Mangroves in NZ – Misunderstandings and Management 1
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Dr Sharon De Luca Boffa Miskell Ltd, Tauranga, New Zealand; sharon.deluca@boffamiskell.co.nz
Keywords: mangrove, policy, ecology, biodiversity, amenity. 1. Introduction The values of mangroves in New Zealand are often misrepresented and misunderstood which has led, in some cases, to mismanagement and unanticipated adverse outcomes. This paper seeks to explore some of the values and perceptions of mangroves in New Zealand, discuss current and historic management practices and consider the changing policy/planning environment around mangroves. Mangroves in New Zealand comprise a single native species (Avicennia marina var. australasica) that has been present for at least 19 million years [10]. Mangroves occur within many sheltered intertidal habitats north of their southernmost natural range of Ohiwa Harbour on the east coast and Kawhia Harbour on the west coast. New Zealand’s mangrove stands and associated biological communities contribute to coastal biodiversity, ecosystem health and services. Mangrove stands provide important habitat for native birds, fish and invertebrates. The natural coastal process of infilling of estuaries and harbours can provide suitable habitat for the establishment and expansion of mangroves. The shape and size of estuaries and harbours, combined with the interaction of stream and river processes, tidal exchange and waves, determines the volume of sediment that can accumulate. The rate of infilling, and subsequent increase in area of mangrove forest, has in many cases been accelerated by land use practices that increase the runoff of sediment from the land to the waterways. Expansion of mangrove forest area is purported to be unnatural. Concerns regarding loss of open water views, habitat diversity, recreational area, and boating access are often raised regarding mangrove expansion in New Zealand. This concern has led to a commonly held view that mangroves are a pest or weed, are of low value and have no place in New Zealand’s harbours and estuaries. Pressure has been, and continues to be, placed on territorial and regional authorities by community groups to remove areas of mangrove forest with the objective of re-creating other coastal habitat types and “improving” amenity values and access to the Coastal Marine Area (CMA). Personal interest goals such as open water views have
tainted the perception of the values of mangroves [8]. Northland Regional Council (NRC), Auckland Council (AC), and Bay of Plenty Regional Council (BOPRC) have policies and plans specifically addressing the management of mangroves. Furthermore, several large-scale mangrove removal proposals have been heard by the Environment Court [9].
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Ecological Values and perceptions of mangroves Public perceptions of mangroves are often polarised, with a strong and vocal contingent believing mangroves are unwanted nuisance and their spread is unreasonable. Others, however, understand the ecological value of mangroves, the role they play in coastal protection and the importance of protecting them. 2.1 Ecological Values Mangroves form an important part of coastal vegetation sequences i.e. seagrass-mangrovesaltmarsh-terrestrial coastal vegetation and estuarine vegetation mosaics within harbours and estuaries. Mangroves provide three-dimensional habitat for a large range of native fish, birds and invertebrates. A range of native fish use mangrove stands at high tide. Those species that use mangrove habitat more than other estuarine or marine habitats include short-fin eel, grey mullet and flounder. A large scale study of fish communities within mangrove stands in a sheltered estuary in the Waitemata Harbour revealed a high abundance of yellow eye mullet and estuarine triplefin, plus flounder, short-fin eel and parore [5]. Pilchards and anchovy are also common within mangrove habitats [1]. A large number of native terrestrial and coastal bird species utilise mangrove stands, including reef heron, white-faced heron, pukeko, New Zealand kingfisher, bittern, white heron, grey warbler, spotless crake, silvereye, fantail, shining cuckoo, royal spoonbill, pied shag, little black shag and banded rail [1, 3, 11, 13]. Reef heron, white-faced heron and pukeko forage in tidal channels within mangrove stands and roost in mangroves [10, 11]. New Zealand kingfisher use mangrove stands for roosting, foraging and breeding habitat and banded rail (an At Risk species) forage extensively
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within mangrove stands [10, 4]. The regular tidal inundation of mangrove stands is likely to provide wading birds that utilise mangrove habitat for roosting and foraging with some protection from predation by mustelids and cats. Many native birds and fish that use mangrove stands feed upon the benthic invertebrates that inhabit the benthos. A large range of invertebrate taxa occur within mangrove stands including numerous species of crab, snail, bivalve, gastropod, worms and crustacea such as snapping shrimp [7, 5, 6]. Mangrove trees themselves provide a substrate for sessile invertebrates such as black mussels, barnacles, rock oysters and Pacific oysters [7]. In addition, the larvae of an endemic moth (Planotortrix avicenniae) are found only on mangroves [2]. Mangroves also play an important role in natural coastal processes such as shoreline protection and trapping of sediment.
De Luca, S Mangrove Management
New Zealand. There is, however, evidence that identifies significant adverse ecological effects associated with mangrove removal [9]. Whilst there may be non-ecological reasons for the removal of mangroves, the ecological benefits often put forward in support of mangrove removal are not supported by robust data or published scientific literature. A recent Environment Court decision refers to the “public dislike” of mangroves and how previous mangrove removal consent approvals [in the Bay of Plenty] are likely to have been granted due a “lack of opposition and desire to support estuary care groups”. The judge concluded in his decision, from the expert evidence put forward, that mangroves have ecological values and provide valuable ecological habitat. The decision also clearly states that removal of mangroves has no ecological benefit, further stating that a change from a habitat previously not inhabited by mangroves to one inhabited by mangroves is merely habitat change and one habitat is not better or worse than the other. The actual benefits of mangrove removal may be non-ecological, e.g. amenity, visual, cultural or recreational.
3. Management practices and outcomes Removal (consented and illegal) of mangroves has occurred in the Northland, Auckland, Bay of Plenty and Waikato Regions. In most cases, large scale removals have not achieved the desired outcome.
Figure 1: Mangrove forest in the Whangateau Harbour (photograph taken by Dr Roger Grace)
2.2 Perceptions Councils are tasked with finding a balance between the objectives of many community groups seeking mangrove removal and the environmental effects of removals. The arguments put forward by mangrove removal proponents are numerous, including visual, amenity, recreational, cultural and ecological. Mangrove removal proponents often state that the ecological benefits from mangrove removal are the maintenance of benthic habitat diversity and ensuring that there is sufficient open mudflat/sandflat habitat for wading and coastal bird foraging. In my experience, neither of these proposed ecological justifications have scientific rigor. There is no published literature that clearly links ecological enhancement with mangrove removal in
Councils themselves are often the resource consent applicant on behalf of community and estuary care groups. Those applications in particular need to be carefully and robustly assessed by independent experts (not only independent hearings commissioners) in order to remove the suggestion of conflicts of interest. To date, the ecological science supporting large scale mangrove removal applications has, in some situations, been insufficiently weak and not adequately independently reviewed. Consented large-scale mangrove removal has occurred in several locations within the Auckland Region, including Pahurehure Inlet and Waiuku. In addition, numerous small scale consents for mangrove removal have been granted, primarily relating to infrastructure and access for boats. Consented mangrove removal has been occurring in the Bay of Plenty for nearly ten years. Consent for mechanical removal of mangroves over 92ha was approved in 2009. This approach caused significant adverse ecological effects to estuaries, primarily due to physical disturbance from heavy
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machinery and to anoxic conditions arising from mangrove mulch smothering the benthos [9]. Removals arising from many previously approved applications have caused significant acute and chronic adverse ecological effects including mass mortality of benthic invertebrates, anoxic sediment, declines in water quality, and loss of foraging habitat [9]. These adverse effects were not identified in the original resource consent applications. Apparently unforeseen significant adverse effects from mangrove removal activities continue to occur. For example, in an estuary in Tauranga in March 2015, where a machine become stuck in benthic sediment, inundated at high tide, and had to be dragged out using a crane. Removal of the machinery is likely have caused significant disturbance and damage to benthos and coastal vegetation (Figure 2). The potential adverse effects of this mangrove removal work are yet to be quantified. Machinery tracks and mangrove vegetation debris are often evident in estuaries for many years after large scale removal activities (Figure 3).
Figure 2: Te Puna Estuary, Tauranga, March 2015.
De Luca, S Mangrove Management
4. Policy and planning environment Resource consent should be sought for any proposed mangrove removal and a site-specific, robust ecological assessment should be provided as part of any such application. This approach is supported by Morrisey et al. (2007) who concluded that evaluation of mangroves should be undertaken on a site-by-site basis in order to understand the processes and effects occurring, whether mangrove populations are stable or dynamic, site specific physical and ecological characteristics, geomorphology, climate, sediment input, nutrient status and hydrodynamics. Mangrove management provisions should be revised to recognise the important ecological values of mangrove forests, including that mangroves are part of a wider vegetation sequence, the lack of ecological benefits from removal, and the significant adverse effects that may continue to occur due to the removal and maintenance methodology used. There is an opportunity to influence perceptions of mangroves through education (of the public and learners at primary, secondary and tertiary providers) and through recognition of ecological values in National and Regional policy and planning documents. It is important that the ecological benefit from their removal is understood at all levels within Government and Regional Councils in order to ensure that future consent applications are appropriately considered and in a scientific manner without prejudgement and bias, and that plans that provide for management of mangroves incorporate an appropriate degree of protection and precaution. There will be challenges in changing existing ill-will towards mangroves. However, in order to breakdown misconceptions around mangroves, information can be shared widely through media brochures, public open days within estuaries boardwalks, information panels, and presentations to interest groups etc. Northland Regional Council currently have a specific policy in their plan re management of mangroves that they have clarified further in a 2010 plan change. The plan states circumstances in which mangrove removal may be needed (primarily around access to structures) and NRC have applied a controlled or restricted discretionary activity status to most removals. Waikato Regional Council do not currently have specific provisions in their plan for mangrove removal, although consents have been granted for large scale removal of mangroves in the Whangamata Harbour and Wharekawa Harbour.
Figure 3: Welcome Bay, Tauranga, March 2015.
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15 - 18 September 2015, Pullman Hotel, Auckland
Mangrove Management
There is currently an opportunity for Auckland Council to develop ecologically sound mangrove management provisions in their Proposed Auckland Unitary Plan (PAUP). Similarly, the Bay of Plenty Regional Council have an opportunity to do the same in their Proposed Coastal Environment Plan (PCEP). Currently, the draft provisions in both of these plans have rules that are relatively permissive for removal of mangroves and seedlings.
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Provisions in the PAUP currently allow for mangrove removal to occur as a permitted activity back to an arbitrary date (i.e. removal permitted back to the extent of mangrove cover in 1996), subject to satisfying certain criteria and does not allow for site specific conditions to be assessed nor assessment of ecological effects.
4. Botha, A., 2009. Foraging distances and habitat preferences of banded rail in the Ohiwa Harbour. Bay of Plenty Regional Council, Environmental Publication 2010/06, Whakatane.
BOPRC’s PCEP currently allows for the removal of mangrove seedlings, small scale removal of adult mangroves under certain criteria and ongoing clearance of seedlings and adult mangroves from previous cleared areas as a permitted activity.
6. De Luca, 2013, State of the Receiving Environment Summary of Survey Data Collected January-February 2013, report prepared for Tauranga City Council.
Submissions have been heard both in support and against the provisions by both Councils. It is hoped that the final provisions for mangrove management in Auckland’s PAUP and the Bay of Plenty’s PCEP will take into account the clear evidence showing that there are negligible ecological benefits for mangrove removal and that the signs of ecological damage experienced to date from removal activities must be avoided. Regulatory authorities must ensure that applications for mangrove removal are scientifically supported and reviewed by independent experts. Proposals must contain provisions to ensure that ecological damage from removal activities does not continue to occur. With such requirements, it is envisaged that in the future mangroves will become widely understood as an important component of our coastal ecosystem and misconceptions will dissipate. It is hoped that attitudes towards mangroves will change over time in the same way that has occurred with wetlands. In past decades, wetlands were widely considered to be wasteland swamps of little value, but are now valued for their significant contribution to indigenous biodiversity in New Zealand. Councils responsible for the CMA need to lead the change through their responsibilities to protect and enhance biodiversity and ecosystem services.
References
1. Anon, undated. New Zealand’s Mangroves. Former Auckland Regional Council. 2. Auckland Regional Council, undated. New Zealand’s mangroves. Auckland Regional Council publication. 3. Beauchamp, A.J., 2012. Statement of Evidence prepared for Environment Court hearing, Mangawhai Harbour Restoration Society Incorporated vs Northland Regional Council.
5. De Luca, S., 2012. Baseline Marine Ecological Monitoring (Marine Reserves Act), Waterview Connection Causeway Project, Report prepared for New Zealand Transport Agency.
7. Green, M., Ellis, J., Schwarz, A., Lind, D., Bluck, B., 2003. For and against mangrove control. NIWA Information Series No. 31. 8. Harty, C., 2009. Mangrove planning and management in New Zealand and South East Australia – A reflection on approaches. Ocean and Coastal Management, 52, 278-286. 9. Lundquist, C., Hailes, S., Cartner, K., Carter, K., Gibbs, M., 2012. Physical and biological impacts associated with mangrove removals using in situ mechanical mulching in Tauranga Harbour. NIWA Technical Report 137, NIWA, Wellington 10. Morrisey et al., 2007. The New Zealand mangrove: review of the current state of knowledge. Auckland Council Technical Publication ARCTP325, Auckland Council. 11. Rayner, M., 2011. Whangamata Harbour Mangrove Removal Assessment: Effects on Avifauna. NIWA Client Report AKL2010-038. 12. Robertson, H.A., Dowding, J.E., Elliott, G.P., Hitchmough, R.A., Miskelly, C.M., O’Donnell, C.F.J., Powlesland, R.G., Sagar, P.M., Scofield, P., Taylor, G.A., 2013. Conservation status of New Zealand birds, 2012. New Zealand Threat Classification Series 4, Department of Conservation, Wellington. 13. Wildland Consultants, 2011. Banded rail survey of Whangamata Harbour, January 2011. Wildland Consultants Ltd, Contract Report No. 2579. Prepared for Waikato Regional Council.