Mangrove Seedling Removal Trial at Whangateau Harbour

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

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

Summary

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.

Background

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:

  1. A mangrove seedling is defined as no more than 60cm tall, has a single supple stem, and has no flowers or propagules (“seeds”).
  2. Seedlings are to be pulled by hand or clipped off at ground level with hand clippers.  No motorized equipment is to be used.
  3. No seedlings to be removed from under mature mangroves.
  4. Seedlings are to be disposed of away from the coast.
  5. No vehicles to be used to recover seedlings (although recovery by boat at high tide may be an option).
  6. No disturbance to salt marsh or seagrass.
  7. No removal from areas where mangroves are helping protect the shore from erosion.
  8. 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: Micah.Butt@aucklandcouncil.govt.nz who could pass you on to the appropriate Council staff if required.

Proposed mangrove seedling removal trial area, approximately 10 hectares

Proposed mangrove seedling removal trial area, approximately 10 hectares

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.

Area successfully cleared on 12 October 2012

Area successfully cleared on 12 October 2012

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.

Comments

  1. Well done all!

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

%d bloggers like this: