28 March – 1st survey, sand harvest impacts


This is a summary of findings from SCUBA diving surveys carried out on 28 March 2019, on coral reefs adjacent to intensive sand-harvesting from 8-27 March off the Waa and Tiwi coral reefs, Kwale County, Kenya (see map below).

Key Findings

  • •  Coral reef communities at Waa/Tiwi are vibrant with a high diversity of coral genera and species, soft corals and other invertebrates.
  • •  The impact of sand harvesting at Block 4, through excess deposition of fine silt on reef surfaces, is extremely high on algae/rock surfaces, with likely impacts on the algae, herbivores, and small invertebrates.
  • •  Impact to hard corals has been low-to-moderate for adults, though early signs of stress are evident, and coral recruits may be strongly impacted.

Recommendations

  • •  Harvesting at Block 4 should be stopped for the next month until the southeast monsoon starts (cooler, rougher conditions).
  • •  To proceed with harvesting at other blocks, there must be stringent monitoring and associated mechanisms for mitigating impacts, initially from precautionary principles. These to be superseded as quickly as possible by more evidence-based and informed limits and thresholds in a revized Environmental Monitoring and Management Plan (EMMP).
  • •  Reducing the intensity of harvesting at individual locations is essential, through multiple mechanisms.

Cautions

  • •  This survey is highly preliminary, restricted in scale (3 sites) and scope (visual observation of corals and the benthos at a single point in time).
  • •  The next few weeks of maximal seawater temperatures, calm wind, high risk of coral bleaching and high microbial activity may strongly exacerbate stress from silt and magnify impacts reported here.
  • •  Compliance to a new improved EMMP will be critical to assure impacts to date are not amplified, and future impacts reduced below current levels .

The findings of the report are summarized below, or click on the icon to see the full report.

Map of the sand harvesting area, showing Blocks 3–6. Blocks 3, 4 and 5 have been the focus of harvesting to date, the duration of harvest (dates and number of harvest tracks in red text) is shown on the right. Survey sites are shown in black, the letters A-C are used in the report text. The lower end of the map is at the Mwachema river between Tiwi and Diani.


Evidence of sediment from sand harvesting

Particularly at sites A and B there was very thick layer of fine silty mud on algae and rock surfaces, from about 5 m (below datum) to the reef base at 18 m. Shallower than 5 m it appears wave energy/surge is sufficient to prevent settling of the silt. The silt is also evident in the layer of water adjacent to the bottom, appearing like a ‘smog’- blanket over the bottom, and a turbid plume coming off the reef as low tide was approached. Exposure was observedly higher at A, then at B, and significantly less at C. See plates on next page.

The most severe impacts/evidence

Sites A and B had a very heavy deposition of white silt on all algal and rock surfaces, only coral and other invertebrates’ surfaces being clear. These two videos show the degree of fine silt trapped in the algal turf on the rock surfaces.

It is highly likely that small/juvenile corals not easily visible to the naked eye are heavily stressed or dying under the silt layers, as well as other small invertebrates and organisms (< 5-10 mm size) living on the bottom.

The heavy silt load in all algae surfaces could have a significant negative effect on herbivorous fish and other consumers, making the algae inedible. This, together with disturbance during sand harvesting, may result in avoidance by fish of the affected area. The sediment plumes to the reef have clearly been high to deposit this much silt, so during the plumes, it is likely that fish and other mobile organisms’ behaviour are also significantly affected, as a result of which there may considerable impacts to fishing activities in the area.

Site C (in block 5) showed much less silt, reflecting a much lower degree of exposure to sand harvesting to date. While in the center of the large patches in Block 5, the amount of harvest (45,000 m3) is less than one eight of the harvest at Block 4 (350,000 m3).

Other impacts

Coral mortality was very in-frequent (small patches on about 10 colonies observed at A (mostly) and B), and stress from the silt on adult corals (pale/bleached tissue low incidence, evidence of disease zero) was apparent on some corals, but not severe. Mucus sheaths on Porites corals and some soft corals were frequent; this is a regular stress response – at high frequency, but not indicative of dangerous stress. In A and B some corals had silt pockets on them, pale/bleached tissue under these pockets, and some associated mortality.

It is clear that the intensity stress at block 5 while high, has not resulted in significant coral mortality to date. Silt layers were much less at site 2 (site with lower exposure), but nevertheless present and coating turf and fleshy algae, but evidence of stress to corals was lower.

 

FOR MORE DETAILS SEE THE REPORT ABOVE

 

BACK TO MAIN PAGE