321
9 Studying and Monitoring Aggregating Species
water column may be important in infl uencing fi sh behaviour. The observation of
Starr et al. ( 2007 ) that Nassau grouper descended for 2 months to depths averaging
about 70 m, after spawning in shallower (20 m) water, would have been nicely
complimented by having temperature information at these different depths.
9.6.14 Water Mass Properties
Shcherbina et al. ( 2008 ) deployed an autonomous underwater vehicle to examine
bathymetric and water column properties around Glover’s Reef, Belize, including a
Nassau grouper aggregation site. While providing an abundance of intriguing data,
the short sampling periods (5 days each in May and February) provide little insight
into the seasonal oceanographic regimes that affect the aggregation site. This study
would have been well complimented by long-term deployment of instruments
(temperature at various depths, current meters) at sites around the atoll. Then the
wealth of detailed data obtained from such a vehicle can be placed in a longer term
perspective obtained from deployed instruments.
9.6.15 Drifters
Information on where eggs and larvae go after spawning is necessary to understand
the importance of aggregations. The water into which gametes are released needs to
be “tagged” in some manner, or the eggs themselves “tagged” (for those with planktonic
eggs). Placing “drifters” into water containing eggs is one method of tracking
propagules, however, there is no guarantee the drifter is actually staying with the
desired parcel of water. Theoretically the gamete release from a spawn in a large
transient aggregation could be tracked by drifter and the eggs and larvae subsequently sampled based on the location of the drifter. Real time verifi cation of the
coupling of drifter and propagules is limited by the inability to rapidly sort and
identify larvae sampled near the drifter. At a later time after larval samples are
processed there should be some ability to determine the connection between drifter
track and larval presence.
The initial advection of eggs after spawning can be critical in determining where
they will end up, either being incorporated into general oceanic circulation or
retained locally by mechanisms associated with near-shore physical dynamics (see
Chap. 6 ). Short term drifters (not satellite tracked) can be useful for tracking initial
trajectories of eggs and larvae (see Fig. 5.16). Drifters can be “inert”, their movement over time determined by following in a boat and logging positions with a hand
held GPS, or “active”, recording their positions in some manner. With a logging
GPS installed on a marker fl oat, recording its position over time, the drifter is recovered after some hours and data downloaded (Colin et al. 2003 ) . Hatcher et al. ( 2004 )
used GPS drifters to track coral spawn. If left at sea for more than a few hours, it is
9 Studying and Monitoring Aggregating Species
water column may be important in infl uencing fi sh behaviour. The observation of
Starr et al. ( 2007 ) that Nassau grouper descended for 2 months to depths averaging
about 70 m, after spawning in shallower (20 m) water, would have been nicely
complimented by having temperature information at these different depths.
9.6.14 Water Mass Properties
Shcherbina et al. ( 2008 ) deployed an autonomous underwater vehicle to examine
bathymetric and water column properties around Glover’s Reef, Belize, including a
Nassau grouper aggregation site. While providing an abundance of intriguing data,
the short sampling periods (5 days each in May and February) provide little insight
into the seasonal oceanographic regimes that affect the aggregation site. This study
would have been well complimented by long-term deployment of instruments
(temperature at various depths, current meters) at sites around the atoll. Then the
wealth of detailed data obtained from such a vehicle can be placed in a longer term
perspective obtained from deployed instruments.
9.6.15 Drifters
Information on where eggs and larvae go after spawning is necessary to understand
the importance of aggregations. The water into which gametes are released needs to
be “tagged” in some manner, or the eggs themselves “tagged” (for those with planktonic
eggs). Placing “drifters” into water containing eggs is one method of tracking
propagules, however, there is no guarantee the drifter is actually staying with the
desired parcel of water. Theoretically the gamete release from a spawn in a large
transient aggregation could be tracked by drifter and the eggs and larvae subsequently sampled based on the location of the drifter. Real time verifi cation of the
coupling of drifter and propagules is limited by the inability to rapidly sort and
identify larvae sampled near the drifter. At a later time after larval samples are
processed there should be some ability to determine the connection between drifter
track and larval presence.
The initial advection of eggs after spawning can be critical in determining where
they will end up, either being incorporated into general oceanic circulation or
retained locally by mechanisms associated with near-shore physical dynamics (see
Chap. 6 ). Short term drifters (not satellite tracked) can be useful for tracking initial
trajectories of eggs and larvae (see Fig. 5.16). Drifters can be “inert”, their movement over time determined by following in a boat and logging positions with a hand
held GPS, or “active”, recording their positions in some manner. With a logging
GPS installed on a marker fl oat, recording its position over time, the drifter is recovered after some hours and data downloaded (Colin et al. 2003 ) . Hatcher et al. ( 2004 )
used GPS drifters to track coral spawn. If left at sea for more than a few hours, it is