185
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
in different amounts of phytoplankton in the water column at different depths.
However, the effects on reef fi sh larval feeding and survival are largely unknown.
Fronts are formed at sea when water masses of different density converge, with
one sinking below the other. Where water masses meet they have on or near the
surface a concentrated line of organisms and materials that fl oat or are able to
swim to stay at or near the surface. Fronts attract a variety of nektonic animals.
Recently, Genin et al. ( 2005 ) confi rmed, using 3D acoustic imaging in the Red
Sea, that the accumulation of high concentrations of zooplankton along a frontal
zone between convergent water masses was due to their directional swimming
behaviour (up to tens of body lengths per second) against upwelling and downwelling currents.
The importance of fronts and eddies in the overall economy of the sea has been
reemphasized recently by Kai et al. ( 2009 ) . The importance of fronts to early stage
larvae of reef fi shes is virtually unknown, but there is strong evidence they have
important infl uences on pre-settlement fi shes (Kingsford et al. 1991 ) . Larvae that
are entrained at a front, of course, are well-placed to fi nd food because they need
only maintain position and wait while a conveyor belt of concentrated food moves
continuously toward them. Fronts are surprisingly common around coral reefs
because almost any given coral head can generate topographic eddies (Hamner and
Hauri 1977 ; Wolanski and Hamner 1988 ; Wolanski 2001 ) and because all eddies
generate fronts along the outside edges of the vortex (White and Wolanski 2007 ) .
Hydrographic structure, such as the slicks of internal waves, tidally generated
eddies, jets, fronts, thermal plumes and Langmuir circulations, may infl uence both
distribution patterns of fi shes and zooplankton, as well as their direction of
movement (Alldredge and Hamner 1980 ; Sakamoto and Tanaka 1986 ; Wolanski
and Hamner 1988 ; Kingsford et al. 1991 ; Wolanski 2001 ) . Whether fronts play any
greater role for aggregation spawned larvae than for any other type of fi sh larvae is
not known, but needs exploration.
Convergence of water masses also generates directional horizontal fl ow along
the front that can transport entrained organisms back toward shore or into a coral
reef lagoon (Largier 1993 ) . Eggleston et al. ( 1998 ) state that since “fronts constrain
cross-frontal fl ow, they serve to defl ect incident fl ow resulting in strong along-frontal
and down-welling fl ows that transport larvae collected at the front.” Strong swimming
larvae in surface waters, such as late stage larval fi shes, may use fronts as ‘larval
conduits’ to intersect adequate settlement habitat.
There are also fronts associated with the coastal boundary layer since these
currents retain their integrity as they fl ow along an unbroken coastline, often with
surface waters slightly warmer than offshore waters on calm warm afternoons.
In Palau, on ebbing tides, warmer water from the lagoon fl ows out across the reef
fl at and it slides seaward over slightly cooler oceanic water (Fig. 6.12 ). The frontal
edge of the buoyant sheet of lagoon water fl owing off the reef carries most of the
eggs spawned on this reef every day at ebb tide by resident spawning aggregations
of surgeonfi shes and parrotfi shes. Given the importance of fronts for larval foraging,
it is surprising that fronts have seldom been carefully sampled.
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
in different amounts of phytoplankton in the water column at different depths.
However, the effects on reef fi sh larval feeding and survival are largely unknown.
Fronts are formed at sea when water masses of different density converge, with
one sinking below the other. Where water masses meet they have on or near the
surface a concentrated line of organisms and materials that fl oat or are able to
swim to stay at or near the surface. Fronts attract a variety of nektonic animals.
Recently, Genin et al. ( 2005 ) confi rmed, using 3D acoustic imaging in the Red
Sea, that the accumulation of high concentrations of zooplankton along a frontal
zone between convergent water masses was due to their directional swimming
behaviour (up to tens of body lengths per second) against upwelling and downwelling currents.
The importance of fronts and eddies in the overall economy of the sea has been
reemphasized recently by Kai et al. ( 2009 ) . The importance of fronts to early stage
larvae of reef fi shes is virtually unknown, but there is strong evidence they have
important infl uences on pre-settlement fi shes (Kingsford et al. 1991 ) . Larvae that
are entrained at a front, of course, are well-placed to fi nd food because they need
only maintain position and wait while a conveyor belt of concentrated food moves
continuously toward them. Fronts are surprisingly common around coral reefs
because almost any given coral head can generate topographic eddies (Hamner and
Hauri 1977 ; Wolanski and Hamner 1988 ; Wolanski 2001 ) and because all eddies
generate fronts along the outside edges of the vortex (White and Wolanski 2007 ) .
Hydrographic structure, such as the slicks of internal waves, tidally generated
eddies, jets, fronts, thermal plumes and Langmuir circulations, may infl uence both
distribution patterns of fi shes and zooplankton, as well as their direction of
movement (Alldredge and Hamner 1980 ; Sakamoto and Tanaka 1986 ; Wolanski
and Hamner 1988 ; Kingsford et al. 1991 ; Wolanski 2001 ) . Whether fronts play any
greater role for aggregation spawned larvae than for any other type of fi sh larvae is
not known, but needs exploration.
Convergence of water masses also generates directional horizontal fl ow along
the front that can transport entrained organisms back toward shore or into a coral
reef lagoon (Largier 1993 ) . Eggleston et al. ( 1998 ) state that since “fronts constrain
cross-frontal fl ow, they serve to defl ect incident fl ow resulting in strong along-frontal
and down-welling fl ows that transport larvae collected at the front.” Strong swimming
larvae in surface waters, such as late stage larval fi shes, may use fronts as ‘larval
conduits’ to intersect adequate settlement habitat.
There are also fronts associated with the coastal boundary layer since these
currents retain their integrity as they fl ow along an unbroken coastline, often with
surface waters slightly warmer than offshore waters on calm warm afternoons.
In Palau, on ebbing tides, warmer water from the lagoon fl ows out across the reef
fl at and it slides seaward over slightly cooler oceanic water (Fig. 6.12 ). The frontal
edge of the buoyant sheet of lagoon water fl owing off the reef carries most of the
eggs spawned on this reef every day at ebb tide by resident spawning aggregations
of surgeonfi shes and parrotfi shes. Given the importance of fronts for larval foraging,
it is surprising that fronts have seldom been carefully sampled.