The longfin eel (Anguilla mossambica), one of four species of eel that utilise rivers draining the east and south-east coasts of Africa, spends most of its life hidden from view of humans. Whether feeding at the bottom of a dark mountain pool, slithering silently up and down rivers at night or spawning secretly at some unknown location out in the vastness of the Indian Ocean, the chances of seeing a longfin eel in the wild are slim.
Over the past few decades there has been growing concern among scientists that eel numbers are plummeting the world over. The main threats to eels include changes in oceanic currents resulting from climate change, chemical and physical pollution of rivers and oceans, over-harvesting, habitat loss resulting from water abstraction in rivers and man-made barriers to migration such as reservoirs and river diversions. Eels are catadromous, meaning that they spawn at sea and feed in fresh water, and rely on rivers as migration corridors between their spawning and feeding grounds. Without healthy rivers it is not possible for eels to complete their life cycle. Consequently the degraded state of many of east and south-east Africa’s river systems is a serious cause for concern.
Eels are unlike other migratory fish in that they can spend up to a decade or more in a single river pool. In rivers they prey opportunistically on anything from bugs and crabs to frogs and fish and may grow to over a metre long before embarking on the long and arduous journey back to the ocean to spawn. Their lengthy stays in rivers may make it difficult to detect critical declines in eel numbers until it is too late. Furthermore, there is a serious lack of knowledge about eel movements and migrations, with a case in point being our inability to locate and monitor their spawning grounds at sea.
Their complex and mysterious life cycles render longfin eels especially difficult animals to study and conserve and there is an urgent need to increase eel-related research, awareness and education. Perhaps satellite tags could be used to track their mysterious oceanic journeys as has recently been done for their cousin the European eel Anguilla Anguilla. Since eels rely on healthy rivers, estuaries and oceans to survive and reproduce, they are excellent barometers of the health of these different aquatic ecosystems and the connectivity between them. We stand to learn a lot from changes in eel behaviour and numbers, and the sooner we start taking note of such changes the better.
The short film e e l s was shot in a tributary of the upper Breede River near Worcester in the Western Cape of South Africa, and aims to raise the profile of African eels in support of World Fish Migration Day. I was interested to see what type of challenges an eel might face when making its way upriver in search of foraging sites, and found signs that obstacles to eel migrations are plentiful and severe, and will no doubt intensify in forthcoming decades. Eels are fragile, misunderstood creatures and without our help we may lose our eels before we even understand them.
For more information on anguillid eels visit: Freshwater Fish Specialist Group.