135
5 Timing and Location of Aggregation and Spawning in Reef Fishes
These data support the belief that large TA aggregators do not migrate between
reefs, islands or atolls separated by deep (several hundred meters or more) water.
However, at locations where reefs exist on a contiguous insular or continental
shelf aggregating fi shes may easily migrate “between reefs” (Sadovy et al. 1994 ;
Zeller 1998 ) .
How do fi shes with large TAs migrating long distances fi nd the sites? It has been
shown in some smaller RA species younger fi shes learn routes of migration and
location from older fi sh (Mazeroll and Montgomery 1998 ; Warner 1988 ) and it is
likely some ability exists in this regard for larger TA species. Similar to RA species,
it is likely TA species would use reef features as a migration guide. Observations of
probable TA migrants swimming along shelf edge reefs (Colin 1992 ; Carter et al.
1994 ; Whaylen et al. 2006 ; Starr et al. 2007 ) indicate that larger fi shes may use shelf
edge, or drop off, contours as “highways” to reach transient aggregation sites. Shelf
edges are essentially a linear environment and at the shelf edge a migrating fi sh can
swim one of two ways along this feature. The possibility of an innate navigational
compass or ability to guide migrations also exists (Helfman and Shultz 1984 ) , but
has not been demonstrated for aggregating fi shes.
Earlier suggestions the fi sh might locate aggregation sites by swimming either
upcurrent (Colin et al. 1987 ) or down-current (Carter 1986 ) along the shelf edge to
the end of an island or to a promontory were overly simplistic. Currents are likely
too variable daily and seasonally to be of great importance in locating aggregations
(Carter et al. 1994 ; Colin 1992 ) . However, Nemeth et al. ( 2008 ) did fi nd that most
red hinds in the Virgin Islands migrated in an up-current direction to reach aggregation sites. In areas where currents are directionally consistent, a correlation of current
and migration directions might exist, however the current alone is probably not the
sole means to locate aggregations. Acoustic tagging studies, in which the migration
direction can be determined by successive detection of fi sh by receivers (Starr et al.
2007 ) , hold promise for providing information on migration, particularly if combined with current measurements.
Given the longevity of many large TA species, an individual fi sh could potentially migrate to the same or multiple aggregations for a decade or more. For Nassau
grouper, with sexual maturity at 4–7 years and a maximum age of nearly 30 years
(Sadovy and Eklund 1999 ) , most participating individuals may well have migrated
to a site numerous times, assuming a yearly migration. With heavy fi shing pressure
the average age of fi sh in aggregations decreases and could reduce the number of
fi sh from which younger ones might learn the locations of aggregations.
5.6 Oceanography of Spawning Sites
The oceanographic conditions at sites have generally been proposed as a critical
factor attracting individuals and promoting site use over time. Physical factors such
as temperature, current speed and direction, light, water clarity, water column
structure, waves, seasonal weather or productivity differences could play a role.
5 Timing and Location of Aggregation and Spawning in Reef Fishes
These data support the belief that large TA aggregators do not migrate between
reefs, islands or atolls separated by deep (several hundred meters or more) water.
However, at locations where reefs exist on a contiguous insular or continental
shelf aggregating fi shes may easily migrate “between reefs” (Sadovy et al. 1994 ;
Zeller 1998 ) .
How do fi shes with large TAs migrating long distances fi nd the sites? It has been
shown in some smaller RA species younger fi shes learn routes of migration and
location from older fi sh (Mazeroll and Montgomery 1998 ; Warner 1988 ) and it is
likely some ability exists in this regard for larger TA species. Similar to RA species,
it is likely TA species would use reef features as a migration guide. Observations of
probable TA migrants swimming along shelf edge reefs (Colin 1992 ; Carter et al.
1994 ; Whaylen et al. 2006 ; Starr et al. 2007 ) indicate that larger fi shes may use shelf
edge, or drop off, contours as “highways” to reach transient aggregation sites. Shelf
edges are essentially a linear environment and at the shelf edge a migrating fi sh can
swim one of two ways along this feature. The possibility of an innate navigational
compass or ability to guide migrations also exists (Helfman and Shultz 1984 ) , but
has not been demonstrated for aggregating fi shes.
Earlier suggestions the fi sh might locate aggregation sites by swimming either
upcurrent (Colin et al. 1987 ) or down-current (Carter 1986 ) along the shelf edge to
the end of an island or to a promontory were overly simplistic. Currents are likely
too variable daily and seasonally to be of great importance in locating aggregations
(Carter et al. 1994 ; Colin 1992 ) . However, Nemeth et al. ( 2008 ) did fi nd that most
red hinds in the Virgin Islands migrated in an up-current direction to reach aggregation sites. In areas where currents are directionally consistent, a correlation of current
and migration directions might exist, however the current alone is probably not the
sole means to locate aggregations. Acoustic tagging studies, in which the migration
direction can be determined by successive detection of fi sh by receivers (Starr et al.
2007 ) , hold promise for providing information on migration, particularly if combined with current measurements.
Given the longevity of many large TA species, an individual fi sh could potentially migrate to the same or multiple aggregations for a decade or more. For Nassau
grouper, with sexual maturity at 4–7 years and a maximum age of nearly 30 years
(Sadovy and Eklund 1999 ) , most participating individuals may well have migrated
to a site numerous times, assuming a yearly migration. With heavy fi shing pressure
the average age of fi sh in aggregations decreases and could reduce the number of
fi sh from which younger ones might learn the locations of aggregations.
5.6 Oceanography of Spawning Sites
The oceanographic conditions at sites have generally been proposed as a critical
factor attracting individuals and promoting site use over time. Physical factors such
as temperature, current speed and direction, light, water clarity, water column
structure, waves, seasonal weather or productivity differences could play a role.