Field observations (most of which are subjective measurements presented as informal
reports) with coincident satellite data are only available for a limited number of years.
These observations consistently indicate that there is a correlation with bleached corals
and a threshold DHW value of 4. By the time DHW values reach 8, widespread
bleaching is likely and some mortality can be expected. CRW has applied these DHW
values for the last two years when generating satellite bleaching warnings and alerts
(Liu et al., 2003).
All of the products discussed in this chapter have been used successfully in
monitoring major coral bleaching episodes around the globe (e.g. Goreau et al., 2000;
Wellington et al., 2001). Since 2000, when the DHW product was being used in an
experimental mode (it became operational along with the HotSpot product in 2001),
CRW has issued over 100 bleaching warnings. On 46 occasions field reports on the
status of the corals were received and on every occasion bleaching was confirmed.
This is not a comprehensive test of the DHW product but is a very good indication that
it is at the very least a very good conservative measure of coral bleaching. Figure 5 is a
plot of the geographic distribution of these reports and demonstrates that the success of
the DHW product is widespread, i.e. not restricted to a regional level.
The data products discussed in this chapter are available as HotSpot and DHW
charts (produced by CRW twice weekly in near-real time from the composite nighttime
AVHRR SST products). These products, along with descriptions of the methodologies,
are Web-accessible at http://orbit-net.nesdis.noaa.gov/orad/coral_bleaching_index.html.
6. Coral Reef Watch in Action
6.1 CASE STUDY: MIDWAY ATOLL
During July of 2002 CRW team members noticed a SST anomaly building in the
central North Pacific Ocean (Figure 3). This anomaly grew until temperatures at
Midway Island on 1
st August passed the threshold for coral bleaching, where they
remained. Based on the accumulated heat stress (recorded in the DHW product), CRW
issued a bleaching warning on 7
th August. There was, however, no one on site to
confirm the existence of bleaching. Continued monitoring revealed that SSTs in this
area remained above the threshold for more than a month, and that SSTs did not drop
back below the threshold until 7
th September (Figure 6).
At this time, the Hawaii Laboratories of NOAA’s National Marine Fisheries
Service (NMFS) were due to lead a multi-agency scientific cruise to the North West
Hawaii Islands (NWHI) as part of their 2002 NWHI Reef Assessment and Monitoring
Program. This cruise was scheduled almost one year prior to this bleaching event as
part of their on-going monitoring efforts. The cruise was conducted between
September 8
th and October 7
th
, 2002. During the cruise, surveys were conducted at
Midway Atoll on September 20
th
, 21
st and 25
th
. Although the CRW bleaching report
did not instigate the cruise, the cruise plan was altered to focus more attention on
conducting surveys in shallow backreef habitats based on the information that the field
team gained from the HotSpot products combined with NMFS in situ buoy data from
the region.
During the field work at Midway Atoll, which took place two weeks after the
thermal stress abated, the survey team found extensive fresh bleaching. In most cases,
the corals were bleached white with no algal cover (indicative of recent bleaching). In a
19
Extreme Events and Perturbations
reports) with coincident satellite data are only available for a limited number of years.
These observations consistently indicate that there is a correlation with bleached corals
and a threshold DHW value of 4. By the time DHW values reach 8, widespread
bleaching is likely and some mortality can be expected. CRW has applied these DHW
values for the last two years when generating satellite bleaching warnings and alerts
(Liu et al., 2003).
All of the products discussed in this chapter have been used successfully in
monitoring major coral bleaching episodes around the globe (e.g. Goreau et al., 2000;
Wellington et al., 2001). Since 2000, when the DHW product was being used in an
experimental mode (it became operational along with the HotSpot product in 2001),
CRW has issued over 100 bleaching warnings. On 46 occasions field reports on the
status of the corals were received and on every occasion bleaching was confirmed.
This is not a comprehensive test of the DHW product but is a very good indication that
it is at the very least a very good conservative measure of coral bleaching. Figure 5 is a
plot of the geographic distribution of these reports and demonstrates that the success of
the DHW product is widespread, i.e. not restricted to a regional level.
The data products discussed in this chapter are available as HotSpot and DHW
charts (produced by CRW twice weekly in near-real time from the composite nighttime
AVHRR SST products). These products, along with descriptions of the methodologies,
are Web-accessible at http://orbit-net.nesdis.noaa.gov/orad/coral_bleaching_index.html.
6. Coral Reef Watch in Action
6.1 CASE STUDY: MIDWAY ATOLL
During July of 2002 CRW team members noticed a SST anomaly building in the
central North Pacific Ocean (Figure 3). This anomaly grew until temperatures at
Midway Island on 1
st August passed the threshold for coral bleaching, where they
remained. Based on the accumulated heat stress (recorded in the DHW product), CRW
issued a bleaching warning on 7
th August. There was, however, no one on site to
confirm the existence of bleaching. Continued monitoring revealed that SSTs in this
area remained above the threshold for more than a month, and that SSTs did not drop
back below the threshold until 7
th September (Figure 6).
At this time, the Hawaii Laboratories of NOAA’s National Marine Fisheries
Service (NMFS) were due to lead a multi-agency scientific cruise to the North West
Hawaii Islands (NWHI) as part of their 2002 NWHI Reef Assessment and Monitoring
Program. This cruise was scheduled almost one year prior to this bleaching event as
part of their on-going monitoring efforts. The cruise was conducted between
September 8
th and October 7
th
, 2002. During the cruise, surveys were conducted at
Midway Atoll on September 20
th
, 21
st and 25
th
. Although the CRW bleaching report
did not instigate the cruise, the cruise plan was altered to focus more attention on
conducting surveys in shallow backreef habitats based on the information that the field
team gained from the HotSpot products combined with NMFS in situ buoy data from
the region.
During the field work at Midway Atoll, which took place two weeks after the
thermal stress abated, the survey team found extensive fresh bleaching. In most cases,
the corals were bleached white with no algal cover (indicative of recent bleaching). In a
19
Extreme Events and Perturbations