178
W.M. Hamner and J.L. Largier
spawn greatly increase the possibility that larvae will eventually be entrained into
topographically generated eddies. Unfortunately, the literature on fl ow interactions
with reef and island topography is incomplete and it is diffi cult to generalize about
the fate of spawn in generic topographic locations – indeed, Bakun ( 2006 ) concurs
that this literature is incomplete, sparse, incomprehensible and confusing. It is
incomplete because fi ne-scale eddies are not addressed by most physical oceanographers, who work preferentially with meso-scale and larger oceanic fl ows. It is
sparse because topographic eddies are not relevant to most fi sheries biologists who
investigate pelagic fi shes that often live far from shore. It is often incomprehensible
to biologists poorly trained in the mathematics of fl uid mechanics, and it is confusing
due to the inconsistent use of terms that are vague or imprecise.
Eddies are ubiquitous in the ocean, at all locations and at all scales. Every obstacle
in the ocean – be it island, reef, or submerged bank, a whale or a copepod – sets up
a downstream eddy fi eld. Usually the fi ne-scale oceanography of a particular area is
not well known when fi eld sampling of spawning fi shes is contemplated and it is
diffi cult to tailor the sampling regime for eddy effects. There are, however, some
general, well known rules associated with fl ow of fl uids around objects; those rules
apply to how water fl ows around coral reefs (Wolanski 2001 ) . For a qualitative
appreciation of eddies and vorticity around coral reefs and their retentive potential
there are fi ve factors of immediate concern: velocity and direction of the current, the
size and shape of the reef, and the depth of the water. Tides alter velocity and direction
of fl ow around a reef every day, with range varying throughout the lunar month.
Within any 6-h tidal period fl ow around a reef might be (a) smooth, or (b) turbulent
with stable eddies, or (c) turbulent with an elongate wake (Fig. 6.8 ). Flow around an
island, headland or reef will differ during fl ood and ebb tide, depending on topography, tidal strength and wind direction. Thus, when a current encounters an abrupt
change in bathymetry such as a submerged bank, a coral reef or an island, it diverges
to fl ow around the obstruction, and eddies form in the lee. Shallow water eddies in
reversing tidal fl ows typically last only about 2–3 h and they transport particles around/
through the reef several times over a period of days, effectively trapping them in the
immediate vicinity of the reef and preventing them being swept away by far-fi eld
currents (Black et al. 1990, 1991 ; Kingsford et al. 1991 ; Burgess et al. 2007 ) . Tidal
eddies have been described at islands (Wolanski et al. 1984 ) , headlands (Alldredge
and Hamner 1980 ) , and coral reefs of the GBR (Hamner and Hauri 1981 ; Wolanski
et al. 1984 ; Young et al. 1993 ; Suthers et al. 2004 ; White and Wolanski 2007 ) .
It is not easy to determine whether transiently aggregating fi shes have spawned
into a linear coastal boundary layer or into a transient, tidal, topographic eddy unless
one simultaneously deploys drogues or obtains aerial images at that specifi c site
because quite different fl ow patterns look exactly the same to an observer at the
edge of the reef. A range of methods can be used to examine water movements. For
small bodies of water fl uorescent dyes are often used, but in many locations it is
possible to visualize fl ow from aircraft because sediments are suspended by strong
tidal currents, as in the Whitsunday Islands, Australia (Hamner and Hauri 1977 ) .
Satellite images also show differences in water temperatures and chlorophyll.
In addition, current meters, acoustic doppler current profi lers, and drifters are
W.M. Hamner and J.L. Largier
spawn greatly increase the possibility that larvae will eventually be entrained into
topographically generated eddies. Unfortunately, the literature on fl ow interactions
with reef and island topography is incomplete and it is diffi cult to generalize about
the fate of spawn in generic topographic locations – indeed, Bakun ( 2006 ) concurs
that this literature is incomplete, sparse, incomprehensible and confusing. It is
incomplete because fi ne-scale eddies are not addressed by most physical oceanographers, who work preferentially with meso-scale and larger oceanic fl ows. It is
sparse because topographic eddies are not relevant to most fi sheries biologists who
investigate pelagic fi shes that often live far from shore. It is often incomprehensible
to biologists poorly trained in the mathematics of fl uid mechanics, and it is confusing
due to the inconsistent use of terms that are vague or imprecise.
Eddies are ubiquitous in the ocean, at all locations and at all scales. Every obstacle
in the ocean – be it island, reef, or submerged bank, a whale or a copepod – sets up
a downstream eddy fi eld. Usually the fi ne-scale oceanography of a particular area is
not well known when fi eld sampling of spawning fi shes is contemplated and it is
diffi cult to tailor the sampling regime for eddy effects. There are, however, some
general, well known rules associated with fl ow of fl uids around objects; those rules
apply to how water fl ows around coral reefs (Wolanski 2001 ) . For a qualitative
appreciation of eddies and vorticity around coral reefs and their retentive potential
there are fi ve factors of immediate concern: velocity and direction of the current, the
size and shape of the reef, and the depth of the water. Tides alter velocity and direction
of fl ow around a reef every day, with range varying throughout the lunar month.
Within any 6-h tidal period fl ow around a reef might be (a) smooth, or (b) turbulent
with stable eddies, or (c) turbulent with an elongate wake (Fig. 6.8 ). Flow around an
island, headland or reef will differ during fl ood and ebb tide, depending on topography, tidal strength and wind direction. Thus, when a current encounters an abrupt
change in bathymetry such as a submerged bank, a coral reef or an island, it diverges
to fl ow around the obstruction, and eddies form in the lee. Shallow water eddies in
reversing tidal fl ows typically last only about 2–3 h and they transport particles around/
through the reef several times over a period of days, effectively trapping them in the
immediate vicinity of the reef and preventing them being swept away by far-fi eld
currents (Black et al. 1990, 1991 ; Kingsford et al. 1991 ; Burgess et al. 2007 ) . Tidal
eddies have been described at islands (Wolanski et al. 1984 ) , headlands (Alldredge
and Hamner 1980 ) , and coral reefs of the GBR (Hamner and Hauri 1981 ; Wolanski
et al. 1984 ; Young et al. 1993 ; Suthers et al. 2004 ; White and Wolanski 2007 ) .
It is not easy to determine whether transiently aggregating fi shes have spawned
into a linear coastal boundary layer or into a transient, tidal, topographic eddy unless
one simultaneously deploys drogues or obtains aerial images at that specifi c site
because quite different fl ow patterns look exactly the same to an observer at the
edge of the reef. A range of methods can be used to examine water movements. For
small bodies of water fl uorescent dyes are often used, but in many locations it is
possible to visualize fl ow from aircraft because sediments are suspended by strong
tidal currents, as in the Whitsunday Islands, Australia (Hamner and Hauri 1977 ) .
Satellite images also show differences in water temperatures and chlorophyll.
In addition, current meters, acoustic doppler current profi lers, and drifters are