16
M.L. Domeier
protocol is scientifi cally sound and consistent (Chap. 9 ). Underwater visual census
will reveal decreases in stock size prior to a drop in CPUE. Hydroacoustic surveys
(assessing stock size via sonar) may allow for the remote monitoring of spawning
aggregations in the future, but the technology and validation techniques are not
yet fully realized. Unfortunately long term standardized monitoring of spawning
aggregations is not widespread, particularly in the Indo-west Pacifi c and Indian
Oceans.
1.8.1 Feeding Aggregations That Form as a Result
of Spawning Aggregations
The black marlin ( Makaira indica ) is an exceptionally large pelagic predator that
gathers along the Great Barrier Reef each year between the months of October and
December. The aggregation is predictable, and until it was protected by the Australian
government, it was once heavily targeted by the Japanese longline fl eet. The aggregation remains the basis of an important recreational fi shery that is now primarily
catch and release (not mandated by law, but exercised by anglers). Although anecdotal information existed that suggested this aggregation was for the purpose of
spawning, the presence of hydrated eggs, post-ovulatory follicles and very early stage
larvae have now confi rmed that this is indeed a black marlin spawning aggregation.
Mature female black marlin are much larger than the males, and multiple males are
frequently observed at the surface following females; presumably a prelude to
spawning (MLD personal observation).
The formation of a spawning aggregation of such a large pelagic fi sh along a
coral reef is unique. What makes this event even more interesting is the fact that large
sharks seem to aggregate along the reef at the same time, possibly to prey on the black
marlin. Black marlin are often attacked by sharks while being captured on rod-andreel; furthermore, the marlin are sometimes consumed after being released by the
anglers. One such event caught on fi lm during a satellite tagging expedition documented a large group of sharks rising from deeper water to attack a black marlin that
was too exhausted to escape (fi lm by Guy Harvey). The sharks entirely consumed
the 200 kg marlin in less than 60 s. To identify and track which species of sharks are
responsible for preying on black marlin, a large hook was baited and dropped in the
water upon releasing a black marlin. One 250 kg bull shark was immediately captured and satellite tagged. The resulting data showed this shark leaving the GBR
when the marlin aggregation dispersed, traveling 500 km south to a river mouth near
Townsville (MLD unpublished data) Although these observations are anecdotal,
further studies may demonstrate that predatory sharks are aggregating to prey on the
spawning black marlin. Although the example described here involves predators
possibly gathering to feed on the spawning adults, better documented cases of the
formation of feeding aggregations over spawning aggregations involve whale sharks
(Fig. 1.3a ) and manta rays (Fig. 1.3b ) feeding on the eggs released by snappers and
surgeonfi sh (respectively).
M.L. Domeier
protocol is scientifi cally sound and consistent (Chap. 9 ). Underwater visual census
will reveal decreases in stock size prior to a drop in CPUE. Hydroacoustic surveys
(assessing stock size via sonar) may allow for the remote monitoring of spawning
aggregations in the future, but the technology and validation techniques are not
yet fully realized. Unfortunately long term standardized monitoring of spawning
aggregations is not widespread, particularly in the Indo-west Pacifi c and Indian
Oceans.
1.8.1 Feeding Aggregations That Form as a Result
of Spawning Aggregations
The black marlin ( Makaira indica ) is an exceptionally large pelagic predator that
gathers along the Great Barrier Reef each year between the months of October and
December. The aggregation is predictable, and until it was protected by the Australian
government, it was once heavily targeted by the Japanese longline fl eet. The aggregation remains the basis of an important recreational fi shery that is now primarily
catch and release (not mandated by law, but exercised by anglers). Although anecdotal information existed that suggested this aggregation was for the purpose of
spawning, the presence of hydrated eggs, post-ovulatory follicles and very early stage
larvae have now confi rmed that this is indeed a black marlin spawning aggregation.
Mature female black marlin are much larger than the males, and multiple males are
frequently observed at the surface following females; presumably a prelude to
spawning (MLD personal observation).
The formation of a spawning aggregation of such a large pelagic fi sh along a
coral reef is unique. What makes this event even more interesting is the fact that large
sharks seem to aggregate along the reef at the same time, possibly to prey on the black
marlin. Black marlin are often attacked by sharks while being captured on rod-andreel; furthermore, the marlin are sometimes consumed after being released by the
anglers. One such event caught on fi lm during a satellite tagging expedition documented a large group of sharks rising from deeper water to attack a black marlin that
was too exhausted to escape (fi lm by Guy Harvey). The sharks entirely consumed
the 200 kg marlin in less than 60 s. To identify and track which species of sharks are
responsible for preying on black marlin, a large hook was baited and dropped in the
water upon releasing a black marlin. One 250 kg bull shark was immediately captured and satellite tagged. The resulting data showed this shark leaving the GBR
when the marlin aggregation dispersed, traveling 500 km south to a river mouth near
Townsville (MLD unpublished data) Although these observations are anecdotal,
further studies may demonstrate that predatory sharks are aggregating to prey on the
spawning black marlin. Although the example described here involves predators
possibly gathering to feed on the spawning adults, better documented cases of the
formation of feeding aggregations over spawning aggregations involve whale sharks
(Fig. 1.3a ) and manta rays (Fig. 1.3b ) feeding on the eggs released by snappers and
surgeonfi sh (respectively).