170
W.M. Hamner and J.L. Largier
off-reef transport of eggs, and although the offshore tidal fl ow was short-lived, it
enhanced along-reef dispersion through inducing shear dispersion. Further, Holm
( 2004 ) obtained fi eld data and model results for the movement of fertilized eggs of
cubera snapper off Belize until the time of hatching (17–20 h after spawning) which
indicate a wide variety of possible larval trajectories (depending on tide/wind/
currents), presenting a broad cloud of larval destinations at the scale of population
dispersal.
In the above section, we have taken the broad-brush approach of scaling length
and time scales from an advection-diffusion approach. In reality, dispersal will be
more complex in space and time, but in the absence of better data and understanding
this provides a rough estimate of the problem. There is much to be discovered relating
to dispersal of early life stages of aggregating reef fi shes, and we expect that fi eld
investigations will yield plenty of surprises in this regard.
6.3 Nearshore Oceanography: Dispersion and Advection
of Early Larvae in Marginal Shallow Waters
Existing marine populations have obviously evolved successful larval strategies,
with both local and distant recruitment occurring within a single population. Looking
at spawning and fl ow across reefs one sees the origins of this bimodal outcome early
in the dispersion of the egg/larvae cloud – where fl ow separates from topography,
leading to the offshore export of some propagules (which may return to the same
reef later or easily be advected across deep offshore waters to a distant reef), while
others remain in the fl ow attached to the topography (greatly increasing the chances
of being retained in a wake or other retention zone long enough to ensure recruitment back to a nearby reef).
A wide diversity of oceanographic features results from the interaction of fl ow
with topography (e.g. structure of the shoreline, variability of benthic topography
and bottom depth) on coral reefs. We expect that three fl ow patterns, coastal boundary
layers (CBL) , lateral trapping in e mbayments , and eddies (gyres) are important in
the transport outcomes for aggregation spawned propagules. Resident and transient
spawning aggregations do not form everywhere but only at quite specifi c locations
on the coral reef and only at particular times of day, tide, and time of year (see
Chap. 5 ).
Resident spawning aggregations on coral reefs are often composed of many species
of small herbivorous fi shes. In Palau, a suite of 30–40 species of surgeonfi shes, parrotfi shes, and wrasses spawn every day for a few hours after high tide along the
forereef throughout the year (Patrick L. Colin personal observation). In particular,
spawning aggregations of the bullethead parrotfi sh, Chlorurus sordidus, and two
surgeonfi shes, the brown surgeonfi sh, Acanthurus nigrofuscus, and striped bristletooth,
Ctenochaetus striatus , occur at intervals of about 50 m along the reef face of the eastern
barrier reef (Colin and Bell 1991 ; Domeier and Colin 1997 ; Hamner et al. 2007 ,
Patrick L. Colin personal observation), aggregating to spawn each day at shallow
W.M. Hamner and J.L. Largier
off-reef transport of eggs, and although the offshore tidal fl ow was short-lived, it
enhanced along-reef dispersion through inducing shear dispersion. Further, Holm
( 2004 ) obtained fi eld data and model results for the movement of fertilized eggs of
cubera snapper off Belize until the time of hatching (17–20 h after spawning) which
indicate a wide variety of possible larval trajectories (depending on tide/wind/
currents), presenting a broad cloud of larval destinations at the scale of population
dispersal.
In the above section, we have taken the broad-brush approach of scaling length
and time scales from an advection-diffusion approach. In reality, dispersal will be
more complex in space and time, but in the absence of better data and understanding
this provides a rough estimate of the problem. There is much to be discovered relating
to dispersal of early life stages of aggregating reef fi shes, and we expect that fi eld
investigations will yield plenty of surprises in this regard.
6.3 Nearshore Oceanography: Dispersion and Advection
of Early Larvae in Marginal Shallow Waters
Existing marine populations have obviously evolved successful larval strategies,
with both local and distant recruitment occurring within a single population. Looking
at spawning and fl ow across reefs one sees the origins of this bimodal outcome early
in the dispersion of the egg/larvae cloud – where fl ow separates from topography,
leading to the offshore export of some propagules (which may return to the same
reef later or easily be advected across deep offshore waters to a distant reef), while
others remain in the fl ow attached to the topography (greatly increasing the chances
of being retained in a wake or other retention zone long enough to ensure recruitment back to a nearby reef).
A wide diversity of oceanographic features results from the interaction of fl ow
with topography (e.g. structure of the shoreline, variability of benthic topography
and bottom depth) on coral reefs. We expect that three fl ow patterns, coastal boundary
layers (CBL) , lateral trapping in e mbayments , and eddies (gyres) are important in
the transport outcomes for aggregation spawned propagules. Resident and transient
spawning aggregations do not form everywhere but only at quite specifi c locations
on the coral reef and only at particular times of day, tide, and time of year (see
Chap. 5 ).
Resident spawning aggregations on coral reefs are often composed of many species
of small herbivorous fi shes. In Palau, a suite of 30–40 species of surgeonfi shes, parrotfi shes, and wrasses spawn every day for a few hours after high tide along the
forereef throughout the year (Patrick L. Colin personal observation). In particular,
spawning aggregations of the bullethead parrotfi sh, Chlorurus sordidus, and two
surgeonfi shes, the brown surgeonfi sh, Acanthurus nigrofuscus, and striped bristletooth,
Ctenochaetus striatus , occur at intervals of about 50 m along the reef face of the eastern
barrier reef (Colin and Bell 1991 ; Domeier and Colin 1997 ; Hamner et al. 2007 ,
Patrick L. Colin personal observation), aggregating to spawn each day at shallow