I mentioned in a previous blog that Indian Ocean coral was severely affected by bleaching and mortality in the 1997 El Nino event, which caused most of the coral in Watamu to die. Fifteen years on many areas in the Indian Ocean have recovered to some degree, but there is a large variation in coral regrowth. Sadly Watamu hasn’t recovered as fast as one would hope and most papers report that it has the slowest regrowth in Kenya. However in the short time I’ve been here I’ve noticed massive variation, even across the park in coral regrowth, with some areas having much more coral cover than others. Not only this but it seems that the areas with good cover are not the established study sites used for published papers to date. Eventually these areas need to be properly surveyed and documented, but before that I decided to start to trying to pull apart why there is a difference.
Many coral species spawn around November to March, sending small larvae into the water column, which eventually settle and grow into new colonies. Success of new colonies is based on the survival of larvae in the water column, currents to carry them to a location and finding suitable substrate, known as pre-settlement mortality rate. After that the coral needs to establishing itself, out-compete other organisms and grow; post-settlement mortality rate. Quite different processes are know to control pre and post-settlement mortality, so in order to understand variation in the park it is first important to know how which kind of mortality is most affecting coral recruits.
In order to do this, I have put settlement plates down. These are pieces of equipment onto which corals can grow. If there is large variation in the number coral recruits on plates when I remove them in March, perhaps this indicates pre-settlement mortality is controlling the flow of larvae to certain locations. If there is little variation, this could suggest that larvae are arriving in all locations, but it is post-settlement mortality in their local environment which either allows or prevents coral growth.
I decided to make plates just before Christmas and it was a mammoth task of designing, testing and then manufacturing many of these small plates to put around the park. I came up with a design based on advice from Tim McClanahan and Austin Humpheries from Wildlife Conservation Society in Mombasa, who also donated the square plates cut out of dead coral head and a big thanks is owed to them. The coral plate was tied to a plastic meshing cage to prevent fish herbivory interfering with settlement variation. This was then sunk into a cement mould, which when solidified, would provide the ballast to keep the plates still on the reef.
The first prototype sat happily on the reef for two days with no issues, so the race was on to make 36 plates before my parents arrived for Christmas! Seemingly no plastic meshing is imported to Kenya, but I discovered some bins in the local plastic market in Timboni, which had just the right kind of holes, and I set about trying to buy enough of these to make the plates. The shop keeper was bemused at why I was so adamant to buy that one kind of bin and why I needed 25 of them! With all the equipment bought I roped in several volunteers and even guests staying at Mwamba at the time, and got to work cutting up the bins and attaching the coral plates. Finally the night before setting them out we sunk them in numerous moulds.
Finally one day before Mum and Dad got here we sank all 36 onto various places on the reef. Success! The plates are all still where I left them just over a month ago and are being settled by a range of organisms. Stayed tuned for more results coming from this research strand, which hopefully will help pull apart one of the most poorly known aspects of reef ecology; how do baby corals grow?