77
3 Why Spawn in Aggregations?
3.7 Conclusions and Future Directions
A few fi sh species that spawn in aggregations have been very well studied. For these
species, it appears that there is no consistent pattern pointing to a single explanation
for the evolution of spawning in aggregations, e.g. social benefi ts of aggregating
versus space limitation. This is not surprising, because there is considerable variation among species in social systems, travel costs, and the nature of the aggregations
themselves. The phylogenetic diversity of aggregation-spawning fi shes (Domeier
and Colin 1997 , Chapter 4) also makes it unlikely that there should be a single evolutionary explanation for aggregation spawning. Most studies lack experimental
manipulations, which has prevented defi nitive conclusions. Although predictions
from competing hypotheses are not fundamentally impossible to test, they are harder
to tackle in some taxa than in others. For example, studies of birds breeding in leks
and in colonies have gone much farther than studies of 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.
To move forward in our understanding of fi sh aggregations, it would be helpful
to study more model systems where individuals can be tracked and their reproductive success recorded. For example, studies of groupers (e.g. Nemeth et al. 2007 ;
Starr et al. 2007 ) have been very informative about travel costs and individual variation
in behaviour, and studies of Atlantic cod in large tanks have been helpful for measuring individual differences in behaviour and reproductive success (Rowe et al.
2008 ) . There is very little direct evidence concerning predation dilution benefi ts of
spawning aggregations. However, as most fi sheries around the world are causing
rapid depletion of top predators, perhaps progress can be made by comparing the
timing, density, and behaviour of fi sh in spawning aggregations inside and outside
of marine protected areas.
Many spawning aggregations are themselves targeted heavily by fi sheries (Chaps.
8 and 11 ). It would be interesting to study changes in spawning behaviour as traditional aggregations become depleted or recover from over-fi shing. For example, it
would be useful to test for Allee effects, also known as depensation in the fi sheries
literature, whereby components of individual fi tness decline with decreasing group
sizes (Courchamp et al. 2008 ) (Chap. 8 ). Again, comparisons of aggregations inside
and outside of marine protected areas or across fi shing gradients would be helpful
here. Such studies would both help elucidate costs and benefi ts of spawning in
aggregations, and they might also shed some light on the specifi c management
needs and the effectiveness of management actions taken to protect species that
exhibit this remarkable behaviour.
Acknowledgements This is a contribution from Project Seahorse at University of British
Columbia, and the Earth2Ocean group at Simon Fraser University. PPM was supported by a
Leverhulme Study Abroad Studentship # 2/SAS/2006/0057 and a Canadian Bureau for International
Education postdoctoral research fellowship. JDR and IMC were supported by NSERC of Canada
Discovery Grants. Thanks to Yvonne Sadovy and Patrick Colin for useful comments on previous
drafts of this chapter.
3 Why Spawn in Aggregations?
3.7 Conclusions and Future Directions
A few fi sh species that spawn in aggregations have been very well studied. For these
species, it appears that there is no consistent pattern pointing to a single explanation
for the evolution of spawning in aggregations, e.g. social benefi ts of aggregating
versus space limitation. This is not surprising, because there is considerable variation among species in social systems, travel costs, and the nature of the aggregations
themselves. The phylogenetic diversity of aggregation-spawning fi shes (Domeier
and Colin 1997 , Chapter 4) also makes it unlikely that there should be a single evolutionary explanation for aggregation spawning. Most studies lack experimental
manipulations, which has prevented defi nitive conclusions. Although predictions
from competing hypotheses are not fundamentally impossible to test, they are harder
to tackle in some taxa than in others. For example, studies of birds breeding in leks
and in colonies have gone much farther than studies of 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.
To move forward in our understanding of fi sh aggregations, it would be helpful
to study more model systems where individuals can be tracked and their reproductive success recorded. For example, studies of groupers (e.g. Nemeth et al. 2007 ;
Starr et al. 2007 ) have been very informative about travel costs and individual variation
in behaviour, and studies of Atlantic cod in large tanks have been helpful for measuring individual differences in behaviour and reproductive success (Rowe et al.
2008 ) . There is very little direct evidence concerning predation dilution benefi ts of
spawning aggregations. However, as most fi sheries around the world are causing
rapid depletion of top predators, perhaps progress can be made by comparing the
timing, density, and behaviour of fi sh in spawning aggregations inside and outside
of marine protected areas.
Many spawning aggregations are themselves targeted heavily by fi sheries (Chaps.
8 and 11 ). It would be interesting to study changes in spawning behaviour as traditional aggregations become depleted or recover from over-fi shing. For example, it
would be useful to test for Allee effects, also known as depensation in the fi sheries
literature, whereby components of individual fi tness decline with decreasing group
sizes (Courchamp et al. 2008 ) (Chap. 8 ). Again, comparisons of aggregations inside
and outside of marine protected areas or across fi shing gradients would be helpful
here. Such studies would both help elucidate costs and benefi ts of spawning in
aggregations, and they might also shed some light on the specifi c management
needs and the effectiveness of management actions taken to protect species that
exhibit this remarkable behaviour.
Acknowledgements This is a contribution from Project Seahorse at University of British
Columbia, and the Earth2Ocean group at Simon Fraser University. PPM was supported by a
Leverhulme Study Abroad Studentship # 2/SAS/2006/0057 and a Canadian Bureau for International
Education postdoctoral research fellowship. JDR and IMC were supported by NSERC of Canada
Discovery Grants. Thanks to Yvonne Sadovy and Patrick Colin for useful comments on previous
drafts of this chapter.