132
P.L. Colin
5.4 Remote Sensing to Identify Aggregation Sites
Attempts to discover unknown aggregation sites using remote sensing rely on
characterizing the geomorphology of known sites, then identifying similar areas as
potential sites (Kobara and Heyman 2008 ) . While appealing in principle, the variability of TA site geomorphology would alone make it unlikely such would be effective
in practice. Even so, the limitations of the commonly used satellite images, often
low resolution Landsat coverage, render such attempts often futile. Many TWA TAs
occur at depths below those visible in satellite images (Colin 1992 ; Sadovy et al.
1994 ; Nemeth et al. 2007 ) . When the bottom cannot be seen, then there is no ability
to discern geomorphology as an indication of possible aggregation occurrence. For
example, for the south Puerto Rico-Virgin Islands shelf, a “low tech” marine chart
shows the basic geomorphology of the area fairly clearly (Fig. 5.13a ) but a multibeam sonar map has much more detail (Fig. 5.13b ). A Landsat 7 image, however,
shows no bottom visible near any aggregation sites (Fig. 5.13c ) and a map interpretation
(Reefbase.org), based on the satellite image, also does not indicate shallow bottom
in the area. The same problem occurs for many Nassau grouper aggregations in the
Bahamas. These limitations argue against using satellite images to identify aggregation sites even if geomorphology alone were a valid indicator (which it is not).
This is discussed further in Chap. 9 .
5.5 Migration to and from Aggregation Sites
To aggregate, individual fi shes have to undergo migrations of varying distances
unless among the small number already resident at the aggregation site. TA migrations are believed to occur at lunar or seasonal time frames while those of RA may
occur daily for periods of weeks or months. Why do fi shes migrate to TAs? Why do
RA fi shes migrate at all?
Migration was discussed in more detail in Chap. 2 (also see Nemeth 2009 ) . The
migration distances for fi shes with RAs are probably limited to a few km, since they
must make the round trip from resting (and possibly feeding) sites to the aggregation site daily. Data are lacking for most species, but brown surgeonfi sh, Acanthurus
nigrofuscus , in the Red Sea have daily migrations between feeding and spawning
sites (Mazeroll and Montgomery 1998 ; Myrberg et al. 1988 ) for much of the year,
following discrete pathways in an organized manner.
Different patterns of migration to RAs may occur on a single reef, the distance
increasing with larger fi sh size. For example, on a shallow reef front in Palau two
patterns of migration were found. In the fi rst, the small bullethead parrotfi sh,
Chlorurus sordidus, striated surgeonfi sh, Ctenochaetus striatus, and brown surgeonfi sh aggregate in many areas on the reef front to spawn, migrating no more than
100–200 m daily (Fig. 5.14a ). They form an almost continuous, band-like, distribution of spawning fi sh on the reef front (PLC unpublished data, Robertson 1983 ) .
P.L. Colin
5.4 Remote Sensing to Identify Aggregation Sites
Attempts to discover unknown aggregation sites using remote sensing rely on
characterizing the geomorphology of known sites, then identifying similar areas as
potential sites (Kobara and Heyman 2008 ) . While appealing in principle, the variability of TA site geomorphology would alone make it unlikely such would be effective
in practice. Even so, the limitations of the commonly used satellite images, often
low resolution Landsat coverage, render such attempts often futile. Many TWA TAs
occur at depths below those visible in satellite images (Colin 1992 ; Sadovy et al.
1994 ; Nemeth et al. 2007 ) . When the bottom cannot be seen, then there is no ability
to discern geomorphology as an indication of possible aggregation occurrence. For
example, for the south Puerto Rico-Virgin Islands shelf, a “low tech” marine chart
shows the basic geomorphology of the area fairly clearly (Fig. 5.13a ) but a multibeam sonar map has much more detail (Fig. 5.13b ). A Landsat 7 image, however,
shows no bottom visible near any aggregation sites (Fig. 5.13c ) and a map interpretation
(Reefbase.org), based on the satellite image, also does not indicate shallow bottom
in the area. The same problem occurs for many Nassau grouper aggregations in the
Bahamas. These limitations argue against using satellite images to identify aggregation sites even if geomorphology alone were a valid indicator (which it is not).
This is discussed further in Chap. 9 .
5.5 Migration to and from Aggregation Sites
To aggregate, individual fi shes have to undergo migrations of varying distances
unless among the small number already resident at the aggregation site. TA migrations are believed to occur at lunar or seasonal time frames while those of RA may
occur daily for periods of weeks or months. Why do fi shes migrate to TAs? Why do
RA fi shes migrate at all?
Migration was discussed in more detail in Chap. 2 (also see Nemeth 2009 ) . The
migration distances for fi shes with RAs are probably limited to a few km, since they
must make the round trip from resting (and possibly feeding) sites to the aggregation site daily. Data are lacking for most species, but brown surgeonfi sh, Acanthurus
nigrofuscus , in the Red Sea have daily migrations between feeding and spawning
sites (Mazeroll and Montgomery 1998 ; Myrberg et al. 1988 ) for much of the year,
following discrete pathways in an organized manner.
Different patterns of migration to RAs may occur on a single reef, the distance
increasing with larger fi sh size. For example, on a shallow reef front in Palau two
patterns of migration were found. In the fi rst, the small bullethead parrotfi sh,
Chlorurus sordidus, striated surgeonfi sh, Ctenochaetus striatus, and brown surgeonfi sh aggregate in many areas on the reef front to spawn, migrating no more than
100–200 m daily (Fig. 5.14a ). They form an almost continuous, band-like, distribution of spawning fi sh on the reef front (PLC unpublished data, Robertson 1983 ) .