A Coral Spawning Calendar for Zanzibar

Determining annual reproductive patterns of reef-building corals in East Africa

Location

Zanzibar, Tanzania

Funding

Sound Ocean Science

(Seeking additional funders)

Years

starting 2024

Partners

marinecultures.org

Chumbe Island Coral Park

Under the Wave

Linda Bahari

For the first time in Zanzibar, this ambitious project will

document coral spawning patterns for the dominant reef-builders, using multiple complementary approaches. In collaboration with local community and conservation partners, we aim to:

Document mass coral spawning in situ for 8-10 dominant species of reef-building hard corals.

Create a spawning calendar for hard corals in Tanzania, in relation to seasonal/celestial cycles (lunar phase, sunset, moonrise), and environmental factors (e.g., temp, light).
Provide baseline data to empower relevant communities, resource managers, NGOs and scientists to propagate coral for restoration efforts, access coral larvae for research, and to better manage human activities that affect coral reproduction.

Have you observed coral spawning in Zanzibar? Report it here!

What does coral spawning look like?

Most scleractinian coral species, the hard corals that build three-dimensional reef habitat, are broadcast spawners, simultaneously releasing eggs and sperm into the water column often only once or twice per year. After release, eggs and sperm fertilize at the sea surface and develop through a larval stage before settling and attaching to the seafloor. These synchronized spawning events are known to occur en masse across vast areas of reef - where dense coral populations exist, large surface slicks of coral gametes can be observed at the water surface following a spawning event.

Spawning for each species is closely timed to solar, lunar, and seasonal cycles, under the influence of environmental factors such as water temperature, solar insulation, and light. For example, an individual species may spawn annually on the 10th night after the full moon in May between 45-60 minutes after sunset. Because gametes only remain viable for a few hours, it is critical to reproductive success that individuals in a coral species spawn in such close coordination.

In the most well-studied regions (e.g., Australia’s Great Barrier Reef), mass spawning of dozens of species has been observed over just one or two nights in a single year ; however, variation in spawning frequency and timing is known to occur across geographies and there are instances of populations with more protracted non-overlapping spawning periods of several months.

Rationale: Why do we want to know when corals spawn?

Establishing baseline data of when and where corals spawn on tropical reefs is fundamental to increasing our knowledge of these essential organisms and to furthering our ability to conserve existing populations or rehabilitate degraded reefs. While coral spawning monitoring has been carried out in other parts of the Indo-Pacific Ocean, producing over 6500 records of spawning dates and times, observations in eastern Africa are particularly scarce. The majority of observations in this ocean basin come from the Red Sea and Australia, with only two single reports from Tanzania, both collected opportunistically at a single location in Zanzibar.

To our knowledge, no dedicated study to directly document coral spawning in situ has been carried out in East Africa. A 2004-06 study in Kenya’s Mombasa Marine Reserve inferred coral spawning indirectly by quantifying gametogenesis in coral tissue samples and monitoring the settlement of coral spat. While this past work does not allow prediction of species’ spawning day and time, it will, along with the two records from Zanzibar, support our study’s design to do so.