571
13 Conclusion
and intriguing question of ‘why spawn in aggregations’. They examine the theoretical
frameworks, costs and benefi ts of fi sh spawning aggregations as well as those of a
wide range of other, non-fi sh, group-breeding taxa and ask what insights can be
gained from examining the habit in other taxa? In non-fi sh species, aggregating for
reproduction evolved due to limitation of suitable space or habitat thereby forcing
animals to occupy higher densities than normal at mating sites. Alternatively there
may be active choice by individuals for proximity to conspecifi cs because of benefi ts
to breeding adults or their young.
Social reproduction benefi ts are in two general categories: enhanced reproduction
and reduced predation. Reduced predation within a breeding group can result from
dilution effects (the likelihood of any individual being the target of a predator
decreases as group size increases), earlier detection of predators (increasing probability of escape) or deterring/confusing predators through cooperative efforts among
the group. In addition, the synchrony of reproductive activities in a group may temporarily satiate predators, leading to lower predation per capita when offspring are
vulnerable. For fi shes, at least, as covered in other chapters, there is little indication
that predation, either on spawning adults or on eggs after release, has a major role
in structuring aggregations, despite the likely benefi t of short-term transport of eggs
away from benthic predators.
From what is known about a few well-studied non-fi sh species, including the
variation in social systems, travel costs, the nature of the aggregations themselves,
and the phylogenetic diversity of aggregation-spawning fi shes, there is unlikely to
be a single evolutionary explanation for aggregation spawning. The lack of studies
with experimental manipulations precludes the evaluation of some hypotheses.
Moreover, although predictions from competing hypotheses are not fundamentally
impossible to test, they are harder to tackle in some taxa than in others. Studies of
bird breeding strategies, for example, have developed much farther than for fi shes
because birds are often easier to mark for individual recognition, the parentage of
the young is easier to determine, they often lend themselves better to experimentation, and unexploited populations are more widely accessible for study. Although in
other ways fi shes lend themselves well to comparative study, it is curious that, unlike
many other taxa, the possible importance of specifi c habitats for marine fi sh aggregations has received little attention.
In Chap. 4 , with a focus on fi sh taxa, Howard Choat examines the biological
features associated with aggregation spawning. He fi nds that maximum body size,
trophic ecology, and anatomy are more predictive of aggregation versus nonaggregation habits than are other life history features. As long as the different
aggregation types (RA and TA) share basic properties of size, nutritional ecology and
anatomy, he found that they manifested similar spawning behaviours regardless of
whether fi sh species are protogynous or gonochoristic, short- or long-lived or have
slow or fast population turnover rates. Neither larval retention nor broad dispersal
are seen as critical elements in the evolution of spawning aggregations. Choat
hypothesizes that differences in aggregate spawning patterns and their underlying
processes will occur in the Pacifi c and Atlantic oceans, a refl ection of the different
histories, oceanic environments and habitat structures of these two ocean basins.
13 Conclusion
and intriguing question of ‘why spawn in aggregations’. They examine the theoretical
frameworks, costs and benefi ts of fi sh spawning aggregations as well as those of a
wide range of other, non-fi sh, group-breeding taxa and ask what insights can be
gained from examining the habit in other taxa? In non-fi sh species, aggregating for
reproduction evolved due to limitation of suitable space or habitat thereby forcing
animals to occupy higher densities than normal at mating sites. Alternatively there
may be active choice by individuals for proximity to conspecifi cs because of benefi ts
to breeding adults or their young.
Social reproduction benefi ts are in two general categories: enhanced reproduction
and reduced predation. Reduced predation within a breeding group can result from
dilution effects (the likelihood of any individual being the target of a predator
decreases as group size increases), earlier detection of predators (increasing probability of escape) or deterring/confusing predators through cooperative efforts among
the group. In addition, the synchrony of reproductive activities in a group may temporarily satiate predators, leading to lower predation per capita when offspring are
vulnerable. For fi shes, at least, as covered in other chapters, there is little indication
that predation, either on spawning adults or on eggs after release, has a major role
in structuring aggregations, despite the likely benefi t of short-term transport of eggs
away from benthic predators.
From what is known about a few well-studied non-fi sh species, including the
variation in social systems, travel costs, the nature of the aggregations themselves,
and the phylogenetic diversity of aggregation-spawning fi shes, there is unlikely to
be a single evolutionary explanation for aggregation spawning. The lack of studies
with experimental manipulations precludes the evaluation of some hypotheses.
Moreover, although predictions from competing hypotheses are not fundamentally
impossible to test, they are harder to tackle in some taxa than in others. Studies of
bird breeding strategies, for example, have developed much farther than for fi shes
because birds are often easier to mark for individual recognition, the parentage of
the young is easier to determine, they often lend themselves better to experimentation, and unexploited populations are more widely accessible for study. Although in
other ways fi shes lend themselves well to comparative study, it is curious that, unlike
many other taxa, the possible importance of specifi c habitats for marine fi sh aggregations has received little attention.
In Chap. 4 , with a focus on fi sh taxa, Howard Choat examines the biological
features associated with aggregation spawning. He fi nds that maximum body size,
trophic ecology, and anatomy are more predictive of aggregation versus nonaggregation habits than are other life history features. As long as the different
aggregation types (RA and TA) share basic properties of size, nutritional ecology and
anatomy, he found that they manifested similar spawning behaviours regardless of
whether fi sh species are protogynous or gonochoristic, short- or long-lived or have
slow or fast population turnover rates. Neither larval retention nor broad dispersal
are seen as critical elements in the evolution of spawning aggregations. Choat
hypothesizes that differences in aggregate spawning patterns and their underlying
processes will occur in the Pacifi c and Atlantic oceans, a refl ection of the different
histories, oceanic environments and habitat structures of these two ocean basins.