572
Y. Sadovy de Mitcheson and P.L. Colin
Differences in fi sh species, refl ecting natural abundance levels and associated
with habitat structure, may also be a factor determining the sizes and even types of
aggregation. It is clear that one major gap in our understanding of the aggregation
habit is how it is related to fi sh abundance and/or densities and what implications
changes in these might have in the case of exploited populations.
Hypotheses regarding the “where and when” of aggregations, covered by Pat
Colin in Chap. 5 , generally focus on whether aggregations produce dispersal or
retention of propagules. In the past, the most common hypothesis was that reef fi sh
populations were selected 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.
The idea was that aggregations form in areas that promote the offshore dispersal of
eggs and larvae into oceanic waters for wide dispersal or perhaps for return by
meso-scale eddies after lengthy periods at sea. More recently, focus shifted to ideas
of larval populations being retained near their sources with aggregation sites and
times possibly playing a role in retention of eggs and larvae, rather than dispersal.
While such hypotheses need testing, it is quite likely that populations exhibit characteristics of both strategies and are quite sensitive to locality differences.
A review of the diversity of locations, timing and conditions between TA and RA
spawning aggregations, as well as comparisons with non-aggregation spawning
sites, provides insight into factors likely to be infl uencing aggregation-spawning at
both local and regional levels. Nearly all TA species either spawn on the reef dropoff or in areas closely connected (channels) with outer reef areas. RA species also
spawn in such areas, but additionally utilize more inshore areas. Different species at
multi-species TA sites do not usually spawn at exactly the same times but all spawning is
completed within a typically restricted spawning season. This suggests that, within a
region, spawning may be structured around local abiotic regimes with good evidence
that temperature is a major shaping factor. Such physical conditions could be cues
for adults in the area or may be important for the success of the larvae, particularly
for feeding, a variation of the famous match/mismatch hypothesis (Sinclair 1988 ) .
At a wider spatial scale, differences between the western Atlantic and the Indo-Pacifi c
regions may also infl uence aggregation patterns. The Atlantic is characterized by
modest tidal amplitudes (1 m or less), predominance of insular shelves without barrier
reefs, presence of few atolls and weak tidal currents on reefs. The Indo-Pacifi c has
generally higher tidal amplitudes (up to several metres), abundant barrier reefs with
channels, many atolls, and strong tidal currents on reefs (due to geomorphology and
tidal amplitude). Such physical differences and how these also might affect fi sh
densities and distributions is a little-considered but potentially very interesting area
for comparative study.
An understanding of why and how fi sh spawning aggregations form and persist
may emerge by examining commonalities in the location and timing of aggregation
and spawning (Chap. 5 ) combined with what is known of the specifi c oceanography
of aggregation sites covered in Chap. 6 by Bill Hamner and John Largier. Transient
and resident aggregations with long-term persistence at specifi c sites are found
throughout the world. The former tend to occur on outer reefs (with or without
promontories), in channels, and at the ends of islands and atolls. The latter occur
Y. Sadovy de Mitcheson and P.L. Colin
Differences in fi sh species, refl ecting natural abundance levels and associated
with habitat structure, may also be a factor determining the sizes and even types of
aggregation. It is clear that one major gap in our understanding of the aggregation
habit is how it is related to fi sh abundance and/or densities and what implications
changes in these might have in the case of exploited populations.
Hypotheses regarding the “where and when” of aggregations, covered by Pat
Colin in Chap. 5 , generally focus on whether aggregations produce dispersal or
retention of propagules. In the past, the most common hypothesis was that reef fi sh
populations were selected 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.
The idea was that aggregations form in areas that promote the offshore dispersal of
eggs and larvae into oceanic waters for wide dispersal or perhaps for return by
meso-scale eddies after lengthy periods at sea. More recently, focus shifted to ideas
of larval populations being retained near their sources with aggregation sites and
times possibly playing a role in retention of eggs and larvae, rather than dispersal.
While such hypotheses need testing, it is quite likely that populations exhibit characteristics of both strategies and are quite sensitive to locality differences.
A review of the diversity of locations, timing and conditions between TA and RA
spawning aggregations, as well as comparisons with non-aggregation spawning
sites, provides insight into factors likely to be infl uencing aggregation-spawning at
both local and regional levels. Nearly all TA species either spawn on the reef dropoff or in areas closely connected (channels) with outer reef areas. RA species also
spawn in such areas, but additionally utilize more inshore areas. Different species at
multi-species TA sites do not usually spawn at exactly the same times but all spawning is
completed within a typically restricted spawning season. This suggests that, within a
region, spawning may be structured around local abiotic regimes with good evidence
that temperature is a major shaping factor. Such physical conditions could be cues
for adults in the area or may be important for the success of the larvae, particularly
for feeding, a variation of the famous match/mismatch hypothesis (Sinclair 1988 ) .
At a wider spatial scale, differences between the western Atlantic and the Indo-Pacifi c
regions may also infl uence aggregation patterns. The Atlantic is characterized by
modest tidal amplitudes (1 m or less), predominance of insular shelves without barrier
reefs, presence of few atolls and weak tidal currents on reefs. The Indo-Pacifi c has
generally higher tidal amplitudes (up to several metres), abundant barrier reefs with
channels, many atolls, and strong tidal currents on reefs (due to geomorphology and
tidal amplitude). Such physical differences and how these also might affect fi sh
densities and distributions is a little-considered but potentially very interesting area
for comparative study.
An understanding of why and how fi sh spawning aggregations form and persist
may emerge by examining commonalities in the location and timing of aggregation
and spawning (Chap. 5 ) combined with what is known of the specifi c oceanography
of aggregation sites covered in Chap. 6 by Bill Hamner and John Largier. Transient
and resident aggregations with long-term persistence at specifi c sites are found
throughout the world. The former tend to occur on outer reefs (with or without
promontories), in channels, and at the ends of islands and atolls. The latter occur