318
P.L. Colin
Egg density and ascent rates
This information allows estimation of the time required for eggs to ascend from the
spawning depth to near the surface, where water transport may be different and
affect the advection of eggs from the spawning site. Most planktonic reef fi sh eggs
are very slightly positively buoyant in seawater. Their density is somewhat less than
seawater, and their buoyancy and depth of release are important in determining how
fast they will rise to the surface after spawning. The small differences between the
density of eggs and that of seawater is not easy to directly determine. Eggs cannot
be accurately weighed, as water clings to them once they are removed from water;
nor can their volume be easily measured to the accuracy required. However, their
density can be quickly determined by putting fertilized eggs into containers with
different densities of seawater (by diluting with fresh water) to see at what salinity
the eggs are neutrally buoyant (Colin et al. 1996 ) . Ascent rates are generally measured by releasing eggs, usually from a pipette, at the bottom of a container with
seawater of the salinity found at the spawning site and then measuring their rise over
time. Great care is needed to ensure that the water in the container is totally still,
with no vertical water movement which would bias results.
9.6.10 Dynamics of Migration
The dynamics of migration to and from aggregations has not received much attention
beyond anecdotal observations, particularly for transient aggregations. Observers
have seen numbers of fi sh moving to (or away) from aggregation sites, but the fi sh
usually are followed for only a few minutes (Colin 1992 , Chap. 2 ). Acoustic tagging
holds great promise for learning more about migrations (Nemeth et al. 2007 ; Starr
et al. 2007 ) , but for some species large acoustic deployment areas are needed to
encompass the entire migration catchment for larger species. For something like
Nassau grouper, this may mean covering in some manner hundreds of km of reef
with receivers. Obviously for such large scale situations, receivers can only be
placed at intervals, but hopefully suffi ciently close that general movements can be
determined and later surveys focused on the areas to which fi sh are migrating
from aggregations.
In addition to the documentation of aggregation catchments, exactly how fi shes
may navigate during migration, particularly the long distance migrations of some
transient aggregators (Colin 1992 ; Bolden 2000 ; Starr et al. 2007 ) , is a matter of
considerable interest. The technology to undertake such studies is now available
through acoustic tagging and receiver arrays (Domeier 2005 ; Nemeth et al. 2007 ;
Starr et al. 2007 ) , but generally has not been deployed on a scale suitable for such
studies. For some transient aggregators, stationary still cameras taking photos at
regular intervals might provide new information on occurrence of fi sh and their
migration behaviour, but locations for such cameras need to be carefully chosen as
it is possible to completely miss a migration if the camera is placed only slightly off
the migration track.
P.L. Colin
Egg density and ascent rates
This information allows estimation of the time required for eggs to ascend from the
spawning depth to near the surface, where water transport may be different and
affect the advection of eggs from the spawning site. Most planktonic reef fi sh eggs
are very slightly positively buoyant in seawater. Their density is somewhat less than
seawater, and their buoyancy and depth of release are important in determining how
fast they will rise to the surface after spawning. The small differences between the
density of eggs and that of seawater is not easy to directly determine. Eggs cannot
be accurately weighed, as water clings to them once they are removed from water;
nor can their volume be easily measured to the accuracy required. However, their
density can be quickly determined by putting fertilized eggs into containers with
different densities of seawater (by diluting with fresh water) to see at what salinity
the eggs are neutrally buoyant (Colin et al. 1996 ) . Ascent rates are generally measured by releasing eggs, usually from a pipette, at the bottom of a container with
seawater of the salinity found at the spawning site and then measuring their rise over
time. Great care is needed to ensure that the water in the container is totally still,
with no vertical water movement which would bias results.
9.6.10 Dynamics of Migration
The dynamics of migration to and from aggregations has not received much attention
beyond anecdotal observations, particularly for transient aggregations. Observers
have seen numbers of fi sh moving to (or away) from aggregation sites, but the fi sh
usually are followed for only a few minutes (Colin 1992 , Chap. 2 ). Acoustic tagging
holds great promise for learning more about migrations (Nemeth et al. 2007 ; Starr
et al. 2007 ) , but for some species large acoustic deployment areas are needed to
encompass the entire migration catchment for larger species. For something like
Nassau grouper, this may mean covering in some manner hundreds of km of reef
with receivers. Obviously for such large scale situations, receivers can only be
placed at intervals, but hopefully suffi ciently close that general movements can be
determined and later surveys focused on the areas to which fi sh are migrating
from aggregations.
In addition to the documentation of aggregation catchments, exactly how fi shes
may navigate during migration, particularly the long distance migrations of some
transient aggregators (Colin 1992 ; Bolden 2000 ; Starr et al. 2007 ) , is a matter of
considerable interest. The technology to undertake such studies is now available
through acoustic tagging and receiver arrays (Domeier 2005 ; Nemeth et al. 2007 ;
Starr et al. 2007 ) , but generally has not been deployed on a scale suitable for such
studies. For some transient aggregators, stationary still cameras taking photos at
regular intervals might provide new information on occurrence of fi sh and their
migration behaviour, but locations for such cameras need to be carefully chosen as
it is possible to completely miss a migration if the camera is placed only slightly off
the migration track.