152
P.L. Colin
5.9 Discussion
As this brief review indicates, our understanding of the reasons for the location
and timing of aggregations is at best preliminary. Only a few species have been
examined in any detail, and at only in a few locations. Interestingly, two
extremes of the aggregation spectrum, the Nassau grouper (a large TA species
with previously massive aggregations) and the bluehead wrasse (a small RA
species often with small aggregations) are perhaps the best-known species.
New information is slowly becoming available; assumptions and conclusions
regarding species and the phenomenon of spawning aggregations in general are
likely to undergo revision.
What is known at present indicates some general working ideas:
There are some clear distinctions in locations and timing between TAs and RAs.
•
The former occurring infrequently, but almost exclusively, on shelf edge areas
while the latter are more numerous and occur at a higher diversity of locations.
They are characterized by different types of fi shes; TAs with larger predatory
species and RAs with herbivores and omnivores. They have different protection
and conservation needs.
The TAs and RAs of the IWP are different from those of the TWA. Differences
•
can possibly be attributed to the higher tidal amplitudes and geomorphology of
barrier reefs and their channels of the IWP with resultant effects on the oceanic
environment.
TA fi shes have longer, less frequent migrations to aggregations while RA species
•
have shorter, more frequent migrations (Chap. 2 ). These are related to the
frequency of spawning and duration of spawning season.
Aggregation sites are stable in location over decades with only slight unexplained
•
variation in some species year to year.
There are no known differences between eggs and larvae of aggregating and
•
non-aggregating species within families (Chap. 7 ).
The currents occurring at TA and RA sites during spawning are just beginning to
•
be elucidated. Several examples with low current speed, termed “quiet currents”,
are now known at the time of spawning for Nassau grouper, red hind and cubera
snapper. Quiet currents tend of reduce chances of offshore dispersal. Currents
during spawning at RA sites are also often weak and would not cause advection
of eggs far off the reef.
The entrainment of propagules into oceanic circulation after spawning is not a
•
given, and at some TA sites there appears a tendency for propagules to be retained
nearshore. Spawn from RAs has a harder time becoming entrained into oceanic
circulation. Geomorphology of the reef and spawning sites is important in this
regard (Chap. 6 ).
Water temperature regimes may be an important determinant of seasonality of
•
spawning and early life history success. Aggregations often occur over a limited
temperature range.
P.L. Colin
5.9 Discussion
As this brief review indicates, our understanding of the reasons for the location
and timing of aggregations is at best preliminary. Only a few species have been
examined in any detail, and at only in a few locations. Interestingly, two
extremes of the aggregation spectrum, the Nassau grouper (a large TA species
with previously massive aggregations) and the bluehead wrasse (a small RA
species often with small aggregations) are perhaps the best-known species.
New information is slowly becoming available; assumptions and conclusions
regarding species and the phenomenon of spawning aggregations in general are
likely to undergo revision.
What is known at present indicates some general working ideas:
There are some clear distinctions in locations and timing between TAs and RAs.
•
The former occurring infrequently, but almost exclusively, on shelf edge areas
while the latter are more numerous and occur at a higher diversity of locations.
They are characterized by different types of fi shes; TAs with larger predatory
species and RAs with herbivores and omnivores. They have different protection
and conservation needs.
The TAs and RAs of the IWP are different from those of the TWA. Differences
•
can possibly be attributed to the higher tidal amplitudes and geomorphology of
barrier reefs and their channels of the IWP with resultant effects on the oceanic
environment.
TA fi shes have longer, less frequent migrations to aggregations while RA species
•
have shorter, more frequent migrations (Chap. 2 ). These are related to the
frequency of spawning and duration of spawning season.
Aggregation sites are stable in location over decades with only slight unexplained
•
variation in some species year to year.
There are no known differences between eggs and larvae of aggregating and
•
non-aggregating species within families (Chap. 7 ).
The currents occurring at TA and RA sites during spawning are just beginning to
•
be elucidated. Several examples with low current speed, termed “quiet currents”,
are now known at the time of spawning for Nassau grouper, red hind and cubera
snapper. Quiet currents tend of reduce chances of offshore dispersal. Currents
during spawning at RA sites are also often weak and would not cause advection
of eggs far off the reef.
The entrainment of propagules into oceanic circulation after spawning is not a
•
given, and at some TA sites there appears a tendency for propagules to be retained
nearshore. Spawn from RAs has a harder time becoming entrained into oceanic
circulation. Geomorphology of the reef and spawning sites is important in this
regard (Chap. 6 ).
Water temperature regimes may be an important determinant of seasonality of
•
spawning and early life history success. Aggregations often occur over a limited
temperature range.