15
1 Revisiting Spawning Aggregations: Defi nitions and Challenges
croaker, Larimichthyes crocea , in China did not result in restoration of viable
populations in the wild (Liu and Sadovy de Mitcheson 2008 ) .
1.8 Spawning Aggregations as Ecological
and Fisheries Indicators
Spawning aggregations can be quickly disrupted and eradicated by intense fi shing
pressure (e.g. Sala et al. 2001 ; Sadovy and Domeier 2005 ) , and once eradicated,
they are not known to recover. In addition, regions of particularly heavy fi shing
pressure tend to be devoid of spawning aggregations (Sadovy de Mitcheson et al.
2008 ) . The presence of a spawning aggregation at a particular site can therefore be
considered an indicator of long-term ecological stability. Moreover, ecosystems that
include the formation of feeding aggregations, predictable in time and space, that
target the aggregating adults or mass release of eggs from a spawning aggregation
may be an indication of apex ecological stability. A spawning aggregation must
form and persist for some length of time before a feeding aggregation could evolve
to target the aggregation. Whale sharks, Rhincodon typus (Heyman et al. 2001 ;
Hoffmayer et al. 2007 ) , and manta rays, Manta birostris (Lance Millbrand personal
communication), have been observed to aggregate over spawning aggregations to feed
on eggs of cubera snapper, Lutjanus cyanopterus , and convict surgeonfi sh, Acanthurus
triostegus , respectively, while cephalopods, elasmobranchs, teleosts, mammals and
birds prey on spawning adults at aggregation sites (Gorka et al. 2000 ; Smale et al.
2001 ; Bogetveit et al. 2008 ) . Similarly, non-resident bull sharks, Carcharhinus leucas ,
appear to aggregate at the Great Barrier Reef during a presumed spawning aggregation of black marlin (MLD unpublished data). Since there is no way to age a spawning
aggregation it is impossible to determine the length of time necessary for predatory
aggregations to coincidentally form over spawning aggregations; whether it takes just
a few, or thousands of generations is an important question.
Finally, spawning aggregations provide a unique opportunity to assess the fi sh
stock. Estimating stocks of non-schooling reef fi shes is very diffi cult, but if they
form spawning aggregations they can be directly counted at the aggregation site.
Landings data from these aggregations, on the other hand, may not be useful for
stock assessment since the stock may overwhelm effort until it is drastically reduced
through hyperstability (Sadovy and Domeier 2005 , Chapter 8, 11). To illustrate this
point, one need only envision a child sticking his arm into a cookie jar that he cannot
see into; each reach into the jar produces a cookie until suddenly the jar is empty.
Similarly, a small fl eet fi shing an aggregation may not show decreased catch per
unit of effort (CPUE) until the aggregation is nearly extirpated. However, structured
underwater visual census can produce valuable monitoring data relevant for
assessing the stock independent of fi shery measures. Conducting annual assessments of the size of an aggregation, even if the assessment is an index rather than
an actual count, can provide valuable real-time data as long as the sampling
1 Revisiting Spawning Aggregations: Defi nitions and Challenges
croaker, Larimichthyes crocea , in China did not result in restoration of viable
populations in the wild (Liu and Sadovy de Mitcheson 2008 ) .
1.8 Spawning Aggregations as Ecological
and Fisheries Indicators
Spawning aggregations can be quickly disrupted and eradicated by intense fi shing
pressure (e.g. Sala et al. 2001 ; Sadovy and Domeier 2005 ) , and once eradicated,
they are not known to recover. In addition, regions of particularly heavy fi shing
pressure tend to be devoid of spawning aggregations (Sadovy de Mitcheson et al.
2008 ) . The presence of a spawning aggregation at a particular site can therefore be
considered an indicator of long-term ecological stability. Moreover, ecosystems that
include the formation of feeding aggregations, predictable in time and space, that
target the aggregating adults or mass release of eggs from a spawning aggregation
may be an indication of apex ecological stability. A spawning aggregation must
form and persist for some length of time before a feeding aggregation could evolve
to target the aggregation. Whale sharks, Rhincodon typus (Heyman et al. 2001 ;
Hoffmayer et al. 2007 ) , and manta rays, Manta birostris (Lance Millbrand personal
communication), have been observed to aggregate over spawning aggregations to feed
on eggs of cubera snapper, Lutjanus cyanopterus , and convict surgeonfi sh, Acanthurus
triostegus , respectively, while cephalopods, elasmobranchs, teleosts, mammals and
birds prey on spawning adults at aggregation sites (Gorka et al. 2000 ; Smale et al.
2001 ; Bogetveit et al. 2008 ) . Similarly, non-resident bull sharks, Carcharhinus leucas ,
appear to aggregate at the Great Barrier Reef during a presumed spawning aggregation of black marlin (MLD unpublished data). Since there is no way to age a spawning
aggregation it is impossible to determine the length of time necessary for predatory
aggregations to coincidentally form over spawning aggregations; whether it takes just
a few, or thousands of generations is an important question.
Finally, spawning aggregations provide a unique opportunity to assess the fi sh
stock. Estimating stocks of non-schooling reef fi shes is very diffi cult, but if they
form spawning aggregations they can be directly counted at the aggregation site.
Landings data from these aggregations, on the other hand, may not be useful for
stock assessment since the stock may overwhelm effort until it is drastically reduced
through hyperstability (Sadovy and Domeier 2005 , Chapter 8, 11). To illustrate this
point, one need only envision a child sticking his arm into a cookie jar that he cannot
see into; each reach into the jar produces a cookie until suddenly the jar is empty.
Similarly, a small fl eet fi shing an aggregation may not show decreased catch per
unit of effort (CPUE) until the aggregation is nearly extirpated. However, structured
underwater visual census can produce valuable monitoring data relevant for
assessing the stock independent of fi shery measures. Conducting annual assessments of the size of an aggregation, even if the assessment is an index rather than
an actual count, can provide valuable real-time data as long as the sampling