African Tigerfish (Hydrocynus sp.)

The Hydrocynus tigerfishes are among the most impressive of Africa’s freshwater fishes. The largest of these, the Goliath Tigerfish (H. goliath), is said to to reach more than four feet in length and over 100 pounds, naturally making it one of the most popular species for sport fishermen. It can also be found on rare occasion within the aquarium trade, where it is highly coveted by those interested in keeping “monster fish”. But today we’ll focus instead on its slightly smaller siblings.

Hydrocynus is a confusing group, though it might come as a bit of a surprise to learn that some of the closest relatives of this ferocious fish are small, peaceful tetras like the Congo Tetra (Phenacogrammus) and the Yellow-tail Congo Tetra (Alestopetersius). These belong to a uniquely African family of tetra-like fishes known as the Alestidae.

Judging by a recent genetic study of Hydrocynus from across the major river drainages of Africa, there are still undescribed species waiting to be recognized. The Goliath Tigerfish forms its own distinct lineage within the genus, having diverged around 11 million years ago. You’ll find this brute only in parts of the Congo river system. All of the other species, both described and undescribed, occur more broadly on the African continent, often as geographically isolated populations.

There are three species in the northern drainages (e.g. the Nile, Gambia, Niger, Sanaga rivers), and, like H. goliath, these have only the lower lobe of the tail red. H. brevis is the larger of these, and it seems to fill a similar ecological role as H. goliath in this part of Africa. The smaller species are H. forskahlii and an undescribed relative from Cameroon’s Sanaga River. Typical aquarium specimens of this fish reach only around 10 inches, making it the most suitable for smaller tanks (though “small” is of course relative here).

When we head further south, the picture gets muddied by the complex geological and evolutionary history of the region. When Africa began to split apart several millions of year ago, forming its famous Rift Valley and corresponding great lakes, it broke apart the rivers that stretched across the continent. This isolated a population of tigerfish in the rivers of Tanzania, giving rise to H. tanzaniae, while the remaining populations of the Congo and Zambezi diverged into a species we call H. vittatus.

Except, it’s not quite so simple. The recent genetic study done on this group showed that there are significant differences between the various populations of H. vittatus. It remains to be seen what sort of morphological traits might separate these fishes, as they all share a similar appearance, having darkened scales along the sides which form a series of parallel stripes, along with a red or yellow caudal fin. This latter feature is said to be sexually dichromatic, with females having yellow tails and males red. Another recent study found some support for this, though it was noted that there are intermediate colorations in both males and females that can blur this sexual distinction.

Aquarists have noted some subtle difference in some of the H. vittatus specimens that enter the trade (usually out of Congo). One form is a bit thicker, having fewer lateral line scales and an extra row or two above the pelvic fins. Another form said to come from the lower portions of the Congo River is notable for having fewer scales, a more angular head, and a tendency towards habitating the bottom of aquariums. These two fishes may correspond to some of the undescribed biodiversity found in this group.

The true H. vittatus was described from the Okavango River, which sits at the southwesternmost portion of this complex’s distribution. Aquarium specimens aren’t likely to ever come from here, which calls into question whether this species is ever actually kept in captivity. Genetically similar populations occur in the neighboring Zambezi River, along with some of the other minor drainages that empty into the Pacific. And it also occurs throughout the Congo, alongside some of these undescribed forms. The situation is confusing at the present time and awaits further study.

As aquarium fishes, Hydrocynus husbandry is comparable to other large apex predators. Massive volumes of water are needed, though the exact dimensions will depend on the species in question. This is why correct identification is of the utmost importance, as one needs to know whether they are dealing with a potential hundred-pounder like H. goliath or a more manageable 10-incher like H. forskahlii. Since most aquarium specimens originate out of the Congo, a maximum size of somewhere between 12-18 inches can be expected, but, again, this depends on which member of the vittatus complex we’re talking about.

Juveniles are social, found in tight shoals along the shoreline of larger rivers and lakes. Tigerfishes at this age feed mostly on insects and even seeds that fall onto the water’s surface. This is of importance to aquarists, as it means that smaller specimens are hard-wired for a diverse diet, whereas adults feed exclusively on live fishes. Smaller specimens are thus more easily weaned onto other foods, like chopped pieces of meat, live worms, or even dry pellets. While it is undeniably entertaining to feed lives fishes to Hydrocynus, an immediate effort should be made to wean juveniles onto a varied diet. Dither fishes can help with this, and Tinfoil Barbs are a recommendable choice.

Coming primarily from the brisk waters of major river channels or the highly oxygenated waters of lakes, high water quality and good flow are necessary for keeping specimens in good health. Aquarium decor should be kept to a minimum, to allow for the maximum flow of water, as well as minimizing the threat of injury that might occur when a boisterous tigerfish thrashes about.

Tankmates are a controversial topic for tigerfishes. Some choose to keep them isolated, as they undeniably pose some threat to other species, especially so as they mature into adulthood. Specimens have been kept alongside all sorts of larger species: catfishes, barbs, peacock basses, stingrays, gar, bowfin, vampire tetras, and even arowana. Bottomdwellers are usually ignored, though at least one aquarist has reported carnage inflicted upon their resident Wolfish.

The determining factor when deciding on keeping an African Tigerfish is whether or not one can adequately house a large predator into adulthood. Hydrocynus might make for a tempting impulse buy, but a minimum aquarium size of 180-240 gallons is needed, and even larger for a species like H. goliath. Be sure you can meet this requirement, as it can be very difficult to rehome a large specimen.