320
P.L. Colin
9.6.12 Currents
If fi sh are spawning 10 m above a 20 m deep bottom, then currents need to be
measured at the 10 m level if possible. A single point current meter could be placed
on a mooring above the bottom. An acoustic doppler current profi ler (ADCP), while
more expensive, mounted on the bottom directly below where the fi sh spawn
provides currents in the entire water column as well as depth and tide data (Fig. 9.2 ).
The fl uid dynamics around aggregation sites can be complicated. For example,
some sites are located near to or at reef promontories, which should produce eddies
nearby as currents move past them.
It would be useful to have multiple current meters at an aggregation site (Colin
1992 ; Heyman et al. 2005 ) . Currents in relation to aggregation sites probably need
to be looked at over several spatial scales (Chap. 6 ). Do the currents at the aggregation site differ from areas just beyond the limits of the aggregation? What is the role
of promontories in infl uencing current patterns. Do currents at sites favour offshore
transport of eggs or their retention? Currents tend to run along reefs, unless there is
a very pronounced projection out from the reef. In such cases, currents (if any are
present) would tend to run along one side of the projection and then move offshore
for a distance, producing a back eddy along the opposite side of the projection.
Measuring currents in channels where spawning might be occurring within the
channel or at its mouth is also complicated (Fig. 9.3 ). If fl ow is strong enough, small
eddies are produced on the edges of the channel and a modest indentation in the
channel edge can produce a back eddy where fl ow is contrary to the general channel
fl ow. This was seen for the blacktail snapper, Lutjanus fulvus , in Palau (Chap. 12 ).
Depending on these small scale effects, the strategy for measuring currents at an
aggregation site may have to be modifi ed signifi cantly.
9.6.13 Temperature
The use of satellite-derived sea surface temperatures to characterize thermal regimes
at aggregation sites should be avoided unless there is no alternative. With present
technology satellite sensors cannot measure the temperature over reefs, instead
relying on nearby areas of open ocean, and surface temperatures may be higher than
those just below the surface. Recording thermographs for underwater use are readily
available, inexpensive and very robust. They should be carefully calibrated before
and after deployments to ensure accuracy, particularly as the differences in temperature over seasons or between sites may be small. Sampling intervals should such
that many measurements are taken per day, ideally every 30 min or less for spawning sites. The variation in temperature over time of days to weeks is also important
as this variation provides a good measure of the amount of upwelling of deeper
water over time to shallow reefs and may indicate seasonal upwelling potentially
associated with increased phytoplankton production. If possible, vertical thermograph arrays also should be installed at shelf edge locations, as the structure of the
P.L. Colin
9.6.12 Currents
If fi sh are spawning 10 m above a 20 m deep bottom, then currents need to be
measured at the 10 m level if possible. A single point current meter could be placed
on a mooring above the bottom. An acoustic doppler current profi ler (ADCP), while
more expensive, mounted on the bottom directly below where the fi sh spawn
provides currents in the entire water column as well as depth and tide data (Fig. 9.2 ).
The fl uid dynamics around aggregation sites can be complicated. For example,
some sites are located near to or at reef promontories, which should produce eddies
nearby as currents move past them.
It would be useful to have multiple current meters at an aggregation site (Colin
1992 ; Heyman et al. 2005 ) . Currents in relation to aggregation sites probably need
to be looked at over several spatial scales (Chap. 6 ). Do the currents at the aggregation site differ from areas just beyond the limits of the aggregation? What is the role
of promontories in infl uencing current patterns. Do currents at sites favour offshore
transport of eggs or their retention? Currents tend to run along reefs, unless there is
a very pronounced projection out from the reef. In such cases, currents (if any are
present) would tend to run along one side of the projection and then move offshore
for a distance, producing a back eddy along the opposite side of the projection.
Measuring currents in channels where spawning might be occurring within the
channel or at its mouth is also complicated (Fig. 9.3 ). If fl ow is strong enough, small
eddies are produced on the edges of the channel and a modest indentation in the
channel edge can produce a back eddy where fl ow is contrary to the general channel
fl ow. This was seen for the blacktail snapper, Lutjanus fulvus , in Palau (Chap. 12 ).
Depending on these small scale effects, the strategy for measuring currents at an
aggregation site may have to be modifi ed signifi cantly.
9.6.13 Temperature
The use of satellite-derived sea surface temperatures to characterize thermal regimes
at aggregation sites should be avoided unless there is no alternative. With present
technology satellite sensors cannot measure the temperature over reefs, instead
relying on nearby areas of open ocean, and surface temperatures may be higher than
those just below the surface. Recording thermographs for underwater use are readily
available, inexpensive and very robust. They should be carefully calibrated before
and after deployments to ensure accuracy, particularly as the differences in temperature over seasons or between sites may be small. Sampling intervals should such
that many measurements are taken per day, ideally every 30 min or less for spawning sites. The variation in temperature over time of days to weeks is also important
as this variation provides a good measure of the amount of upwelling of deeper
water over time to shallow reefs and may indicate seasonal upwelling potentially
associated with increased phytoplankton production. If possible, vertical thermograph arrays also should be installed at shelf edge locations, as the structure of the