further spread can be arrested by regular removal of seedlings.

The Auckland Regional Council instigated a major technical review of mangroves with results contained in a substantial report.   A summary of the major findings was published by the Auckland Regional Council around 2009 in a small booklet entitled “New Zealand’s Mangroves” (undated, ARC).  This includes discussion on the botanical status of mangroves, their important ecological role in estuaries, what controls their distribution, effects of recent expansion, and management issues.

The Auckland Regional Council, and now Auckland Council, developed and adopted a policy on mangroves, recognizing their important ecological roles but also their potential to affect wading bird feeding areas and areas of human amenity and recreational use.  The policy includes procedures to be followed in the event people seek to remove mangroves.  Generally the removal of mature mangroves requires a Resource Consent.  The removal of mangrove seedlings, however, can be carried out without a resource consent, subject to certain rules.

The rules for removal of mangrove seedlings are detailed in ARC Coastal Fact Sheet CE8 “Removing mangrove seedlings”, available from Auckland Council and on their website.  In particular I note the following:

  • A mangrove seedling is defined as no more than 60cm tall, has a single supple stem, and has no flowers or propagules (“seeds”).
  • Seedlings are to be pulled by hand or clipped off at ground level with hand clippers. No motorized equipment is to be used.
  • No seedlings to be removed from under mature mangroves.
  • Seedlings are to be disposed of away from the coast.
  • No vehicles to be used to recover seedlings (although recovery by boat at high tide may be an option).
  • No disturbance to salt marsh or seagrass.
  • No removal from areas where mangroves are helping protect the shore from erosion.
  • Auckland Council to be notified at least 3 days prior to a planned seedling removal effort by individuals or groups.

If groups or individuals are considering removing mangrove seedlings in their area, prior discussion with Council staff would be appreciated as the activity is prohibited in some specific areas.  Initial contact could be with Compliance Officer Ms. Micah Butt, email: who could pass you on to the appropriate Council staff if required.

Field Activities

On Friday 12th October, eight people from Whangateau HarbourCare Group, Omaha Beach Community, Point Wells Residents and Ratepayers, and Omaha Shorebirds Protection Trust, and others, met at the eastern end of the causeway on a lovely fine and calm morning.  Armed with gloves, clippers and bags we walked south along the firm sand flat for a kilometre or so to the target trial area, passing small groups of pied stilts and a white-faced heron on the way.  On the western side of the channel there was a flock of around 30 godwits feeding in the rich seagrass bed near low tide.

At the site we spread out across the shore, but clearly the main area of mangrove seedlings was amongst the existing mature mangroves, with increasing numbers of seedlings further up the shore.  With the objective being to “hold the line”, the most important seedlings to target were those near the outer edge of the band of mature mangroves and any visible beyond the edge out on the sand flats.  Thus we soon abandoned the very large numbers of seedlings well up the shore amongst the larger mangroves with their dense intervening cover of breathing roots, and concentrated our efforts in the more sparse areas of mature plants and the open sand flats.

Some of us used secateurs or small loppers to snip off the seedlings at ground level, while others pulled seedlings from the sand.  Seedlings were dropped into bags as we went.

After an hour or so our bags were getting full and we debated whether to drag them back to the causeway or stash them high to be retrieved by boat in a day or so.  We decided to drag the bags back to

the causeway which proved to be quite a heavy job and may have been the wrong decision.

We arranged to dump the nine bags of seedlings on a fire pile on a large property at Point Wells.

What We Learned

The job took a lot longer than anticipated.  Although the marked area we have cleared looks like two hectares on the plan, in fact the area we worked on was more like one hectare.  Basically using these methods the rate of clearance of the appropriate target areas looks like about one hectare per hour for ten people.

At this time of year the number of seedlings is probably at a minimum.  So September to end of December seems a good season to target, after any winter die-off, but before the influx of new seedlings next season.

Snipping the seedlings off at ground level is a lot quicker than pulling them out of the ground.  It also results in substantially less material to cart away as the bulky root system is left in the ground to rot.

Snipping causes much less disturbance to the muddy sand, as pulling out the root system leaves quite a hole which could be re-filled but adds significantly to the time required.  We have not determined how long it would take for the disturbance to disappear under the activities of crabs and wave action.

(We understand from Council literature that there is no chance of re-growth if the seedlings are snipped off at ground level.  On the other hand seedlings pulled up by the roots will readily put down new roots if they are simply left lying on the ground).

Dragging away the sacks of seedlings is a hard job.  This could be avoided by stashing the sacks in a group high on the shore, or in the branches of larger mangrove trees so they can be found easily and removed on a subsequent high tide by boat.

The Future

Consideration might be given to leaving snipped-off seedlings in place where they have been taken from amongst the mature mangroves.  Seedlings taken from open areas between sparse mangroves or from open sand flats should still be taken away, or perhaps could be spread amongst the nearby denser mangroves.

There would appear to be some justification for this approach from comments in the Council booklet “New Zealand’s Mangroves”.  “The study (Matapouri estuary) revealed that fresh mangrove material was present in sediment next to the mangroves, but not in a sandflat further down the estuary.  This suggests that distribution of fresh mangrove material is localized.  Decomposing mangrove litter appeared to be more widely distributed and available to a range of animals in the surrounding estuary via the food web.” [New Zealand’s Mangroves, page 6 “Food source”].

I see little ecological difference between natural leaf-fall amongst mature mangroves and snipped mangrove seedlings being left to rot amongst the mature mangroves.  Leaving snipped seedlings on open sandflats, however, would be contrary to the natural occurrence of freshly dropped mangrove leaves.

I believe this approach could significantly increase the areas that could be cleared satisfactorily given limited volunteer time.

If we were to limit the areas of clearance effort to areas where mature mangroves are currently sparse, and to open areas being actively colonized, and chose to abandon areas where large mature mangroves are dominant and the ground is covered with breathing roots, we are looking at approximately 50 to 70 hectares of area in the southern arm of the Whangateau Harbour south of the causeway.  To clear this area by the techniques adopted for this trial (1 ha/10 persons/ hr), that would take approximately 500 to 700 volunteer hours for one clearance.  To be effective at “holding the line” this amount of effort would be required on an annual basis.  It would thus appear to be worthwhile exploring more rapid methods of mangrove seedling removal.

Roger Grace

13 October 2012

 Our band of seven (plus the photographer) with our sacks of seedlings.

Our band of seven (plus the photographer) with our sacks of seedlings.

Proposed mangrove seedling removal trial area, approximately 10 hectares.

Proposed mangrove seedling removal trial area, approximately 10 hectares.

Area successfully cleared on 12 October 2012.

Area successfully cleared on 12 October 2012.

Annexure C




of the Resource Management Act 1991 (“the Act”)
IN THE MATTER of an appeal against a decision of the Northland Regional Council pursuant to section 120 of the Act
Section 274 parties


Extracted from Bellingham’s evidence:

1.1 The changing state of mangrove and saltmarsh habitats on Mangawhai Harbour and other harbours in Northland and northern Auckland, and the effects of stock grazing

Optimum mangrove conditions

1. Mangroves grow between mean high tide and 0.4m above low tide in sheltered intertidal waters in Northland (Morrisey et al 2007). In Northland harbours within this zone they may be absent if there are strong currents, excess sedimentation, pesticides or stock grazing affecting their growth. Over the past 30 years the health of the vegetated tidal communities in Northland has improved with the cessation of stock grazing. This has occurred progressively throughout Northland since the 1980s, and now stock grazing is a prohibited activity in the Northland coastal marine area. There are still some parts of Northland where landowners have yet to fence their stock out of the CMA and damage to mangrove and salt marshes still occurs. During the last 30 years the quality of the wildlife habitat in these harbour communities has also improved considerably.

Changes in mangrove growth at Mangawhai Harbour

2. The application proposed that the harbour was sandy in 1946, but there is no evidence for that in an aerial photograph at that time that shows a harbour mainly bare of mangroves, but not necessarily sandy in the upper harbour. In my assessment the 1946 photograph shows a severely grazed harbour that has lost most of its natural mangrove and saltmarsh cover.

3. I first came to Mangawhai in 1975. Some parts of the upper harbour and southern shores were being grazed at that time. In my view, active clearing prior to 1946 may not necessarily explain the difference between the 1880s range of mangroves (Bader para. 26, Wild para.23) and the 1946 photo (LaBonte Para. 5.1, Cureen para.6.6 and attachment P18), as I have found that elimination of mangroves can be quite rapid (5 – 10 years) in other harbours in Northland, with stock grazing, particularly by cattle. This often happens when a change of land ownership has resulted in unfenced and ungrazed shoreline areas being grazed.

4. At Mangawhai, shoreline grazing appears to have progressively reduced during the 1980s and 1990s as farming was replaced by coastal subdivisions. I have observed this land use pattern throughout Northland and the upper North Island. Urban land use (subdivisions), more enforcement of plan rules and awareness of coastal values have reduced stock grazing in estuaries. I consider that this has allowed the Mangawhai Harbour ecosystem to recover and restore the natural mangrove/saltmarsh cover. As Ms Wild’s draft evidence notes the shallowing of the harbour from the construction of the two causeways, has provided more habitat for mangroves. I note that Dr Bader’s evidence outlines the likely previous extent of mangroves in the harbour and this is supported by Ms Wild.

Changes in mangrove growth at Kaipara Harbour
5. I have documented these changes in the Kaipara Harbour. In 2008 I surveyed the shoreline of the southern Kaipara Harbour for the Auckland Regional Council, where I compared the changes in the state of estuarine vegetation from this field survey in 2008 to a previous field survey in 2002. It was the first study to quantify the spatial extent of the effects of livestock grazing and changes of grazing impact on New Zealand mangroves on the ground. Cattle and sheep grazing were previously recorded during an estuarine vegetation survey on the Kaipara Harbour in 2002 (Davis 200210). In 2008 I found that there was a considerable area of the coastal margins of the southern Kaipara Harbour still being grazed by farm livestock (cattle and sheep). The area and length of coastline accessible to livestock and being grazed has reduced slightly since the previous survey in 2002 as some land use had changed from farming to lifestyle blocks, where coastal fencing was required.

6. I have observed a number of places over the past two years throughout Northland and the Kaipara Harbour where there is still extensive damage from stock grazing. One example can still be seen at Stables Landing, at the Tauhoa River end of Witheford Road east of Wellsford on the Kaipara Harbour, where cattle grazing on an adjacent council recreation reserve have eliminated most of the mangroves (photos A & B below). On the adjacent property downstream (separated by a fence out into the mangroves), cattle and sheep have eliminated all of the small mangroves, regrowth and seedlings (photo C).

7 A Davis 2002 Kaipara Ecological District Ecological Survey. Draft 2. June 2002. Report prepared for Auckland Regional Council


  1. Very impressive

  2. Jo Richards says:

    An objective and substantial piece of work from an expert in this field

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