Coral decline or recovery?

Reefs that were heavily smothered by silt on 28 March are clearing, and stress to the corals seems to be over with little additional mortality. The coming rough and cool seas of the southeast monsoon should return conditions to normal.

This is a report of a field survey conducted on 11 April 2019 to the coral reefs at Waa/Tiwi. Sand harvesting started on 5 March 2019, was stopped on 27 March, and restarted on 4th April, at other locations. This is the second survey led by CORDIO East Africa, the first was conducted on 28 March 2019.  

This survey was run to assess the fate of coral communities most impacted by harvesting prior to 27 March. Block 4 was the most heavily harvested location, adjacent to sites a1 and a3 in the survey today, and sites a4 and a5 were added, where sand harvesting was done up to 3 weeks ago (mainly).

The last 2 weeks have experienced the change in monsoon winds from the calm season to the southeast monsoon (kusi). The wind has been SE consistently since about 1 April, though is not yet strong (in general, strong winds are good for the corals as this disperses sediment), but water temperatures have not yet declined (not good for corals as they continue to be stressed by high temperature and sediment combined).

In broad terms, we found the following:

  • ** Coral reef communities exposed to high silt stress from sand harvesting up to 27 March are showing an end to stress and mortality of corals, and reduction of silt on the reef surface.
  • ** Bleaching and mortality of corals is exacerbated by high temperatures due to seawater warming. This has compounded sediment stress in the last month. But this is about to end as temperatures will decrease within days to weeks.
  • ** Impact to hard corals from the level of harvesting done to 27 March resulted in low levels of mortality. But accumulation of silt in reef algae suggests this came close to a limit that could have driven much greater impact and loss of corals.
  • ** The declining silt in algae should be beneficial for fish and other herbivores, but this effect is difficult to evaluate.

It seems clear that a catastrophic impact on the coral reefs from sand harvesting was averted by the cessation of harvesting on 27 March. It may have happened after a further few days or weeks of sand harvesting, if continued in the same way.

So the question now is “can sand harvesting be done in ways that are not harmful to the reef?”

Clearly, the primarily goal must be to stop the delivery of fine silt to the reef, to zero if possible. Mechanisms to do this are under development, and with the right monitoring and operational procedures, it may be possible that sand harvesting can be done without impacting reef health. Only real trials and learning can test this, as theoretical studies and modelling will only indicate what MAY happen, not what WILL happen.

Having looked at this issue from many sides in the last weeks I do think this is possible, but requires hard and fast commitments from all those involved (government and contractors), eliciting confidence from stakeholders that their key interests and assets will not be damaged through sloppy or incomplete implementation of the full mitigation and management actions required.