317
9 Studying and Monitoring Aggregating Species
Petersen et al. ( 1992 ) provide interesting information on collection and handling of
eggs in fertilization studies. Because of the positive buoyancy of planktonic reef fi sh
eggs the mixing of surface layers is particularly important as wind induced shears
and formation of slicks can greatly affect transport of eggs. Appeldoorn et al. ( 1988 )
examined dye, surface drifters, drogues and particles as potential tracers. None was
ideal and some (plastic particles mimicking fi sh eggs) are highly labour intensive to
track. They suggest a combination of methods may be useful (using dye to track
dispersal of beads, etc.). Small drifting items (drift cards, drift bottles, drift vials –
see Domeier 2004 ) can also serve as egg and larval tracers, in those cases relying on
eventual discovery of the item by other people, usually from beaches, followed by
reporting of their discovery.
The spawning of a large reef fi sh aggregation can be a spectacular event. In the
case of large transient aggregators, such as Nassau grouper and cubera snapper,
many millions of fertile eggs can be produced in just a few minutes in a very small
volume of water. While questions of what the concentrated spawning of aggregations might mean to early life history are addressed in Chap. 6 , the huge spawnings
also provide an opportunity to conduct studies that would otherwise be diffi cult.
Egg numbers and gamete cloud size
Beyond the number of fi sh present, it is useful to know the possible contribution of
a spawning aggregation to the future generations of fi sh by knowing something
about how many eggs are produced by an aggregation during the period it occurs.
This information provides a “starting point” for any future analysis of life history
parameters of an aggregating fi sh, stock assessment, etc. Batch fecundity information provides some idea of the numbers of eggs a given female can produce upon
spawning, but is not a direct measure of the numbers of eggs spawned by all females
in an aggregation. Sampling and quantifying the eggs released is one way to do this,
but is diffi cult to do effectively. The eggs are tiny, nearly transparent and drift away
within seconds to minutes with the current. The water where spawning occurred
would have to “tagged” with a drogue or dye and then a grid around the supposed
centre of eggs sampled using standard plankton sampling techniques. If spawned
below the surface, time would be required for the eggs to fl oat to near the surface
(where sampling is easier). The eggs would disperse horizontally outward from the
location of release with the patch expanding quickly in the hours after spawning
(Chap. 6 ), but would be limited in their horizontal distribution by their slight buoyancy tending to keep them near the surface.
Where there are predictable currents downstream of spawning fi shes, moored
plankton nets can be used to quantitatively sample the eggs streaming away from
the spawning. For example, Hamner et al. ( 2007 ) moored plankton nets about 80 m
downcurrent of a site where both resident aggregating and non-aggregating fi shes
were spawning just after high tide on a reef in Palau. They found at least 50% of
eggs captured came from spawning aggregations of a few species of parrotfi sh and
surgeonfi shes, and fi sh eggs made up to 90% of the zooplankton exported from this
reef on falling tides.
9 Studying and Monitoring Aggregating Species
Petersen et al. ( 1992 ) provide interesting information on collection and handling of
eggs in fertilization studies. Because of the positive buoyancy of planktonic reef fi sh
eggs the mixing of surface layers is particularly important as wind induced shears
and formation of slicks can greatly affect transport of eggs. Appeldoorn et al. ( 1988 )
examined dye, surface drifters, drogues and particles as potential tracers. None was
ideal and some (plastic particles mimicking fi sh eggs) are highly labour intensive to
track. They suggest a combination of methods may be useful (using dye to track
dispersal of beads, etc.). Small drifting items (drift cards, drift bottles, drift vials –
see Domeier 2004 ) can also serve as egg and larval tracers, in those cases relying on
eventual discovery of the item by other people, usually from beaches, followed by
reporting of their discovery.
The spawning of a large reef fi sh aggregation can be a spectacular event. In the
case of large transient aggregators, such as Nassau grouper and cubera snapper,
many millions of fertile eggs can be produced in just a few minutes in a very small
volume of water. While questions of what the concentrated spawning of aggregations might mean to early life history are addressed in Chap. 6 , the huge spawnings
also provide an opportunity to conduct studies that would otherwise be diffi cult.
Egg numbers and gamete cloud size
Beyond the number of fi sh present, it is useful to know the possible contribution of
a spawning aggregation to the future generations of fi sh by knowing something
about how many eggs are produced by an aggregation during the period it occurs.
This information provides a “starting point” for any future analysis of life history
parameters of an aggregating fi sh, stock assessment, etc. Batch fecundity information provides some idea of the numbers of eggs a given female can produce upon
spawning, but is not a direct measure of the numbers of eggs spawned by all females
in an aggregation. Sampling and quantifying the eggs released is one way to do this,
but is diffi cult to do effectively. The eggs are tiny, nearly transparent and drift away
within seconds to minutes with the current. The water where spawning occurred
would have to “tagged” with a drogue or dye and then a grid around the supposed
centre of eggs sampled using standard plankton sampling techniques. If spawned
below the surface, time would be required for the eggs to fl oat to near the surface
(where sampling is easier). The eggs would disperse horizontally outward from the
location of release with the patch expanding quickly in the hours after spawning
(Chap. 6 ), but would be limited in their horizontal distribution by their slight buoyancy tending to keep them near the surface.
Where there are predictable currents downstream of spawning fi shes, moored
plankton nets can be used to quantitatively sample the eggs streaming away from
the spawning. For example, Hamner et al. ( 2007 ) moored plankton nets about 80 m
downcurrent of a site where both resident aggregating and non-aggregating fi shes
were spawning just after high tide on a reef in Palau. They found at least 50% of
eggs captured came from spawning aggregations of a few species of parrotfi sh and
surgeonfi shes, and fi sh eggs made up to 90% of the zooplankton exported from this
reef on falling tides.