169
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
The above discussion considers a patch moving along the reef, but the situation
is notably different at a reef passage or at the sudden end of a topographic reef feature,
where fl ow may separate from the shoreline with some of the larva-laden water
being exported offshore while other parts of the larval cloud are retained nearshore.
If one considers the associated jet and wake fl ow features of scale >100 m and fl ow
speeds >0.1 m/s as part of the dispersion process, then values of K would approach
100 m
2 /s, similar to those values used for representing tidal dispersion in similarly
scaled estuary fl ows. In this scenario, one would see the small 100–1,000 m cloud
of larvae rapidly ripped apart into swirls and whirls that could be represented by a
cloud of order 10 km or larger. In most spawning situations there are strong topographic features (Colin 1992 ; Heyman et al. 2005 ; Hamner et al. 2007 ) and the larval
cloud is likely to reach such a separation point within hours or a day after being
spawned – suggesting that the dense concentrations of eggs/larvae produced by
aggregation spawning on a reef will be diluted down to more typical non-aggregation
concentrations within less than a few days. Later herein we discuss such oceanographic features and note that some features also present opportunities for retention
of a larval cloud.
The above discussion has focused on diffusion – the spreading out and dilution
of a cloud of eggs/larvae due to zero-mean fl ow variability, whether due to smaller
scale turbulence or larger scale fl ow features like eddies. This diffusion is superimposed on a background mean fl ow that advects the cloud along the reef – and may
elongate the cloud formed during prolonged spawning (e.g. Nassau grouper
spawning for 30 min will yield a cloud 100–200 m long in an advective fl ow of
0.05–0.1 m/s). The phenomena of advection and diffusion are related because stronger
fl ow over the reef and past small-scale topography will yield shear and eddies that
account for stronger diffusion, exceeding background values expected from the
“4/3-law” values. Further, the comparison between reef length and advection
velocity gives an estimate of how long the cloud diffuses in the vicinity of the reef
and when it will reach a topographic break (due to its advection along the reef).
At the break the associated fl ow separation, through shear effects, can literally tear
the cohesive cloud apart, causing the cloud to become more dispersed. These events,
which destroy the continuity of the cloud, can occur rapidly after spawning (such
as along the edge of a tidal channel with outfl owing current) or take longer (cloud
is slowly moved as a discrete, slowly diffusing, mass along a continuous outer
reef face).
Over time scales of a few hours, tidal fl ows act as advection (moving the eggs or
larvae horizontally). However, dispersion over time scales of days or multiple events
(on different days) will see tidal fl ows as zero-mean fl ow variability and a key
contributor to the total diffusion. For multiple spawn bursts over less than an hour
(e.g. cubera snapper, Heyman et al. 2005 ) , or for multiple spawning events over the
reproductive time scale of a given population , it is the ensemble average of spawning
outcomes that is of interest and tidal fl ows will be quasi-random time-varying fl ows
that affect dispersal variance (i.e. a diffusion effect). For example, Hensley et al.
( 1994 ) observed that bluehead wrasse tended to group spawn at sites that favour
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
The above discussion considers a patch moving along the reef, but the situation
is notably different at a reef passage or at the sudden end of a topographic reef feature,
where fl ow may separate from the shoreline with some of the larva-laden water
being exported offshore while other parts of the larval cloud are retained nearshore.
If one considers the associated jet and wake fl ow features of scale >100 m and fl ow
speeds >0.1 m/s as part of the dispersion process, then values of K would approach
100 m
2 /s, similar to those values used for representing tidal dispersion in similarly
scaled estuary fl ows. In this scenario, one would see the small 100–1,000 m cloud
of larvae rapidly ripped apart into swirls and whirls that could be represented by a
cloud of order 10 km or larger. In most spawning situations there are strong topographic features (Colin 1992 ; Heyman et al. 2005 ; Hamner et al. 2007 ) and the larval
cloud is likely to reach such a separation point within hours or a day after being
spawned – suggesting that the dense concentrations of eggs/larvae produced by
aggregation spawning on a reef will be diluted down to more typical non-aggregation
concentrations within less than a few days. Later herein we discuss such oceanographic features and note that some features also present opportunities for retention
of a larval cloud.
The above discussion has focused on diffusion – the spreading out and dilution
of a cloud of eggs/larvae due to zero-mean fl ow variability, whether due to smaller
scale turbulence or larger scale fl ow features like eddies. This diffusion is superimposed on a background mean fl ow that advects the cloud along the reef – and may
elongate the cloud formed during prolonged spawning (e.g. Nassau grouper
spawning for 30 min will yield a cloud 100–200 m long in an advective fl ow of
0.05–0.1 m/s). The phenomena of advection and diffusion are related because stronger
fl ow over the reef and past small-scale topography will yield shear and eddies that
account for stronger diffusion, exceeding background values expected from the
“4/3-law” values. Further, the comparison between reef length and advection
velocity gives an estimate of how long the cloud diffuses in the vicinity of the reef
and when it will reach a topographic break (due to its advection along the reef).
At the break the associated fl ow separation, through shear effects, can literally tear
the cohesive cloud apart, causing the cloud to become more dispersed. These events,
which destroy the continuity of the cloud, can occur rapidly after spawning (such
as along the edge of a tidal channel with outfl owing current) or take longer (cloud
is slowly moved as a discrete, slowly diffusing, mass along a continuous outer
reef face).
Over time scales of a few hours, tidal fl ows act as advection (moving the eggs or
larvae horizontally). However, dispersion over time scales of days or multiple events
(on different days) will see tidal fl ows as zero-mean fl ow variability and a key
contributor to the total diffusion. For multiple spawn bursts over less than an hour
(e.g. cubera snapper, Heyman et al. 2005 ) , or for multiple spawning events over the
reproductive time scale of a given population , it is the ensemble average of spawning
outcomes that is of interest and tidal fl ows will be quasi-random time-varying fl ows
that affect dispersal variance (i.e. a diffusion effect). For example, Hensley et al.
( 1994 ) observed that bluehead wrasse tended to group spawn at sites that favour