13
1 Revisiting Spawning Aggregations: Defi nitions and Challenges
spawning site, and (4) how far do the offspring disperse from the aggregation site?
Although these questions may superfi cially appear esoteric, they are not. There are
serious fi sheries and economic issues that could draw immediate benefi t if we knew
the answers. For example, once a spawning aggregation is fi shed into extinction,
will it ever recover? If it will recover, how long will that take? And, will it reassemble at the very same aggregation site? What if we learned so much about spawning aggregations that we could move one from an unprotected site to a protected
site, or cause one to spontaneously form within a protected site, or create one from
restocked animals? It would seem easier to simply protect the best spawning aggregation sites, but sometimes the necessary policy is too late or politically impossible.
Chapters in this book address some of these intriguing questions in much more
detail, but for the sake of argument I will highlight some of my personal favourite
hypotheses.
Numerous hypotheses have been proposed to address why spawning aggregations
form at all. These can be broken down into two main themes: those that address
benefi ts to the larvae and those that address benefi ts to adults. Presumably the selective
pressures that led to the evolution of aggregative spawning resulted in a reproductive
strategy that presents a relatively high level of reproductive success for each individual. Past discussions of spawning aggregations have listed numerous hypotheses
as to why marine animals aggregate to spawn; all of them extremely diffi cult to test
(e.g. Shapiro et al. 1988 ; Mora and Sale 2002 ) . When considering the overall life
history of reef fi shes, it is the larval phase that is most subject to mortality, creating
a situation where very small benefi ts to the larval phase could lead to signifi cantly
more recruitment, particularly for species with high fecundity (Chap. 4 ). Therefore,
it is likely that benefi ts to the larvae are an important driving force behind the evolution of the spawning aggregation; however, other hypotheses cannot be discounted.
The selection of spawning aggregation sites and times is one of the most intriguing
phenomena related to this reproductive strategy. If we continue to assume, as I do,
that benefi ts to larvae, expressed by increased recruitment, are driving the selection
of spawning aggregation sites, then we are faced with two scenarios that might lead
to their selection. (1) the process involves selecting sites that generate the highest
level of local recruitment, and (2) a combination of certain topographic and oceanographic conditions predictably leads to increased recruitment at a very general level
that is independent of spatial scale. Under the fi rst scenario local recruitment is a
signifi cant proportion of overall recruitment generating a feedback loop to guide
site selection. Under the second scenario site selection would be guided purely by
instinctual/genetically determined detection of the relevant local conditions leading
to a predictable choice of spawning site.
What is the catalyst for the formation of a spawning aggregation in a region
where one did not previously exist? Although such a genesis has never been documented, it certainly has occurred on an evolutionary time scale. Some if not all
species that aggregate to spawn are capable of spawning outside of an aggregation.
If this were not true, species that form aggregations would not spawn in pairs or
small numbers in captivity, and yet they do [e.g. mutton snapper, Lutjanus analis
(Watanabe et al. 1998 ) , mangrove red snapper, L. argentimaculatus (Emata 2003 ) ,
Nassau grouper, Epinephelus striatus (Manday and Fernandez 1966 ; Tucker et al. 1996 )
1 Revisiting Spawning Aggregations: Defi nitions and Challenges
spawning site, and (4) how far do the offspring disperse from the aggregation site?
Although these questions may superfi cially appear esoteric, they are not. There are
serious fi sheries and economic issues that could draw immediate benefi t if we knew
the answers. For example, once a spawning aggregation is fi shed into extinction,
will it ever recover? If it will recover, how long will that take? And, will it reassemble at the very same aggregation site? What if we learned so much about spawning aggregations that we could move one from an unprotected site to a protected
site, or cause one to spontaneously form within a protected site, or create one from
restocked animals? It would seem easier to simply protect the best spawning aggregation sites, but sometimes the necessary policy is too late or politically impossible.
Chapters in this book address some of these intriguing questions in much more
detail, but for the sake of argument I will highlight some of my personal favourite
hypotheses.
Numerous hypotheses have been proposed to address why spawning aggregations
form at all. These can be broken down into two main themes: those that address
benefi ts to the larvae and those that address benefi ts to adults. Presumably the selective
pressures that led to the evolution of aggregative spawning resulted in a reproductive
strategy that presents a relatively high level of reproductive success for each individual. Past discussions of spawning aggregations have listed numerous hypotheses
as to why marine animals aggregate to spawn; all of them extremely diffi cult to test
(e.g. Shapiro et al. 1988 ; Mora and Sale 2002 ) . When considering the overall life
history of reef fi shes, it is the larval phase that is most subject to mortality, creating
a situation where very small benefi ts to the larval phase could lead to signifi cantly
more recruitment, particularly for species with high fecundity (Chap. 4 ). Therefore,
it is likely that benefi ts to the larvae are an important driving force behind the evolution of the spawning aggregation; however, other hypotheses cannot be discounted.
The selection of spawning aggregation sites and times is one of the most intriguing
phenomena related to this reproductive strategy. If we continue to assume, as I do,
that benefi ts to larvae, expressed by increased recruitment, are driving the selection
of spawning aggregation sites, then we are faced with two scenarios that might lead
to their selection. (1) the process involves selecting sites that generate the highest
level of local recruitment, and (2) a combination of certain topographic and oceanographic conditions predictably leads to increased recruitment at a very general level
that is independent of spatial scale. Under the fi rst scenario local recruitment is a
signifi cant proportion of overall recruitment generating a feedback loop to guide
site selection. Under the second scenario site selection would be guided purely by
instinctual/genetically determined detection of the relevant local conditions leading
to a predictable choice of spawning site.
What is the catalyst for the formation of a spawning aggregation in a region
where one did not previously exist? Although such a genesis has never been documented, it certainly has occurred on an evolutionary time scale. Some if not all
species that aggregate to spawn are capable of spawning outside of an aggregation.
If this were not true, species that form aggregations would not spawn in pairs or
small numbers in captivity, and yet they do [e.g. mutton snapper, Lutjanus analis
(Watanabe et al. 1998 ) , mangrove red snapper, L. argentimaculatus (Emata 2003 ) ,
Nassau grouper, Epinephelus striatus (Manday and Fernandez 1966 ; Tucker et al. 1996 )