71
3 Why Spawn in Aggregations?
3.4 Survival Benefi ts and Costs
3.4.1 Dilution of Predation on Adults and Eggs
Spawning aggregations are conspicuous phenomena. In addition to the increased
density of fi shes, mating behaviours exhibited by the attending individuals are often
elaborate and showy (Clifton and Robertson 1993 ) . Furthermore, aggregations are
spatially and temporally predictable. As such, aggregations may be particularly
attractive to predators of both eggs and spawning adults. For example, whale sharks,
Rhincodon typus , are known to aggregate around snapper spawning aggregations in
Belize, where they feed on recently spawned gametes (Heyman et al. 2001 ) .
Similarly, Nassau grouper aggregations around St Thomas, BVI, attract shark predators (Olsen and LaPlace 1979 ) , although this may have been a consequence of the
fi shing activity, rather than the aggregation per se (Chap. 5 ).
However, although aggregative spawning might increase detection by predators,
doing so may reduce per capita predation rates on adult and/or egg. Although adult
and egg predation by fi shes at spawning aggregations has been reported (e.g. Colin
1978 ; Olsen and LaPlace 1979 ; Bell and Colin 1986 ; Colin et al. 1987 ; AguilarPerera 1994 ; Samoilys and Squire 1994 ; Samoilys 1997 ; Craig 1998 ; Pelaprat 1999 ;
Sancho 2000 ; Heyman et al. 2005 ) , predation rates are usually low (Colin and
Clavijo 1988 ; Colin and Bell 1991 ; Sancho et al. 2000 ) and, perhaps as such, have
rarely been studied in detail (Chap. 5 ). Sancho et al. ( 2000 ) quantifi ed predation on
a wide range of species spawning at an aggregation site at Johnston Atoll in the
Central Pacifi c. Two predatory species, the bluefi n trevally, Caranx melampygus ,
and the small-toothed jobfi sh, Aphareus furca , concentrated their attacks on individuals participating in spawning rushes. A total of 2.3% of all spawning rushes
were attacked, and 4% of attacks were successful, resulting in a rate of successful
predation on spawning rushes of 0.1% for 33 species combined. Species-specifi c
rates are much lower. Per capita predation rates were not reported but they are likely
to be extremely small because attacks were concentrated on spawning groups of
four or more individuals. Similarly, despite the ubiquity of egg predation at spawning grounds and the appeal of the idea that spawning aggregations form to increase
egg survivorship, per spawner egg predation rates have not been quantifi ed.
To obtain such data, more detailed comparisons of predation rates on and off
aggregations are required. Ideally, these data should come from observations of
spawning individuals. These studies should focus on species known to spawn in
groups and in isolated pairs. Examples include various parrotfi shes and wrasses
(Randall and Randall 1963 ; Robertson and Warner 1978 ; Warner and Robertson
1978 ; Colin and Clavijo 1988 ) , dog snapper, Lutjanus jocu (Krajewski and Bonaldo
2005 ) , and several surgeonfi shes (Acanthuridae) (Colin and Clavijo 1988 ) .
Nevertheless, such studies will be hampered by how seldom spawning and predation
co-occur.
It is also important to remember that although spawning aggregations may have
evolved, at least in part, because of predation dilution advantages, this survivorship
3 Why Spawn in Aggregations?
3.4 Survival Benefi ts and Costs
3.4.1 Dilution of Predation on Adults and Eggs
Spawning aggregations are conspicuous phenomena. In addition to the increased
density of fi shes, mating behaviours exhibited by the attending individuals are often
elaborate and showy (Clifton and Robertson 1993 ) . Furthermore, aggregations are
spatially and temporally predictable. As such, aggregations may be particularly
attractive to predators of both eggs and spawning adults. For example, whale sharks,
Rhincodon typus , are known to aggregate around snapper spawning aggregations in
Belize, where they feed on recently spawned gametes (Heyman et al. 2001 ) .
Similarly, Nassau grouper aggregations around St Thomas, BVI, attract shark predators (Olsen and LaPlace 1979 ) , although this may have been a consequence of the
fi shing activity, rather than the aggregation per se (Chap. 5 ).
However, although aggregative spawning might increase detection by predators,
doing so may reduce per capita predation rates on adult and/or egg. Although adult
and egg predation by fi shes at spawning aggregations has been reported (e.g. Colin
1978 ; Olsen and LaPlace 1979 ; Bell and Colin 1986 ; Colin et al. 1987 ; AguilarPerera 1994 ; Samoilys and Squire 1994 ; Samoilys 1997 ; Craig 1998 ; Pelaprat 1999 ;
Sancho 2000 ; Heyman et al. 2005 ) , predation rates are usually low (Colin and
Clavijo 1988 ; Colin and Bell 1991 ; Sancho et al. 2000 ) and, perhaps as such, have
rarely been studied in detail (Chap. 5 ). Sancho et al. ( 2000 ) quantifi ed predation on
a wide range of species spawning at an aggregation site at Johnston Atoll in the
Central Pacifi c. Two predatory species, the bluefi n trevally, Caranx melampygus ,
and the small-toothed jobfi sh, Aphareus furca , concentrated their attacks on individuals participating in spawning rushes. A total of 2.3% of all spawning rushes
were attacked, and 4% of attacks were successful, resulting in a rate of successful
predation on spawning rushes of 0.1% for 33 species combined. Species-specifi c
rates are much lower. Per capita predation rates were not reported but they are likely
to be extremely small because attacks were concentrated on spawning groups of
four or more individuals. Similarly, despite the ubiquity of egg predation at spawning grounds and the appeal of the idea that spawning aggregations form to increase
egg survivorship, per spawner egg predation rates have not been quantifi ed.
To obtain such data, more detailed comparisons of predation rates on and off
aggregations are required. Ideally, these data should come from observations of
spawning individuals. These studies should focus on species known to spawn in
groups and in isolated pairs. Examples include various parrotfi shes and wrasses
(Randall and Randall 1963 ; Robertson and Warner 1978 ; Warner and Robertson
1978 ; Colin and Clavijo 1988 ) , dog snapper, Lutjanus jocu (Krajewski and Bonaldo
2005 ) , and several surgeonfi shes (Acanthuridae) (Colin and Clavijo 1988 ) .
Nevertheless, such studies will be hampered by how seldom spawning and predation
co-occur.
It is also important to remember that although spawning aggregations may have
evolved, at least in part, because of predation dilution advantages, this survivorship