[Coral-List] East African Connectivity
Hogan D
hoganh at uwindsor.ca
Sun Jul 9 10:48:45 EDT 2006
Dear Richard and listers,
I have a few comments about your post regarding East African
connectivity, coral dispersal and the placement of marine reserves.
At first thought it seems like a reasonable assumption that coral
planulae will be carried in currents of up to 3 m/s (The East African
Coastal Current) for a minimum of 4 hours, and simple math lead will
lead to a linear distance of 40 km traveled by a passive particle. I
think these are assumptions that are made often, in fact this is the
same dilemma that reef fish biologists deal with as well, when
studying connectivity. However, if one wants a reasonable estimate of
dispersal distance, there are a few faults with the above
assumptions.
First of all is the issue of scale. The East African Coastal Current
I assume is the main offshore current in the region. However,
dispersal of coral planulae will be most affected by near-field
currents, currents that are in-shore, the speed and direction of which
will be determined by bathymetry and topographic complexity of the
reef (see Black et al, 1990 Coral Reefs; Limouzy-Paris et al. 1999
Bulletin of Marine Science). These near-shore currents are often
slower than large regional-scale currents, and the flow of water
around reefs can often create eddies or other oceanographic features
that entrain particles and prevent advection.
Secondly, there is an issue of behaviour of coral planulae. Coral
planulae can vertically migrate in the water column, and this
behaviour responds to a number of cues including UV (Gleason et al.,
2006 Marine Biology) and salinity (Vermeij et al. 2006 Marine Ecology
Progress Series). Vertical migration can alter dispersal of planulae
in two ways; first, current speeds tend to be slower at depth than on
the surface, and second, currents tend to change direction with depth
(i.e. Ekman Spirals). A recent study by Paris and Cowen (2004
Limnology and Oceanography) showed that particles can be retained near
a natal reef for up to 20 days simply through the combination of
vertical migration and near-shore currents.
In order to create a network of MPAs (ie. to connect populations
between MPAs), then I suppose more information about the local
oceanography of your area, in terms of local currents and conditions
and how these might affect advection and vertical migration of the
planulae, needs to be gathered. If your goal is to make an MPA
network such that there is spillover of corals from one to the next,
an estimate of 40km separation may turn out to be too far for coral
planulae to realistically traverse in such a short period of time.
J. Derek Hogan
PhD. Candidate
University of Windsor
401 Sunset Ave.
Windsor, Ontario, Canada
More information about the Coral-List
mailing list