161
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
horizontal swimming, navigation or schooling behaviours are suffi cient to potentially
enable late larval and juvenile stages to detect and move towards suitable settlement
habitat (see Chap. 7 ).
We address the fi rst few days after spawning using scale modelling to obtain
estimates of dispersal of the initially highly concentrated cloud of sperm and eggs
via turbulence in the upper water column above a spawning aggregation. We address
later dispersal and advection of primarily passive, pre-fl exion, larvae noting the
importance of well-known near-shore fl ow features that reduce the extent of advection into offshore far-fi eld currents. It is important to note that our knowledge of
nearshore coral reef oceanography is incomplete, particularly for windward reefs,
and much of this chapter is directed at what is expected rather than what has
been proved.
6.2 The Start of Early Life History-Modelling Dispersion
of Fertilized Eggs from Aggregation Sites
Population continuity following spawning is a function of (1) spawning location,
time of spawning, and numbers of eggs spawned, (2) the dispersion (i.e., transport
and mixing) and survival of propagules from a single or multiple spawning sites that
determine the distribution of potential settlers, and (3) the presence of suitable
settlement habitat and the density of settlers (Botsford et al. 2009 ) . Among aggregation spawning species, the fi rst two components, in particular, may differ from other
reef fi shes (see also Chap. 7 ). The role of aggregation-spawning in dispersal outcomes
has been largely ignored. Do aggregations, for example, produce large cohorts
recruiting at the same time, as is considered in Chap. 7 , or do these aggregations,
rather, confer benefi ts on eggs and especially early stage larvae (this Chapter)?
Since the specifi cs of spawning, the starting point of ELH, determine where and
when larvae develop at sea, the physical factors impacting the eggs and early stage
larvae, are important. The geomorphology, timing and local environmental conditions at aggregation spawning sites have been described in Chap. 5 , but it is important also to understand how rapidly recently fertilized eggs become part of the
planktonic ecosystem and how they are thereafter dispersed by patterns of water fl ow
interacting with larval behaviour. Rather than thinking of the eggs from aggregationspawning as a point source (except in a broad geographic sense), a more comprehensive view of dispersal can be conceptualised as a dispersal kernel , which
expresses the probability that a larva released from a particular location will be
transported to, and successfully settle at, other specifi c locations where adult habitat
is available (Largier 2003 ) . Dispersal kernels describe a continuum of dispersal,
which is comprised of dispersal patterns from a variety of sources each of which
typically approximates a bell-shaped curve of larval distributions as a function of
distance from the spawning source (Fig. 6.1 ). While diffusion (due to variable
currents) accounts for this spreading out of larvae from a common source, advection
(due to the mean current) will result in an offset of this bell-shaped curve from the
6 Oceanography of the Planktonic Stages of Aggregation Spawning Reef Fishes
horizontal swimming, navigation or schooling behaviours are suffi cient to potentially
enable late larval and juvenile stages to detect and move towards suitable settlement
habitat (see Chap. 7 ).
We address the fi rst few days after spawning using scale modelling to obtain
estimates of dispersal of the initially highly concentrated cloud of sperm and eggs
via turbulence in the upper water column above a spawning aggregation. We address
later dispersal and advection of primarily passive, pre-fl exion, larvae noting the
importance of well-known near-shore fl ow features that reduce the extent of advection into offshore far-fi eld currents. It is important to note that our knowledge of
nearshore coral reef oceanography is incomplete, particularly for windward reefs,
and much of this chapter is directed at what is expected rather than what has
been proved.
6.2 The Start of Early Life History-Modelling Dispersion
of Fertilized Eggs from Aggregation Sites
Population continuity following spawning is a function of (1) spawning location,
time of spawning, and numbers of eggs spawned, (2) the dispersion (i.e., transport
and mixing) and survival of propagules from a single or multiple spawning sites that
determine the distribution of potential settlers, and (3) the presence of suitable
settlement habitat and the density of settlers (Botsford et al. 2009 ) . Among aggregation spawning species, the fi rst two components, in particular, may differ from other
reef fi shes (see also Chap. 7 ). The role of aggregation-spawning in dispersal outcomes
has been largely ignored. Do aggregations, for example, produce large cohorts
recruiting at the same time, as is considered in Chap. 7 , or do these aggregations,
rather, confer benefi ts on eggs and especially early stage larvae (this Chapter)?
Since the specifi cs of spawning, the starting point of ELH, determine where and
when larvae develop at sea, the physical factors impacting the eggs and early stage
larvae, are important. The geomorphology, timing and local environmental conditions at aggregation spawning sites have been described in Chap. 5 , but it is important also to understand how rapidly recently fertilized eggs become part of the
planktonic ecosystem and how they are thereafter dispersed by patterns of water fl ow
interacting with larval behaviour. Rather than thinking of the eggs from aggregationspawning as a point source (except in a broad geographic sense), a more comprehensive view of dispersal can be conceptualised as a dispersal kernel , which
expresses the probability that a larva released from a particular location will be
transported to, and successfully settle at, other specifi c locations where adult habitat
is available (Largier 2003 ) . Dispersal kernels describe a continuum of dispersal,
which is comprised of dispersal patterns from a variety of sources each of which
typically approximates a bell-shaped curve of larval distributions as a function of
distance from the spawning source (Fig. 6.1 ). While diffusion (due to variable
currents) accounts for this spreading out of larvae from a common source, advection
(due to the mean current) will result in an offset of this bell-shaped curve from the