149
5 Timing and Location of Aggregation and Spawning in Reef Fishes
100–1,000 spawns although potential predators are often present near areas where
reef fi shes are spawning (Colin 1978 ; Colin and Clavijo 1988 ; Clifton and
Robertson 1993 ; Craig 1998 ; Sancho et al. 2000b ) . Some authors have failed to
observe, despite the presence of potential predators (sharks and large fi shes) any
attacks on aggregated fi shes (Colin and Clavijo 1978 ; Colin and Bell 1991 ; Colin
1992 ; Kadison et al. 2006 ) during many hours of observation. Some reliable
reports (e.g. Jim Forrest 2010) do indicate occasional predation by sharks on
aggregated or spawning fi shes. However, it is very diffi cult to observe such events,
as they are probably rare, and just the presence of a human observer may disturb
the activity. It has been suggested reef fi shes often take on a “stupor”-like condition
to allow spawning to continue despite possible risks due to the presence of and
attacks by predators (Johannes 1981 ) , but there is little to support this conjecture
and much that argues against it (see following section).
Despite its occurrence, it seems likely that predation on spawning adults plays
only a minor role in structuring aggregation time and location. In nearly all cases
spawning fi shes are very aware of their surroundings and the sequence of events
leading to spawning can be easily interrupted in its preliminary phases. For example, Colin and Clavijo ( 1978 ) saw no predation attempts on aggregated goatfi shes
in the Virgin Islands, but the simple appearance (no attack) of a large mackerel
( Scomberomorus sp.) caused the entire spawning population (100’s of fi sh) to hide
and not reappear for several minutes. If attacked, the group may be disturbed for a
short period and cease spawning, but quickly resume once the possible danger of
predators has passed. Most potential predators are highly mobile and active at
nearly all times of day and if aggregations shifted to different areas or times, perhaps to reduce predation risks, predators would be easily able to follow such
changes and the risk of predation would remain unchanged. Rather it appears that
individual and collective behaviour and awareness is the observed defence against
adult predators.
Fishing activity on TA’s changes the natural relationship between spawning fi sh
and potential predators. It is believed such activity on TA’s often draws in predators, such as sharks, that might otherwise not occur there (Olsen and LaPlace 1979 ;
Colin 1992 ) and may produce the impression that predators are more prevalent than
they would be in an undisturbed situation. Heyman and Kjerfve ( 2008 ) report various
predators present at the Gladden Spit aggregation site, Belize, with “sharks and
bottlenose dolphins preying on aggregating snappers and groupers, the former, most
commonly after aggregating fi sh were hooked on a fi shing line”.
Predation on eggs after their release occurs relatively often, orders of magnitude
more frequently than predation on spawning adults, but a wide range of egg predation occurrence is documented, making it premature to generalize based on only a
single or few sites (Chap. 2 , Table 2.2). For some RA species a high percentage of
spawns is attacked, probably due to predictability of spawning occurrence and limited height above the bottom where eggs are released. This risk almost certainly
infl uences exactly where and when gametes are released (Colin and Bell 1991 ) , but
would serve to reduce egg predation rather than prevent it. For aggregations egg
5 Timing and Location of Aggregation and Spawning in Reef Fishes
100–1,000 spawns although potential predators are often present near areas where
reef fi shes are spawning (Colin 1978 ; Colin and Clavijo 1988 ; Clifton and
Robertson 1993 ; Craig 1998 ; Sancho et al. 2000b ) . Some authors have failed to
observe, despite the presence of potential predators (sharks and large fi shes) any
attacks on aggregated fi shes (Colin and Clavijo 1978 ; Colin and Bell 1991 ; Colin
1992 ; Kadison et al. 2006 ) during many hours of observation. Some reliable
reports (e.g. Jim Forrest 2010) do indicate occasional predation by sharks on
aggregated or spawning fi shes. However, it is very diffi cult to observe such events,
as they are probably rare, and just the presence of a human observer may disturb
the activity. It has been suggested reef fi shes often take on a “stupor”-like condition
to allow spawning to continue despite possible risks due to the presence of and
attacks by predators (Johannes 1981 ) , but there is little to support this conjecture
and much that argues against it (see following section).
Despite its occurrence, it seems likely that predation on spawning adults plays
only a minor role in structuring aggregation time and location. In nearly all cases
spawning fi shes are very aware of their surroundings and the sequence of events
leading to spawning can be easily interrupted in its preliminary phases. For example, Colin and Clavijo ( 1978 ) saw no predation attempts on aggregated goatfi shes
in the Virgin Islands, but the simple appearance (no attack) of a large mackerel
( Scomberomorus sp.) caused the entire spawning population (100’s of fi sh) to hide
and not reappear for several minutes. If attacked, the group may be disturbed for a
short period and cease spawning, but quickly resume once the possible danger of
predators has passed. Most potential predators are highly mobile and active at
nearly all times of day and if aggregations shifted to different areas or times, perhaps to reduce predation risks, predators would be easily able to follow such
changes and the risk of predation would remain unchanged. Rather it appears that
individual and collective behaviour and awareness is the observed defence against
adult predators.
Fishing activity on TA’s changes the natural relationship between spawning fi sh
and potential predators. It is believed such activity on TA’s often draws in predators, such as sharks, that might otherwise not occur there (Olsen and LaPlace 1979 ;
Colin 1992 ) and may produce the impression that predators are more prevalent than
they would be in an undisturbed situation. Heyman and Kjerfve ( 2008 ) report various
predators present at the Gladden Spit aggregation site, Belize, with “sharks and
bottlenose dolphins preying on aggregating snappers and groupers, the former, most
commonly after aggregating fi sh were hooked on a fi shing line”.
Predation on eggs after their release occurs relatively often, orders of magnitude
more frequently than predation on spawning adults, but a wide range of egg predation occurrence is documented, making it premature to generalize based on only a
single or few sites (Chap. 2 , Table 2.2). For some RA species a high percentage of
spawns is attacked, probably due to predictability of spawning occurrence and limited height above the bottom where eggs are released. This risk almost certainly
infl uences exactly where and when gametes are released (Colin and Bell 1991 ) , but
would serve to reduce egg predation rather than prevent it. For aggregations egg