73
3 Why Spawn in Aggregations?
Slotte and Fiksen 2000 ; Peres and Klippel 2003 ) . In addition, individuals may be
more conspicuous to predators while travelling to spawning grounds along migration
routes, which are often highly predictable (Starr et al. 2007 and references therein).
Many aggregation-spawning groupers show non-spawning home-range fi delity (e.g.
Beets and Hixon 1994 ; Lembo et al. 1999 ; Kaunda-Arara and Rose 2004 ) and may
return to the same non-spawning site after spawning (Waschkewitz and Wirtz 1990 ) .
Attending spawning aggregations can result in displacement from non-spawning
territories (e.g. Nassau grouper, Patrick Colin personal communication) (Chap. 2 ).
3.5 Habitat Limitation as a Cause of Spawning Aggregations
The spatial rarity of spawning aggregations in many cases, along with their apparent
site traditionality, might suggest that these aggregations form around specifi c features
that are limited in availability (Chap. 5 ). These sites may have inherent characteristics
that are benefi cial to spawning fi sh, regardless of group size.
Hydrographic conditions that promote the swift removal of spawned eggs from
the reef and its associated planktivorous predators have long been assumed, although
not demonstrated, to be important determinants of spawning aggregation locations
and timing (Johannes 1978 ; Lobel 1978 ) . The same conditions could also enhance
dispersal of larvae into new habitats (Barlow 1981 ; Doherty et al. 1985 ) or, alternatively, increase larval retention into areas that were suitable for the parents (Johannes
1978 ; Lobel 1978 ; Lobel and Robinson 1988 ) . If so, spawning aggregations should
occur at sites and times with, for example, tidal fl ows, non-tidal current speed and/
or direction that differ in a predictable manner from random sites (Chaps. 5 and 6 ).
Scepticism regarding these hypotheses and, more notably, the efforts taken to test
them was raised over 20 years ago (Shapiro et al. 1988 ) . Our understanding of the
role of hydrographic conditions in determining where spawning aggregations are
located has not improved greatly since then (Chap. 6 ). Comparisons of spawning to
non-spawning sites have only been made for a few species. For example, spawning
aggregations of brown puller damselfi sh, Chromis hypsilepis , do not coincide with
tidal regimes that appear to facilitate either early survival or dispersal of larvae
(Gladstone 2007b ) . Similarly, Colin ( 1992 ) found that currents at Nassau grouper
aggregation sites do not favour offshore transport of eggs. Drazen et al. ( 2003 )
hypothesised (but did not test) that blob sculpin aggregate on seamounts to spawn
due to the faster current fl ow across these mounts. Releasing eggs high above the
substratum has also been suggested as being important, and the perception that
spawning aggregations are often located around promontories appears to be consistent with this idea. Nevertheless, there is little evidence supporting this idea (Shapiro
et al. 1988 , Chapter 6, 7). For example, starting heights and lengths of spawning
rushes varied widely among 45 Indo-Pacifi c species of wrasses and parrotfi sh at
Enewetak Atoll, Marshall Islands, implying no inherent benefi t associated with
spawning at a particular height (Colin and Bell 1991 ) . More detailed consideration
of topographic and oceanographic features of spawning sites is given in Chap. 5 .
3 Why Spawn in Aggregations?
Slotte and Fiksen 2000 ; Peres and Klippel 2003 ) . In addition, individuals may be
more conspicuous to predators while travelling to spawning grounds along migration
routes, which are often highly predictable (Starr et al. 2007 and references therein).
Many aggregation-spawning groupers show non-spawning home-range fi delity (e.g.
Beets and Hixon 1994 ; Lembo et al. 1999 ; Kaunda-Arara and Rose 2004 ) and may
return to the same non-spawning site after spawning (Waschkewitz and Wirtz 1990 ) .
Attending spawning aggregations can result in displacement from non-spawning
territories (e.g. Nassau grouper, Patrick Colin personal communication) (Chap. 2 ).
3.5 Habitat Limitation as a Cause of Spawning Aggregations
The spatial rarity of spawning aggregations in many cases, along with their apparent
site traditionality, might suggest that these aggregations form around specifi c features
that are limited in availability (Chap. 5 ). These sites may have inherent characteristics
that are benefi cial to spawning fi sh, regardless of group size.
Hydrographic conditions that promote the swift removal of spawned eggs from
the reef and its associated planktivorous predators have long been assumed, although
not demonstrated, to be important determinants of spawning aggregation locations
and timing (Johannes 1978 ; Lobel 1978 ) . The same conditions could also enhance
dispersal of larvae into new habitats (Barlow 1981 ; Doherty et al. 1985 ) or, alternatively, increase larval retention into areas that were suitable for the parents (Johannes
1978 ; Lobel 1978 ; Lobel and Robinson 1988 ) . If so, spawning aggregations should
occur at sites and times with, for example, tidal fl ows, non-tidal current speed and/
or direction that differ in a predictable manner from random sites (Chaps. 5 and 6 ).
Scepticism regarding these hypotheses and, more notably, the efforts taken to test
them was raised over 20 years ago (Shapiro et al. 1988 ) . Our understanding of the
role of hydrographic conditions in determining where spawning aggregations are
located has not improved greatly since then (Chap. 6 ). Comparisons of spawning to
non-spawning sites have only been made for a few species. For example, spawning
aggregations of brown puller damselfi sh, Chromis hypsilepis , do not coincide with
tidal regimes that appear to facilitate either early survival or dispersal of larvae
(Gladstone 2007b ) . Similarly, Colin ( 1992 ) found that currents at Nassau grouper
aggregation sites do not favour offshore transport of eggs. Drazen et al. ( 2003 )
hypothesised (but did not test) that blob sculpin aggregate on seamounts to spawn
due to the faster current fl ow across these mounts. Releasing eggs high above the
substratum has also been suggested as being important, and the perception that
spawning aggregations are often located around promontories appears to be consistent with this idea. Nevertheless, there is little evidence supporting this idea (Shapiro
et al. 1988 , Chapter 6, 7). For example, starting heights and lengths of spawning
rushes varied widely among 45 Indo-Pacifi c species of wrasses and parrotfi sh at
Enewetak Atoll, Marshall Islands, implying no inherent benefi t associated with
spawning at a particular height (Colin and Bell 1991 ) . More detailed consideration
of topographic and oceanographic features of spawning sites is given in Chap. 5 .