153
5 Timing and Location of Aggregation and Spawning in Reef Fishes
The daily and lunar timing of aggregation spawning may be related to needs of
•
pelagic life history.
Predation on spawning adults is rare while predation of released eggs is common.
•
Neither factor is believed to limit or structure aggregations.
The hypotheses about the “where and when” of aggregations have tended to
focus on whether aggregations produce dispersal or retention of propagules, essentially following the same arc as the theories about reef fi sh populations in general.
Previously, it was thought that reef fi sh populations were selected to be “open”; to
spread their progeny across areas as wide as possible to ensure that at least some of
the population fi nds conditions suitable for survival. Similarly it was initially proposed (Johannes 1978, 1981 ; Barlow 1981 ) that aggregations formed in areas that
promoted the offshore dispersal of eggs and larvae into oceanic waters to either be
dispersed widely or potentially returned by mesoscale eddies after lengthy periods
at sea, rotating on a schedule in synch with larval development times, to their natal
region (Johannes 1978, 1981 ; Lobel and Robinson 1986 ) . While innately appealing,
these hypotheses had little supporting data, but this did not prevent them being
widely interpreted as solid facts in the literature and not subjected to the rigorous
testing necessary to fully evaluate (Shapiro et al. 1988 ) . More recently, focus has
shifted to ideas of larval populations being retained near their sources (Swearer
et al. 1999 ; Jones et al. 1999, 2005 ) . Aggregations may play a role in retention of
eggs and larvae, rather than dispersal, with locations and times of spawning preventing or at least limiting eggs from entering oceanic circulation (Colin 1992 ; Whaylen
et al. 2004 ; Hamner et al. 2007 ) . Arguments have ranged back and forth, and much
of the time results are inconclusive. Black or white arguments may be futile in the
end, since populations are most likely to exhibit characteristics of both strategies
and are likely to be quite sensitive to locality differences.
New data argue that at least some, if not all, TA spawning occurs at times and at
sites which do not favour dispersal of propagules (Colin 1992 ; Whaylen et al. 2004 ;
Nemeth et al. 2007 ; Heyman and Kjerfve 2008 ; Kadison et al. 2009 ) . It previously
assumed that spawning, particularly for TA species, would occur at times of
relatively strong currents potentially favouring offshore dispersal of eggs (Johannes
1978, 1981 ; Barlow 1981 ) . The realization in the last decade that a number of TA
fi shes spawn at times when currents are minimal, rather than maximal, has been
important in understanding that TAs may help to retain larvae, rather than disperse
them. Certain spawning sites, particularly bank reefs and narrow islands with small
shelves, do favour dispersal and entrainment of propagules into oceanic circulation
(Domeier 2004 , Grouper Moon Project unpublished data), so it is clear both dispersal
and retention mechanisms are operating, often at different sites and potentially in
the same area at different times (Colin 1995 ) .
A review of the diversity of locations, timing and conditions between TA and RA
spawning aggregations, as well as comparison to non-aggregation spawning sites,
provides some insight into the factors likely to be infl uencing aggregation-spawning.
Nearly all TA species either spawn on the reef drop-off or in areas closely connected
(channels) with outer reefs. RA species also spawn in such areas, but additionally
5 Timing and Location of Aggregation and Spawning in Reef Fishes
The daily and lunar timing of aggregation spawning may be related to needs of
•
pelagic life history.
Predation on spawning adults is rare while predation of released eggs is common.
•
Neither factor is believed to limit or structure aggregations.
The hypotheses about the “where and when” of aggregations have tended to
focus on whether aggregations produce dispersal or retention of propagules, essentially following the same arc as the theories about reef fi sh populations in general.
Previously, it was thought that reef fi sh populations were selected to be “open”; to
spread their progeny across areas as wide as possible to ensure that at least some of
the population fi nds conditions suitable for survival. Similarly it was initially proposed (Johannes 1978, 1981 ; Barlow 1981 ) that aggregations formed in areas that
promoted the offshore dispersal of eggs and larvae into oceanic waters to either be
dispersed widely or potentially returned by mesoscale eddies after lengthy periods
at sea, rotating on a schedule in synch with larval development times, to their natal
region (Johannes 1978, 1981 ; Lobel and Robinson 1986 ) . While innately appealing,
these hypotheses had little supporting data, but this did not prevent them being
widely interpreted as solid facts in the literature and not subjected to the rigorous
testing necessary to fully evaluate (Shapiro et al. 1988 ) . More recently, focus has
shifted to ideas of larval populations being retained near their sources (Swearer
et al. 1999 ; Jones et al. 1999, 2005 ) . Aggregations may play a role in retention of
eggs and larvae, rather than dispersal, with locations and times of spawning preventing or at least limiting eggs from entering oceanic circulation (Colin 1992 ; Whaylen
et al. 2004 ; Hamner et al. 2007 ) . Arguments have ranged back and forth, and much
of the time results are inconclusive. Black or white arguments may be futile in the
end, since populations are most likely to exhibit characteristics of both strategies
and are likely to be quite sensitive to locality differences.
New data argue that at least some, if not all, TA spawning occurs at times and at
sites which do not favour dispersal of propagules (Colin 1992 ; Whaylen et al. 2004 ;
Nemeth et al. 2007 ; Heyman and Kjerfve 2008 ; Kadison et al. 2009 ) . It previously
assumed that spawning, particularly for TA species, would occur at times of
relatively strong currents potentially favouring offshore dispersal of eggs (Johannes
1978, 1981 ; Barlow 1981 ) . The realization in the last decade that a number of TA
fi shes spawn at times when currents are minimal, rather than maximal, has been
important in understanding that TAs may help to retain larvae, rather than disperse
them. Certain spawning sites, particularly bank reefs and narrow islands with small
shelves, do favour dispersal and entrainment of propagules into oceanic circulation
(Domeier 2004 , Grouper Moon Project unpublished data), so it is clear both dispersal
and retention mechanisms are operating, often at different sites and potentially in
the same area at different times (Colin 1995 ) .
A review of the diversity of locations, timing and conditions between TA and RA
spawning aggregations, as well as comparison to non-aggregation spawning sites,
provides some insight into the factors likely to be infl uencing aggregation-spawning.
Nearly all TA species either spawn on the reef drop-off or in areas closely connected
(channels) with outer reefs. RA species also spawn in such areas, but additionally