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W.M. Hamner and J.L. Largier
to deep, often oceanic water. They generally have two or more short spawning periods
per year, with a strong lunar component, implying a critical role of tidal phase and
amplitude (Colin 1992 ; Heyman et al. 2005 ) . They usually spawn at dusk (and in a
few cases at night or dawn) at 10–30 m depth near topographic projections of the
reef. If their propagules drift along the reef in the alongshore current, they are usually
entrained into topographic eddies generated at sudden changes in reef form.
Alternatively, or even simultaneously, other propagules may be advected into oceanic
waters via a separated jet. Thus, the fates of the eggs spawned by these transient
aggregators often quickly diverge and appear variable, alternating between export
to the ocean and retention near the reef.
6.3.1 Resident Aggregations Spawn into Coastal
Boundary Layers
Csanady ( 1972 ) and Largier ( 2002, 2003, 2004 ) used the phrase coastal boundary
layer for the steady, directional fl ow of water that moves along an unbroken beach
or straight rocky shoreline. A key difference between a coastal boundary layer and
eddies (discussed later) is that CBL fl ow is primarily linear and tidally reversing
whereas fl ow in topographic eddies is rotational. Largier ( 2003 ) noted that coastal
boundary currents are both slow and retentive, and argued that most larvae spawned
into CBLs will take several days to disperse offshore. For many populations, larvae
may spend a major portion of their early life history in the CBL (Largier 2003 ) .
Although coastal boundary currents are characteristic of linear coastlines, most
coral reefs include embayments of various sizes and shapes; typically semi-enclosed
bodies of water that are fully open on only one of four sides, bounded by headlands
or promontories on the reef face, or segmented by passes through the coral reef or
by rivers or estuaries that preclude through-fl ow of water. Coastal boundary currents
in embayments are usually dominated by reversing tidal fl ows (indeed, mean currents
are often near-zero and more attention must be given to tidal boundary layers along
the edge of coral reefs; Hamner et al. 2007 ) . The extent to which tidal dispersion
contributes to the fl ushing of an embayment depends on the ratio of the typical
length scale of topographic variation to the tidal excursion distance (Geyer and
Signell 1992 ) . However, viewed at a larger spatial scale (in which bays are seen as
bumps along the coast), the zero/slow mean along-coast fl ow through the bay may
be viewed as a region where the CBL is extra wide (order 10 km rather than more
usually order 1 km) (Largier 2004 ) .
Larval retention zones in tropical reef environments are not well-known (Largier
2003 ) , but at least two examples have since been described from Palau associated
with resident spawning aggregations of small herbivorous fi shes. Both of these
retention areas occur in deeply incised embayments in the barrier reef (Fig. 6.4 ),
with larval retention zones defi ned by movements of drifters released when resident
fi shes aggregated for spawning (Fig. 6.5 ). The east side retention zone had tidal
currents which moved water and propagules off and onshore (returning toward or
W.M. Hamner and J.L. Largier
to deep, often oceanic water. They generally have two or more short spawning periods
per year, with a strong lunar component, implying a critical role of tidal phase and
amplitude (Colin 1992 ; Heyman et al. 2005 ) . They usually spawn at dusk (and in a
few cases at night or dawn) at 10–30 m depth near topographic projections of the
reef. If their propagules drift along the reef in the alongshore current, they are usually
entrained into topographic eddies generated at sudden changes in reef form.
Alternatively, or even simultaneously, other propagules may be advected into oceanic
waters via a separated jet. Thus, the fates of the eggs spawned by these transient
aggregators often quickly diverge and appear variable, alternating between export
to the ocean and retention near the reef.
6.3.1 Resident Aggregations Spawn into Coastal
Boundary Layers
Csanady ( 1972 ) and Largier ( 2002, 2003, 2004 ) used the phrase coastal boundary
layer for the steady, directional fl ow of water that moves along an unbroken beach
or straight rocky shoreline. A key difference between a coastal boundary layer and
eddies (discussed later) is that CBL fl ow is primarily linear and tidally reversing
whereas fl ow in topographic eddies is rotational. Largier ( 2003 ) noted that coastal
boundary currents are both slow and retentive, and argued that most larvae spawned
into CBLs will take several days to disperse offshore. For many populations, larvae
may spend a major portion of their early life history in the CBL (Largier 2003 ) .
Although coastal boundary currents are characteristic of linear coastlines, most
coral reefs include embayments of various sizes and shapes; typically semi-enclosed
bodies of water that are fully open on only one of four sides, bounded by headlands
or promontories on the reef face, or segmented by passes through the coral reef or
by rivers or estuaries that preclude through-fl ow of water. Coastal boundary currents
in embayments are usually dominated by reversing tidal fl ows (indeed, mean currents
are often near-zero and more attention must be given to tidal boundary layers along
the edge of coral reefs; Hamner et al. 2007 ) . The extent to which tidal dispersion
contributes to the fl ushing of an embayment depends on the ratio of the typical
length scale of topographic variation to the tidal excursion distance (Geyer and
Signell 1992 ) . However, viewed at a larger spatial scale (in which bays are seen as
bumps along the coast), the zero/slow mean along-coast fl ow through the bay may
be viewed as a region where the CBL is extra wide (order 10 km rather than more
usually order 1 km) (Largier 2004 ) .
Larval retention zones in tropical reef environments are not well-known (Largier
2003 ) , but at least two examples have since been described from Palau associated
with resident spawning aggregations of small herbivorous fi shes. Both of these
retention areas occur in deeply incised embayments in the barrier reef (Fig. 6.4 ),
with larval retention zones defi ned by movements of drifters released when resident
fi shes aggregated for spawning (Fig. 6.5 ). The east side retention zone had tidal
currents which moved water and propagules off and onshore (returning toward or