[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

   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

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