573
13 Conclusion
across a wider range of habitats including broad insular shelves and inner reefs.
The multi-species use of both types of aggregation sites is common, suggesting,
perhaps, something special about the sites themselves. There is intriguing evidence
that migration routes to and from aggregation sites are learned from older fi shes, with
shelf edges or other prominent reef features used as a “highway” for long-distance
movements. This suggests that not only should workers pay attention to the possible
presence of juveniles at aggregation sites but that such highways may also need to
be managed; certainly some are known to fi shers and are actively exploited.
Although very little is known of the specifi c oceanography associated with
aggregation spawning, Chaps. 6 and 7 highlight the various methods available and
identify questions to be explored. Initial modelling of egg/larval dispersal during
the days prior to larval initiation of feeding, for example, indicates that the larval
density starting from a concentrated near-point source of spawning aggregations
soon approaches that of larvae from fi shes with more widespread spawning. This
suggests that a point source of large numbers of eggs or larvae has little to do with
the evolution of aggregation-spawning. Moreover, it seems unlikely that food density becomes more critical for the survival of aggregation-produced larvae than for
other fi sh larvae and hence that there is no apparent disadvantage to higher larval
numbers. Although it is highly unlikely that coherent clouds of larvae stay together
for extended periods of time, some physical mechanisms may concentrate larvae
when many are in the water at the same time and in the same general area. Several
aggregating species are reported to settle in particularly large cohorts (e.g. Nassau
grouper) or form the basis for capture-based aquaculture (e.g. certain mullets, rabbitfi sh, bonefi sh and groupers) whereby settling fi sh can be caught in enormous numbers
in specifi c areas and in short time frames. These concentrations have enabled the
establishment of fi sheries that catch large numbers of very young animals and then
grow them out in captivity until they reach market size (Lovatelli and Holthus 2008 ) .
The factors that determine the timing of spawning, at each of annual, diel, and
lunar scales, is of much interest and was variously addressed in several chapters.
Tropical fi sh species in general have extended spawning seasons, probably because
of the relative lack of seasonal changes compared to temperate regions. However,
aggregating species, and most transient aggregators in particular, are interesting in
that their spawning patterns tend to be very limited seasonally unlike other tropical
species; indeed such species may only spawn for a few hours over a few days in a
couple of months each year. What determines this annual seasonality is not known,
but temperature may be one factor (as suggested by data on the Nassau grouper and
camoufl age grouper, Epinephelus polyphekadion ) and the availability of larval food,
another. Regarding the lunar cycle, most TA species have a strong lunar component
to the timing of spawning which can be full or new moon related. Some species, like
the camoufl age grouper, might even cue to a different lunar phases in different areas.
Lunar synchronicity could be determined by optimum times for hatching and/or
larval settlement, or it could simply be a cue for coordinating reproductively ripe
adults. RA species tend to show more of a tidal component to the timing of spawning
than TA species. As for the diel cycle, present knowledge indicates that most TA
species spawn at dusk, during the night or at dawn, while RA species spawn during
13 Conclusion
across a wider range of habitats including broad insular shelves and inner reefs.
The multi-species use of both types of aggregation sites is common, suggesting,
perhaps, something special about the sites themselves. There is intriguing evidence
that migration routes to and from aggregation sites are learned from older fi shes, with
shelf edges or other prominent reef features used as a “highway” for long-distance
movements. This suggests that not only should workers pay attention to the possible
presence of juveniles at aggregation sites but that such highways may also need to
be managed; certainly some are known to fi shers and are actively exploited.
Although very little is known of the specifi c oceanography associated with
aggregation spawning, Chaps. 6 and 7 highlight the various methods available and
identify questions to be explored. Initial modelling of egg/larval dispersal during
the days prior to larval initiation of feeding, for example, indicates that the larval
density starting from a concentrated near-point source of spawning aggregations
soon approaches that of larvae from fi shes with more widespread spawning. This
suggests that a point source of large numbers of eggs or larvae has little to do with
the evolution of aggregation-spawning. Moreover, it seems unlikely that food density becomes more critical for the survival of aggregation-produced larvae than for
other fi sh larvae and hence that there is no apparent disadvantage to higher larval
numbers. Although it is highly unlikely that coherent clouds of larvae stay together
for extended periods of time, some physical mechanisms may concentrate larvae
when many are in the water at the same time and in the same general area. Several
aggregating species are reported to settle in particularly large cohorts (e.g. Nassau
grouper) or form the basis for capture-based aquaculture (e.g. certain mullets, rabbitfi sh, bonefi sh and groupers) whereby settling fi sh can be caught in enormous numbers
in specifi c areas and in short time frames. These concentrations have enabled the
establishment of fi sheries that catch large numbers of very young animals and then
grow them out in captivity until they reach market size (Lovatelli and Holthus 2008 ) .
The factors that determine the timing of spawning, at each of annual, diel, and
lunar scales, is of much interest and was variously addressed in several chapters.
Tropical fi sh species in general have extended spawning seasons, probably because
of the relative lack of seasonal changes compared to temperate regions. However,
aggregating species, and most transient aggregators in particular, are interesting in
that their spawning patterns tend to be very limited seasonally unlike other tropical
species; indeed such species may only spawn for a few hours over a few days in a
couple of months each year. What determines this annual seasonality is not known,
but temperature may be one factor (as suggested by data on the Nassau grouper and
camoufl age grouper, Epinephelus polyphekadion ) and the availability of larval food,
another. Regarding the lunar cycle, most TA species have a strong lunar component
to the timing of spawning which can be full or new moon related. Some species, like
the camoufl age grouper, might even cue to a different lunar phases in different areas.
Lunar synchronicity could be determined by optimum times for hatching and/or
larval settlement, or it could simply be a cue for coordinating reproductively ripe
adults. RA species tend to show more of a tidal component to the timing of spawning
than TA species. As for the diel cycle, present knowledge indicates that most TA
species spawn at dusk, during the night or at dawn, while RA species spawn during