MANGROVE REHABILITATION IN A FORMER AQUACULTURE POND: September 2013

Bang Lang Da Village, Krabi Estuary, Thailand

DW with Jim Enright of MAP and three little Rhizophora mucronata in the pond 2009

Same view, same three trees 2013

Mid-dig in an abandoned shrimp pond, I would not normally have answered the phone. Number blocked. As it turned out, it was a friend of mine from the UK wishing me a happy birthday. I had completely forgotten. Kind of her to think of me, and we chatted for a while about the stresses of a London existence and running her business, mad traffic, the price of shoes and the flattening UK weather.

‘You're really living the life!’ she concluded.

In order to avoid my phone being filled with rain, I'd been bent double and twisted over to shelter it. The rain was stinging me so hard I used a shovel as an umbrella. The wind had picked up making me shiver involuntarily as I was sodden from sweat and rain. Mercifully, that day's two hour digging session was being drawn to a close by the evening call to prayer from Bang Lang Da's mosque and the gloaming. I was shattered - hands, back and shoulders aching - and still had 26k to scooter home in the hard rain, along the side of a two-lane intercity speedway. Living the life, indeed.

A spot of rain. 2009

In 2008 NGO Wetlands International - Thailand (WI-T) had identified this former shrimp pond for a demonstration mangrove restoration project. In Thailand, land tenure is the most difficult issue, before social challenges and technical details. So it took nine months to find the owner and get an MoU signed. Only then could the team of Jim and Ning Enright from Mangrove Action Project, K' Donnapat (Dos) Tamornsuwan and I (WI-T) talk with the local people about the history of the 0.7ha pond and what they wanted from it.

Meeting the community

The area had previously been mangrove but around 2001 had been 'converted', as the euphemism goes, by an ‘outsider’ into a pond for rearing shrimp. However, as is common with much aquaculture in Southeast Asia, soon production dropped off due to a combination of limited technical ability, high stocking density, poor water quality and shrimp disease. Unable to pay the shrimp feed bill, the owner lost control of the pond, and it lay idle for a few years.

When we first saw the pond, the action of rain, tides and burrowing crabs had eroded the mud around the boarded-up sluice gate and the tide was flushing the pond well. This was fortunate as reconnection with the outside hydrology is normally the obligatory first step for pond restoration. Flushing removes the toxic residue of uneaten feed, dead shrimp, chemicals, antibiotics and acid-sulphate soils that remain after shrimp farming has been abandoned. Sea water or brackish water rebalances the pH and carries in with it useful micro-biota and mangrove propagules. Despite this the mangroves were not regenerating. With available propagules and appropriate conditions mangroves should be able to naturally regenerate and colonise new areas.

Mapping and measuring the pond's spot heights with an auto-level confirmed our suspicion that the lack of regeneration was due to the soil being too low relative to sea-level – effectively mudflat. At low water small pools of water remained and the soil was constantly waterlogged.

Setting up for measuring spot heights across the pond, 2009

Following the natural channels that were still visible, I slowly started to improve the drainage by deepening and widening these channels, and used the spoil to form hillocks, similar to mud-lobster mounds, which formed areas of higher elevation suitable for mangroves. The hills of mud (here with a very high clay content) stabilised fairly quickly, after the water had been squeezed out, and appeared to lose only 1cm a month from subsidence and erosion.

The hillocks provided the opportunity to test lots of ideas, all of which villagers could do on their own and at very low cost. This included leaving some of the hillocks blank, dibbling (direct insertion) all manner of seeds and propagules into other mounds, transplanting mangroves I had been growing at home and testing the use of damaged Rhizophora propagules with their apical buds broken to see if they would survive (they did).

Dibbling straight lines only to reveal damage / missing propagules

Short sections of the mangrove associate herb Sesuvium were dibbled into one hillock and a pond wall and these also started growing happily.

Testing and learning was not confined to what we could do with plants on hillocks, but also the actual implementation of the work within a budget any village could produce. From the very real problem of appropriate footwear, to best channel shapes, which shovel would last more than four hours (we tested five) and to how to label individual plants without damaging them in these harshest of conditions.

Deepening and widening channels sounds perfunctory. What it actually meant was cutting dense heavy clay blocks like an Irish peat cutter and shovelling them up high enough to form a mound of the appropriate height, far enough away so that the mound would not fall back into the channel or collapse the edge. In order to make explanation of all this testing easier to bemused villagers and visitors, signs were inserted into the hills, channels and plots with an identifying letter. At mid-tide, with water covering the pond floor but not the hillocks, the site took on the appearance of a golf driving range.

Seven control plots were added at various heights, though some of the village children needed more than one discussion to understand their function.

Singing in our control plots

Unfortunately, after a year, the control plots' metal rods proved too tempting for some miscreant. Monitoring the control plots showed that during 2009-2010 there was no successful natural regeneration. We also tested dibbling directly into the pond floor in a dense 3x3m block. To the surprise of both the local conservation group leader, Bang (Mr.) Don and the team, these clumps of 50 R. apiculata propagules survived and thrived, whereas singletons which drifted in on their own elsewhere in the pond almost always failed.

A constant problem on the site was the amount of debris. Floating debris can collide with and physically damage young mangrove plants. Returning to the site this month (Sept. 2013) we were delighted to see how much the dibbled mangroves had grown on the hills and how many volunteers were now thriving in places where they had repeated failed. This mangrove growth was now effectively trapping the floating debris, without being damaged by it. Also encouraging was the continued erosion of the pond walls at the opposite end to the sluice gate. We were prohibited from making another breach of the walls. Nature seems to be doing it for us.

2013

Same view 2009

Sadly what did not survive was Wetlands International - Thailand due to serious financial management issues. But the pond still lives and the local people told us on our last visit that netting the sluice gate at high water allows them to collect an amazing amount of fish and other creatures from the pond as the tide runs out.
Thank you to the people of Bang Lang Da for their tolerance, to APFED for the funding and to K’ Ning, Jim and K’ Dos for their support and good humour.