This is the first of a series of coral monitoring reports we will send around to keep track of indicators of coral stress during the dredging works. The measurements are being taken by Paeniu Lopati and Filipo Makolo of the Fisheries Department using the Coral Watch methodology. The aim is to use corals as sensitive indicators of ecosystem health in the places being impacted by dredging. By reactively monitoring their bleaching, mucus production and the amount of sediments settling on them, we will be in a position to take action if there is a significant threat of loss.
A total of 80 corals of 4 species of Acropora are being monitored. This includes 2 branching and 2 plating species found growing on lagoon reefs in Funafuti.
The survey is being undertaken at 4 sites along Fogafale and Tegako, with coral measurements being recorded 3 times per week on Mondays, Wednesdays and Fridays. After each survey data are entered into a purpose-built database from which instantaneous graphs (see below) can be generated.
There are no absolute controls in this study because habitats change significantly once we are away from Fogafale and Tegako. Instead, we are relying on these 4 sites to act as partial controls through time. While dredging occurs at Site 1 North End, we would expect no dredging impacts at Sites 3 and 4 in the south. Once the dredge moves south, we would expect to see the opposite response. This is not ideal, it would be preferable to have absolute controls for this monitoring if suitable reef areas existed.
We propose the trigger for action should bleaching threaten the condition of ecosystems should include the following criteria:
- Average of 50%+ of corals showing bleaching to levels 1-5 (6 is not bleached)
- This occurs for 3 consecutive surveys
- Occurs differentially at sites close to dredge and not at sites remote from the dredge on those days (defined by non-overlap of standard error bars).
Dredging began on 29th April 2015. All dredging to date has been concentrated in the area between Site 1 and Site 2 near the northern end of Tegako islet.
The graph below was generated on 21st May after the first month of monitoring. The graph shows bleaching through time separately for each site. The bars are a breakdown for each of the average bleaching levels for the corals, with a perfect coral 100% at level 6 and a totally bleached coral 100% at level 1. The colours used in the chart mirror the scale for D coloured corals in the Coral Watch Chart above.
The data show recent bleaching occurring at all 4 sites, but especially at Sites 3 and 4 which are the most remote from the dredging work. In comparison, the graphs from 28-29 April at all sites are about normal with around 1% bleaching at level 1, and the rest of the coral at unbleached level 6 in the Coral Watch chart (healthy condition).
The corals are on watch, but the high levels of bleaching seen at Site 3 Catalina Patch, a current control, are not likely to be related to dredging. The trigger criteria have not been exceeded at Sites 1 and 2 near the dredging. The high levels of bleaching at Sites 3 and 4 are likely to be due to other factors.
Graph: Dredging is now occurring near the North end of Tegako. Bleaching has been increasing since the beginning of May and is currently least at Sites 1-2 near the dredging and highest at Catalina and South End (Sites 3 and 4) which are the temporal controls. This suggests that bleaching is unrelated to the dredging works and is the result of some other process.