Coral Spawning and Bleaching
sgittings at ocean.nos.noaa.gov
Mon Sep 18 15:22:47 EDT 1995
On September 16, eight evenings after the September 8 full moon, the corals of
the East Flower Garden Bank (northwest Gulf of Mexico) spawned in mass. The
timing and species involved were similar to some previous years. This was
after August spawning reported over seven evenings between August 16 and 22.
Unfortunately, we were not able to make observations on Sept. 15 or 17.
Diploria strigosa brain corals began spawning at about 8:00 p.m. (CDT) on the
16th and continued until around 10:00 p.m. Sperm release from male Montastrea
cavernosa colonies was observed between 9:00 and 10:30. No egg release by
females was reported. Montastrea annularis (most descriptions indicated M.
faveolata form was the predominant participant) star corals spawned between
9:40 and 10:15.
A dense spawn slick was evident from 8:00 to 8:30, then again from 9:15 to
10:35, and a final small, but dense slick passed the ship at 11:05. There was
a lull in activity on the bottom between 8:30 and 9:15. The decrease in the
slick density occurred at nearly the same time.
Also observed spawning at 10:30 was a male Christmas tree worm (Spirobranchus
giganteus; no egg release by females was witnessed in September), and the
sponge Ectyoplasia ferox (observations at 3:30 and 10:30).
Associated observations: Ruby brittle stars (Ophioderma rubicundum) were
active, but unlike previous years, no spawning was witnessed. Brittle stars
were on the tops of corals. Some collected gamete bundles and scurried for
cover, as usual, when hit by video or dive lights. A manta ray was seen by
two divers around 9:15, but no mention was made of it feeding on gamete
The water was unusually green over the banks during the week before, and
during, the spawning. Visibility was less than 70 feet, and currents on the
bottom just under a knot. Blue water usually surrounds the Flower Gardens and
visibility is typically over 100 feet. A swarm of 100s of moon jellys
(Aurelia aurita) passed over the site at 9:15, also an unusual sighting, one
probably associated with the atypical water mass. Seas during the spawing
were less than two feet and winds were from the south at less than 5 knots. A
1 knot bottom currents was heading east and the surface current was nearly
slack, but heading northeast.
Things to consider: There is still a lot to learn before we can accurately
predict the expected timing and extent of spawning on various reefs and for
different species. Could it be that the relatively early August full moon
"confused" the corals into spawning erratically over seven nights, even though
the majority happened on the 8th evening after the full moon? The early Sept.
full moon was followed eight evenings later by a dramatic mass spawning. In
1990, a dramatic "main event" was witnessed on August 13, three nights before
this years first observations, so there is obviously more to the picture than
the date. Next year there is a very late July full moon and a late August
full moon (similar to 1993 when the main event was seen on Sept. 8, after the
August 31 full moon). Based on observations at the Flower Gardens since 1990,
I am betting on the September spawning date. If no unusual environmental
events intervene, it also seems likely that the event will be quite focused
and perhaps more intense than this year's activity.
Water temperature was 30.0#161#C on September 16th, and considerable bleaching
was in progress for three species (Millepora alcicornis, Montastrea cavernosa,
and Stephanocoenia michelini, while a number of other species appeared to be
much paler than normal). This high a temperature this late in the year is
unusual for the Flower Gardens and is very similar to patterns of 1990, the
last bleaching year at the site. Even so, my visual estimate is that extreme
bleaching is affecting less than 10% of the coral cover (coral cover is about
50%). This may, however, exceed 1990 levels, so it bears watching. Corals
are being monitored at 80 repetitive photographic stations, each eight square
meters in size.
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