11
1 Revisiting Spawning Aggregations: Defi nitions and Challenges
(Drazen et al. 2003 ) . Although orange roughy produce a planktonic egg, both the
sculpin and cephalopod have demersal eggs that are guarded.
1.6 Methods for Documenting a Spawning Aggregation
To properly distinguish a spawning aggregation from other forms of aggregations
(e.g. feeding aggregations, shelter aggregations etc.), it is important to carefully document evidence of spawning. To this end, Colin et al. ( 2003 ) identifi ed a suite of direct
and indirect indications of spawning. The following three criteria have been used to
verify directly that the fi sh are gathering for the purpose of spawning: (1) undisputed
spawning observations, (2) females with hydrated eggs and (3) presence of postovulatory follicles in the ovaries of aggregating females. A fourth means of directly
documenting the presence of a spawning aggregation is added here: (4) identifi cation
of very early stage eggs and larvae that can be positively associated with the aggregating species. Recent work on black marlin, Makaira indica (Domeier and Speare
unpublished data), was able to confi rm spawning through the presence of hydrated
eggs, post-ovulatory follicles and the presence of larvae from 0–13 days post hatching. Plankton tows can now be considered a valuable means of verifying the act of
spawning over an aggregation site, without the need to sacrifi ce any spawning adults.
If none of the direct signs of spawning are observed, indirect signs can be used
to document new aggregations for species already proven to form spawning aggregations. Indirect signs can include behaviours or colour patterns, if these are demonstrably known to be associated only with spawning, as well as gonadosomatic index
(GSI) (Chap. 9 ) data or the presence of swollen abdomens (indicating the presence
of hydrated eggs) in a large percentage of the aggregated individuals. In the absence
of witnessing the spawning event, it is not realistically possible to gather enough
information to document spawning without sampling ovaries or larvae. Testes
are not good indicators of the precise timing of spawning since males are running
ripe prior to, and after, the actual spawning events. Sample collection should be a
very high priority for all studies of spawning aggregations that involve new species,
or unusual examples of species already known to aggregate (e.g. uncharacteristic
site or season).
Beyond the documentation of spawning, it is also important, under the current
defi nition, to document that spawning is occurring in densities of fi sh at least four
times greater than that of the non-reproductive season/habitat, and that the spawning aggregation is predictably repeated in time and space. Methods for conducting
underwater surveys have been described in a comprehensive methods manual (Colin
et al. 2003 , Chapter 9).
The scientifi c and grey literature on spawning aggregations contains many examples
of poorly documented ‘spawning aggregations’ that are then perpetuated when
they are cited, illustrating the need for rigorous fi eld methods and for peer-review
of studies that claim to document a spawning aggregation. The Society for the
Conservation of Reef Fish Aggregations has compiled a global database of spawning
1 Revisiting Spawning Aggregations: Defi nitions and Challenges
(Drazen et al. 2003 ) . Although orange roughy produce a planktonic egg, both the
sculpin and cephalopod have demersal eggs that are guarded.
1.6 Methods for Documenting a Spawning Aggregation
To properly distinguish a spawning aggregation from other forms of aggregations
(e.g. feeding aggregations, shelter aggregations etc.), it is important to carefully document evidence of spawning. To this end, Colin et al. ( 2003 ) identifi ed a suite of direct
and indirect indications of spawning. The following three criteria have been used to
verify directly that the fi sh are gathering for the purpose of spawning: (1) undisputed
spawning observations, (2) females with hydrated eggs and (3) presence of postovulatory follicles in the ovaries of aggregating females. A fourth means of directly
documenting the presence of a spawning aggregation is added here: (4) identifi cation
of very early stage eggs and larvae that can be positively associated with the aggregating species. Recent work on black marlin, Makaira indica (Domeier and Speare
unpublished data), was able to confi rm spawning through the presence of hydrated
eggs, post-ovulatory follicles and the presence of larvae from 0–13 days post hatching. Plankton tows can now be considered a valuable means of verifying the act of
spawning over an aggregation site, without the need to sacrifi ce any spawning adults.
If none of the direct signs of spawning are observed, indirect signs can be used
to document new aggregations for species already proven to form spawning aggregations. Indirect signs can include behaviours or colour patterns, if these are demonstrably known to be associated only with spawning, as well as gonadosomatic index
(GSI) (Chap. 9 ) data or the presence of swollen abdomens (indicating the presence
of hydrated eggs) in a large percentage of the aggregated individuals. In the absence
of witnessing the spawning event, it is not realistically possible to gather enough
information to document spawning without sampling ovaries or larvae. Testes
are not good indicators of the precise timing of spawning since males are running
ripe prior to, and after, the actual spawning events. Sample collection should be a
very high priority for all studies of spawning aggregations that involve new species,
or unusual examples of species already known to aggregate (e.g. uncharacteristic
site or season).
Beyond the documentation of spawning, it is also important, under the current
defi nition, to document that spawning is occurring in densities of fi sh at least four
times greater than that of the non-reproductive season/habitat, and that the spawning aggregation is predictably repeated in time and space. Methods for conducting
underwater surveys have been described in a comprehensive methods manual (Colin
et al. 2003 , Chapter 9).
The scientifi c and grey literature on spawning aggregations contains many examples
of poorly documented ‘spawning aggregations’ that are then perpetuated when
they are cited, illustrating the need for rigorous fi eld methods and for peer-review
of studies that claim to document a spawning aggregation. The Society for the
Conservation of Reef Fish Aggregations has compiled a global database of spawning