Depending on the region where you dive, the reef may have a seasonal life cycle, but not always. If you're a regular diver in the Red Sea, for example, you'll be lucky enough to find the same moray eel, under the same rock, year after year. But what happens in Mauritius, where the reefs change throughout the seasons?
As we have just suggested, many fish species are migratory, so few fish are sedentary, apart from a few species that live on coral reefs or in enclosed waters. The term ‘migratory’ in this case means ‘coming and going with the seasons’. It's worth pointing out, however, that not all species will travel thousands of kilometres. In fact, migration can take place over periods ranging from a few days to several years, and cover distances ranging from a few metres to several thousand kilometres. Among the largest migratory species are tuna, bigeye tuna, sharks, dolphins and other cetaceans such as whales. In Mauritius, the movements of humpback whales, which have travelled more than 5,000km from the Antarctic, where they feed, to the warm tropical and sub-tropical waters of the Indian Ocean, where they breed, provide an interesting example of a well-defined migration.
The large migratory species are called ‘oceanodromous’ (which means that they are fish that migrate while remaining in a marine environment) and migrate significant distances. They migrate to feed or reproduce across the oceans. As a result, highly migratory species are particularly sensitive to overfishing, poaching, pollution and impassable obstacles such as dams. Catching an adult interrupts the cycle, as it has probably not yet reached its reproduction site, which can compromise the renewal of the species and threaten its long-term survival.
Let's take a look at what's happening in the reefs around Mauritius. During the summer, generally from December to April, the coastal waters will be warmer (rising from 20°C at their lowest to 28°C at their highest) and therefore highly productive during the summer. These waters will therefore be richer for fish to feed on during the summer and cold and unattractive in winter. As a result, the fish retreat to deeper waters. Other seasonal changes, such as changes in the depth of the thermocline, can cause migrations between inshore and offshore waters.
Now that we've talked about stereotypical fish, let's look at other marine species such as nudibranchs and seahorses, which are more common in Mauritius during the summer.
What happens during the winter when they seem to ‘disappear’ from the reefs?
To begin with, marine species do not hibernate, but they do follow well-defined life cycles. In fact, some have a birth-growth-adult-death cycle that extends over a year, coinciding with the seasons and environmental variations. In the case of nudibranchs or sea slugs, unlike many marine species that provide some parental care, they follow a different strategy. Their life cycle is closely linked to the availability of food. When food is scarce, some species go into dormancy: they take shelter in the reef, reducing their activity to save energy. This phenomenon explains why they are less common at certain times of the year. In this way, nature naturally regulates their populations: the adults lay their eggs before dying, without looking after them. When conditions become favourable again, the eggs hatch, giving rise to larvae that begin their development. Depending on the species, egg size and water temperature, this larval phase can last from a few weeks to several months. As for the adults, their life expectancy rarely exceeds one year.
As far as the presence of seahorses is concerned, they are subject to marine currents, which influence their movements and can lead to a form of seasonal migration. Their distribution and abundance vary from season to season, mainly due to their reproductive behaviour, which plays a key role in these fluctuations. Certain areas, such as bays and estuaries, can serve as essential natural breeding sites. The introduction of closed seasons for fishing during the breeding season, and the creation of marine protected areas, could therefore encourage their conservation.
Temperature also plays a major role in the visibility of other species on the reef. For other species present on the reef, such as sea urchins, temperature is a determining factor that influences their abundance over the seasons. Depending on their metabolism, they can be negatively affected, particularly when temperatures become too high and exceed their tolerance limits, a phenomenon accentuated by global warming.
On the other hand, some species, such as scorpionfish and moray eels, tend to fear the cold. They remain present during the winter, but they simply become more difficult to observe. In periods of cold, they take refuge in cavities in the reef, where the temperature is more stable and slightly warmer than that of the water column. As they are not warm-blooded, their metabolism slows considerably when the temperature drops, even by just a few degrees. Less active, they spend more time hiding.
Finally, the breeding season for some fish (such as clownfish and blennies) also influences their apparent presence. In other words, during this period, the adults devote themselves to protecting their eggs, often remaining hidden near their spawning site, making them less visible on the reef.
Conclusion:
The life cycle of marine species on the reefs of Mauritius is complex and influenced by a multitude of factors. Water temperature, the abundance of food, the metabolism of each species and the seasonal or occasional migrations of fish play a key role in the dynamics of the ecosystems. While some fish migrate on a regular basis, often dictated by their reproductive or feeding needs, others are less mobile, adapting to temperature variations by becoming more discreet and thus ‘disappearing’ from the reef in our eyes. Understanding these factors and their interactions is therefore essential to preserving these fragile ecosystems.
Note: big thanks to Olly who always makes sure my knowledge is up-to-date.
