One of the first interesting facts that staff at the Save Our Seas Foundation D’Arros Research Centre told me about the adjacent St Joseph Atoll was that the whole atoll ecosystem is immensely affected by the continuous change of the tides. Before working at St Joseph, the study site for my PhD project, I didn’t quite understand the full meaning of what they were saying. But now I’ve spent four months on the remote atoll, catching juvenile blacktip reef sharks Carcharhinus melanopterus and sicklefin lemon sharks Negaprion acutidens for my project, and I not only understand what they meant, but have even learned to use the changing tides in my favour.

St Joseph Atoll is subject to the patterns of a mixed semi-diurnal tidal cycle, which means that the magnitude of the high and low tides differs throughout the month, depending on the lunar cycle. Extreme high and low tides, called spring tides, occur around full and new moon, while moderate tides, called neap tides, occur around half moon. Therefore the appearance of St Joseph Atoll changes completely from one extreme to the other within a single month. At spring low tide, large sections of the reef flats are exposed to the heat of the day and transform into a hot, desert-like habitat. At spring high tide, the atoll fills completely with water and nearly disappears into the surrounding ocean. Also, the lagoon’s colour changes with the large amount of water that enters from the surrounding ocean, transforming from bluish turquoise to a much darker greenish colour.

Animals that live at St Joseph Atoll have adapted to these extreme changes in their environment by using different feeding strategies and foraging grounds at different tidal stages. At low tide, while my field team and I are struggling to drag our kayaks through the atoll’s shallow waters, we suddenly see seabirds all around us. They are feeding on worms and other creatures hidden in the freshly exposed and still-wet sand flats. However on an incoming spring high tide, the inflowing water quickly fills this exact same place and transforms it into a feeding ground for predatory fish and adult and sub-adult reef sharks. This hectic aggregation of foraging fish quickly turns the water into a murky soup to the extent that we are unable to see the substrate below us.