Written by: Novia Yip
Four distinct species of freshwater eels are known in South Africa, two of which have been living in the spring at the Palinggat Homestead – where the Stillbaai Tourism Bureau is located – for the past 125 years. Adult freshwater eels migrate by natural instinct from freshwater into the sea, where they spawn, with the next generation migrating back to freshwater habitats. However, the natural reproductive ecology of freshwater eels remains a mystery.
In the spring at Palinggat Homestead, where the Stillbaai Tourism Bureau is located, two species of these freshwater eels can be found, the longfin eel and the Madagascar mottled eel. Their distribution is limited by the availability of suitable oceanic conditions, in which adults can spawn and larvae can develop.
These eels possess certain adaptations that help them move extremely fast through winding movements. The soft dorsal, anal and caudal fins are joined together to form a long tail fin. It is this body shape that gives the eels the advantage of wriggling in-between plant growth or between rocky, coral crevices in which they hide and wait for their prey to pass.
Normally these eels feed on crabs and tadpoles, their natural food in the pond. However, the eels are also fed chicken liver by the Stillbaai Tourism staff, once a day. When it is not feeding time, one cannot see the eels easily, as they hide between the crevices of rocks. However when it is feeding time, and the oils from the chicken liver are deluded into the spring, the eels smell it quickly and sneak out towards their tasty treat.
As long as a river or stream complies with the temperature and oxygen need of eels, they can live in almost any habitat. In general, the deeper the water, the larger the size of the eels.
Because eels prefer warmer waters, they feed more actively in summer than in winter. Low environmental temperatures causes their metabolic rate to decrease and when water temperatures drop below 10℃, all feeding activities are terminated. Eels find their prey by smelling, however, they must almost touch the prey in order to detect it. Eels give preference to slow-moving prey, and organisms occurring in large numbers.
The breeding ground of freshwater eels is located east of Madagascar in the Indian Ocean. Although mature eels live in freshwater, or river estuaries, they migrate to the sea to spawn by natural instinct, often during floods or strong river flows after heavy rains. Before the mature eels enter the sea, they undergo physio-morphological changes to adapt to ocean life. Their eyes enlarge to about double their size, so as to facilitate vision in the deep water. Their retina changes from purple to a golden color, their snout becomes more pointed and their body fat levels increase. These changes help the eels to survive and thus to reproduce.
A female eel can lay up to 10 000 000 eggs, though millions of larvae are caught or perish before they reach their destination, fortunately, many of them still remain. The larvae are carried to the mainland of Africa along different ocean currents. In the coastal waters, the leaf-shaped transparent larvae can grow until between 5 and 6cm long. Then, the larvae start swimming in the direction of freshwater, enter the estuaries and swim upstream. They ascend waterfalls by slowly wriggling up the rocks, moss and crevices, and if this is not done during their larvae state, they can become too big or too heavy to ascend high rocks or walls.
What’s interesting about the Palinggat Homestead, where the Stillbaai eels live is that each time the mature eels leave the pond to spawn in the Indian Ocean, only three or four of their next generation of offspring return, with their mothers never coming back at all. How the eels are able to navigate back to the original habitat of the generation before them remains a mystery and even though biologists have theories as to why they might be capable of such incredibly difficult navigation, they may never know.