Annual Coral Mass Spawning in Western Australia
One of the most spectacular natural events in the Ningaloo Marine Park is the annual mass spawning of corals. Over 250 species of coral have been identified to occur in the Ningaloo Marine Park and the majority of these species mass spawn.
Most coral species are colonial animals made up of millions of anemone-like creatures called polyps. Most are hermaphrodites (i.e. having both male and female gonads) but some are either male or female. During the mass spawning events, coral eggs and sperm are released into the water and float to the surface. Fertilization then takes place and the process of larval development begins. The larvae continue developing for the next 4-6 days while at the whim of the ocean currents until they are ready to find a place to settle on the ocean floor. At this time the larvae actively swim down to the seabed and search for a suitable place to settle. Upon attachment to the seabed the larvae change their form to that of the adult animals known as polyps.
The major period of coral mass spawning on Western Australian reefs mainly occurs after the full moon in March or April. Minor spawning events also occur after the full moons in February and May and after the new moons from February to May. In some years spawning is ‘split’ between the March and April full moons. Spawning can occur on any of several nights from between 4 -14 nights after new or full moons. However, most often spawning occurs 6-10 nights after the moons (i.e. on neap tides).
For people wanting to observe the phenomenon this year in Ningaloo Marine Park, 7-9 nights after the full moon in March or April are the best dates. On these nights spawning will begin around 8pm and last until about 10pm. Dates of major spawning may vary slightly at different reefs along the WA coastline. The major coral-spawning event this year is expected to occur between:
Minor Spawn March 2011 and Major Spawn April 2011*
Mass spawning is a mechanism to facilitate successful reproduction. By spawning at the same time and when water movement is minimal (i.e. neap tides), corals maximize the concentration of eggs and sperm thereby increasing the chances of fertilization. The simultaneous spawning of many different species at night ensure enough coral larvae, or planulae, survive by inundating predators with an excess of food over a short period, thereby minimizing the impact of predation by fishes on the survivorship of larvae.
Coral mass spawning coincides approximately with the autumn intensification of the southward flowing Leeuwin Current which flows poleward along the continental shelf break (i.e. 200m bathymetric contour) off the Western Australian coastline. The Current transports the larvae of corals and other animals down the WA coastline providing a mechanism potentially connecting reefs in the tropical north, such as at the Montebello Islands, to reefs, such as Ningaloo Reef, which are further south. This mechanism is also responsible for the occurrence of coral reefs, such as the Abrolhos Islands, at latitudes where they do not typically occur.
Coral spawning often occurs during periods of calm seas and light on-shore winds that are typical of the autumn period in WA. These conditions can result in coral spawn not being dispersed leading to the formation of coral spawn slicks which are often pushed onshore by winds and currents. This may cause the surrounding water to become oxygen depleted due to biological processes in the water column causing death of coral, fish and other biota. Reefs near Coral Bay have had major recorded major mortality events in 1989 and 2002 due to this process. Monitoring of the reef by DEC and the Australian Institute of Marine Science scientists is providing information on the capacity of Ningaloo Reef to recover from catastrophic events such as these.
Coral slicks are often confused with the naturally occurring blue-green algae (Trichodesmium) blooms and can form large slicks stretching for kilometres. They can occur throughout the year along the WA coastline usually in calm, hot weather. They are frequently reddish-pink or brown when they start decomposing. At this stage they are often confused with oil slicks. They are commonly referred to as red-tides or sea sawdust. True coral slicks will only be seen in narrow windows of 7-11 days after the full moon. The two are easily distinguished by shape when observed very closely, Trichodesmium are splinter-like whereas planulae are globular.
Observations on coral spawning in Western Australia have been carried out each year since 1984. Previous observations by the public along the W.A coastline have provided a valuable insight into the geographical synchrony of coral spawning in W.A and the variability between years at the same location. It is important to continue these observations each year, as this information is important to the long-term management of W.A coral reefs.
*Please note that as with all natural occurences the prediction of an exact dates is not 100% guaranteed.