515
12 Species Case Studies
aggregations (not verifi ed) in the same general area. Moorish idol aggregations
occur at several sites along the western barrier reef in Palau; mostly at spots used for
dive tourism (which is presumably why the observations were made). These aggregations probably occur throughout the Palau barrier reef system and likely number
a hundred or more. At Blue Corner, the best-known site, schools of about 500 to a
few thousand fi sh mill about variously over a distance of approximately 200 m.
When the fi sh move horizontally they do so in unison as a school, all oriented in the
same direction (Fig. 12.53 ), an impressive sight.
In areas such as “Blue Corner” and Saes Corner a variety of predators have been
observed “hunting” both aggregated Moorish idols and surgeonfi shes (Fig. 12.36 ).
These predators include twin-spot snapper ( Lutjanus bohar ), giant trevally ( Caranx
ignobilis ), humphead wrasse ( Cheilinus undulatus ), grey reef sharks ( Carcharhinus
amblyrhynchus ) and white tip reef sharks ( Triaenodon obesus ) (Fig. 12.53c , see
also Fig. 12.32 ). Often multi-species groups of predators pursue the school of moorish idols and surgeonfi shes, trying to separate individuals which are then attacked
and eaten. Sometimes the pursued fi sh will take shelter under a rock and the predators will then focus their attention on that site, trying to scare or drive the single
unfortunate fi sh out where it can be taken easily. Groups of sharks sometimes drive
the aggregations of Moorish idols as a whole away from the reef, and force the
school towards the surface, where they attack the Moorish idols, taking many individuals. At Blue Corner several dive boats have observed schools of Moorish idols
at the surface being “herded” and eaten by grey reef sharks with their dorsal fi ns
breaking the surface and Moorish idols jumping on the surface of the water trying
to avoid the predators. Whether these were fi sh that had left the shelter of the reef to
spawn is not known, but possible. This may occur at or near the time of spawning,
but is incompletely documented as the behaviour is both diffi cult and dangerous for
divers to observe in the water. Moorish idols are small compared to the sharks
attacking them, and it is somewhat surprising that the grey reef sharks are so vigorous in pursuing such small prey.
Overall the temporary presence of the schools of Moorish idols and surgeonfi shes in an area where they normally do not occur in abundance and in areas that
lack large amounts of shelter probably provides an opportunity for predation potentially associated with spawning aggregations, but not targeted specifi cally on courting or spawning fi shes.
A possible spawning movement was observed on 4 Feb 2009, the day after the
fi rst quarter moon. The Blue Corner aggregation was watched from about 9 am until
noon. The fi sh moved back and forth along the seaward drop-off within 1–3 m of the
bottom (Fig. 12.53 ) between about 6 and 20 m depth. However, just after 11 am and
without any interaction with any predators, the entire group of about 430 fi sh swam
into open water up and away from the reef, reaching near the surface some distance
from the drop off (Fig. 12.54 ). It is presumed the fi sh were preparing to spawn,
however, once near the surface a dive boat ran through the middle of the school,
temporarily dispersing them, and then strong currents made it risky to continue to
remain with the group, so actual spawning was not observed.
12 Species Case Studies
aggregations (not verifi ed) in the same general area. Moorish idol aggregations
occur at several sites along the western barrier reef in Palau; mostly at spots used for
dive tourism (which is presumably why the observations were made). These aggregations probably occur throughout the Palau barrier reef system and likely number
a hundred or more. At Blue Corner, the best-known site, schools of about 500 to a
few thousand fi sh mill about variously over a distance of approximately 200 m.
When the fi sh move horizontally they do so in unison as a school, all oriented in the
same direction (Fig. 12.53 ), an impressive sight.
In areas such as “Blue Corner” and Saes Corner a variety of predators have been
observed “hunting” both aggregated Moorish idols and surgeonfi shes (Fig. 12.36 ).
These predators include twin-spot snapper ( Lutjanus bohar ), giant trevally ( Caranx
ignobilis ), humphead wrasse ( Cheilinus undulatus ), grey reef sharks ( Carcharhinus
amblyrhynchus ) and white tip reef sharks ( Triaenodon obesus ) (Fig. 12.53c , see
also Fig. 12.32 ). Often multi-species groups of predators pursue the school of moorish idols and surgeonfi shes, trying to separate individuals which are then attacked
and eaten. Sometimes the pursued fi sh will take shelter under a rock and the predators will then focus their attention on that site, trying to scare or drive the single
unfortunate fi sh out where it can be taken easily. Groups of sharks sometimes drive
the aggregations of Moorish idols as a whole away from the reef, and force the
school towards the surface, where they attack the Moorish idols, taking many individuals. At Blue Corner several dive boats have observed schools of Moorish idols
at the surface being “herded” and eaten by grey reef sharks with their dorsal fi ns
breaking the surface and Moorish idols jumping on the surface of the water trying
to avoid the predators. Whether these were fi sh that had left the shelter of the reef to
spawn is not known, but possible. This may occur at or near the time of spawning,
but is incompletely documented as the behaviour is both diffi cult and dangerous for
divers to observe in the water. Moorish idols are small compared to the sharks
attacking them, and it is somewhat surprising that the grey reef sharks are so vigorous in pursuing such small prey.
Overall the temporary presence of the schools of Moorish idols and surgeonfi shes in an area where they normally do not occur in abundance and in areas that
lack large amounts of shelter probably provides an opportunity for predation potentially associated with spawning aggregations, but not targeted specifi cally on courting or spawning fi shes.
A possible spawning movement was observed on 4 Feb 2009, the day after the
fi rst quarter moon. The Blue Corner aggregation was watched from about 9 am until
noon. The fi sh moved back and forth along the seaward drop-off within 1–3 m of the
bottom (Fig. 12.53 ) between about 6 and 20 m depth. However, just after 11 am and
without any interaction with any predators, the entire group of about 430 fi sh swam
into open water up and away from the reef, reaching near the surface some distance
from the drop off (Fig. 12.54 ). It is presumed the fi sh were preparing to spawn,
however, once near the surface a dive boat ran through the middle of the school,
temporarily dispersing them, and then strong currents made it risky to continue to
remain with the group, so actual spawning was not observed.