Freshwater Dolphin Fish (Mormyrus Longirostris): Ultimate Guide


Freshwater Dolphin Fish are a species of fish that use small electrical pulsations to find food and defend themselves. It is found throughout the streams, rivers, and lakes in West African countries, such as The Democratic Republic of the Congo, Malawi, Mozambique, Tanzania, Zambia, and Zimbabwe. It is also referred to as the Eastern Bottlenose Mormyrid, Eastern Bottlenose, Elephant-snout, and “Baby Dolphin” by hobbyists. It gets its names from its similar appearance to a bottle-nosed dolphin, with its gray, smooth body and elongated snout. The Freshwater Dolphin Fish is frequently confused with other species and subspecies, such as the Mormyrus Rume, Mormyrus Kannume, and Mormyrus Caschive, as they are almost indistinguishable when young, which makes purchasing difficult. The Freshwater Dolphin Fish is known for being intelligent, inquisitive, and outgoing, and keepers report the fish will play with toys and learn tricks. Due to its electrical pulsations, it can give a mild shock when caught.

Freshwater Dolphin Fish Care

Freshwater Dolphin Fish can be difficult to keep, and it is very important to keep them in the correct water conditions. The optimal range for the Freshwater Dolphin Fish is 77°F to 82°F. While they can tolerate temperatures up to 82°F, Freshwater Dolphin Fish are most comfortable at 78°F. When kept at higher temperatures, keepers noticed the fish were significantly agitated and breathing heavier. To keep them happiest, it’s best to keep the tank at the lower temperature range.

The pH levels for Freshwater Dolphin Fish should be kept between 6.5 and 8.0, with the consensus being that they do best from 6.5 to 7.2. The most important factor is to always keep the pH level consistent. Similar to the higher water temperature, the fish’s behavior changes greatly if pH levels are off slightly, so maintaining a stable pH level is vital to keeping them healthy long term. Regular water changes should be done to keep the water in pristine condition. It’s worth noting that under 10 inches, the Freshwater Dolphin Fish is very sensitive to changes, while many keepers note that fish greater than 10 inches become hardier and more accepting of fluctuations in tank parameters and can tolerate less frequent water changes. When maintaining the aquarium’s water condition, no chemicals should be used. The Freshwater Dolphin Fish is a scaleless species and, because of this, is highly intolerant to water treatments, medications, and salt.

Freshwater Dolphin Fish (Mormyrus Longirostris)
Freshwater Dolphin Fish (Mormyrus Longirostris)

Freshwater Dolphin Fish Size

The Freshwater Dolphin Fish can grow to a large size, with the largest specimens measuring up to 30 inches (75cm) and weighing up to 22 pounds (10kg). There is no differentiation in size between male and female fish, although females may appear larger around the middle when they are carrying eggs.

Freshwater Dolphin Fish Tank Size

While Freshwater Dolphin Fish can be kept in smaller tanks to begin with, they will eventually need a larger tank due to accommodate their growth. When purchased small, the Freshwater Dolphin Fish can be kept in a 20-gallon tank, with most sellers suggesting a 55-gallon tank, but as they grow the tank size will need to increase, and by the time the fish reaches 10-inches, they will need at least a 100-gallon tank. Keepers have reported their Freshwater Dolphin Fish being happiest in 250-gallon tanks, particularly if there are other fish living in the tank. They need ample room for movement because they are active swimmers.

Feeding Freshwater Dolphin Fish

Freshwater Dolphin Fish are voracious, yet picky eaters. Most keepers have been unsuccessful in feeding pellets of dry foods, with fish ingesting out of curiosity and quickly spitting it out. Emulating the natural diet of the Freshwater Dolphin Fish in the wild will gain the best results. They should have a primary diet of live foods, including bloodworms, blackworms, white worms, grindal worms, larvae, and small crustaceans. Some Freshwater Dolphin fish will eat frozen foods but are more likely to accept live foods at first as they may be extremely picky when settling into a tank. Offering bloodworms will almost always get them to eat, however, this should not be their primary diet for an extended amount of time, as bloodworms have little nutritional value. It is suggested that the bloodworms be soaked in fish vitamins, and as the fish becomes more comfortable with eating, gradually increasing the diversity of food. Freshwater Dolphin Fish have a fast metabolism and are constantly active, because of this smaller fish should be fed 4 to 5 times a day until at least 8 inches in length, after that the feedings can be reduced to 3 times daily.

Freshwater Dolphin Fish Lifespan

There is no definitive lifespan on record for the Freshwater Dolphin Fish. While one source lists their lifespan as 15 years, the generally accepted lifespan for a Freshwater Dolphin Fish is between 6 to 10 years. A study conducted on the lifespan of Mormyrus Rume in the wild found the average fish lived to 6 years, so it can be assumed that the Freshwater Dolphin Fish has a similar lifespan. However, kept in optimal conditions in captivity there is the potential for the fish to live longer.

Freshwater Dolphin Fish Tank Setup

Having the appropriate tank set up is vital for keeping Freshwater Dolphin Fish healthy. In the wild, the Freshwater Dolphin Fish is found in caves and deep waters with soft, muddy bottoms, and does best in aquariums with soft substrate. They naturally root for food, so a sand bottom, with little or no gravel, is best for the Freshwater Dolphin Fish. Dense gravel bottoms or hard substrate can potentially injure the fish’s nose while it is foraging. Freshwater Dolphin Fish require ample places for hiding, such as caves and plants, but also need sections of open water for swimming and rooting in the sand. They are regularly caught in deep gill nets and appear to prefer deeper waters, with their activity levels peaking at night, because of this low lighting conditions are best for the Freshwater Dolphin Fish. Bright lights will intimidate them, they will spend the majority of their time hiding and will not show their foraging behaviors as consistently as they would with dimmer lighting. Plants should be included in the Freshwater Dolphin Fish’s environment but choosing live plants can be tricky. Due to the digging behaviors of the fish, it is best to avoid delicate plants that will be easily torn up while the fish is foraging for food. Fake plants that may leach metals into the water should not be added to the Freshwater Dolphin Fish’s aquarium, because they are highly sensitive to metals.

Breeding Freshwater Dolphin Fish

There are no known reports of successful breeding of the Freshwater Dolphin Fish in captivity, and the majority are wild-caught. Studies conducted in the wild suggest that the fish have an extended breeding period, potentially the entire year, with a heightened period from August to October. Breeding grounds for the Freshwater Dolphin Fish are thought to be in deep waters. It is very difficult to distinguish between male and female fish, however, females may become plumper when ready to breed. Female Freshwater Dolphin Fish can carry between 10,000 and 70,000 eggs at a time after breeding.

Freshwater Dolphin Fish and Disease

Freshwater Dolphin Fish are susceptible to diseases and parasites, particularly worms and protozoa. Many keepers describe having to nurse their fish back to health when first purchased due to a combination of parasites from the wild and new diseases caught during captivity. This can be incredibly difficult to treat because of the Freshwater Dolphin Fish’s intolerance to medications and water treatments. With optimal water conditions and care, keepers reported their fish were able to heal themselves. However, smaller fish will likely not be as hardy, and it is recommended to purchase at 6-inches or bigger to avoid poor health and potential death.

Are Freshwater Dolphin Fish Aggressive?

Freshwater Dolphin Fish are aggressive fish and should be kept either solo, in a larger group of themselves, or with fish that will not mind their bullying. When small, they tend to be shy, but as they age and grow, they become highly aggressive and will harass other fish. The Freshwater Dolphin fish is known to chase smaller fish, while also using its intelligence to bully bigger fish, for this reason, most believe it should be kept by itself.

Freshwater Dolphin Fish are found in shoals in the wild and can be kept in larger groups of their own kind when small, but they will need to be separated as they grow because they will start to pick off the weaker and smaller fish in the group. Some keepers have had success keeping them with calmer fish, or with other fish species found in their natural environment. Any fish added to a Freshwater Dolphin Fish aquarium should be monitored closely for bullying, while it does not have teeth and may not do much physical damage, it can significantly stress other fish with its constant harassment.

Tank Mates for Freshwater Dolphin Fish

The Freshwater Dolphin Fish is not compatible with most fish because of its indiscriminate bullying. In particular, Freshwater Dolphin Fish should not be kept with other electric fish that may interfere with their own electrical currents. Several keepers reported housing their fish with Black Ghost Knife Fish, another electrical fish, however, it’s advised to keep them separated. The Freshwater Dolphin Fish is not compatible with tankmates that are smaller than its mouth, as it is thought to eat small fish as part of its diet in the wild.

Where to Find Freshwater Dolphin Fish for Sale

Freshwater Dolphin Fish are rare in the fish trade but can be found occasionally in a live fish store. The difficulty becomes identifying exactly which species is being sold because misidentification is so common. They are available through online live fish suppliers, and their price ranges widely from $15.00 up to $160 for a single fish.

Here’s a few online retailers that currently list Freshwater Dolphin Fish for sale:

The Wet Spot sells Freshwater Dolphin Fish for $15.00 for a singly 1.5-inch to 2-inch fish. (Please visit their site for most current prices.)

AquariumFish.net sells Freshwater Dolphin Fish for $43.29. (Please visit their site for most current prices.)

Predatory Fins sells Freshwater Dolphin Fish for $160 for a single 9-inch to 10-inch fish, and $120 for a single 6-inch to 7-inch fish. (Please visit their site for most current prices.)


References

Bell-Cross, G. and Minshull, J.L. 1988. The fishes of Zimbabwe. National Museums and Monuments of Zimbabwe.

Konings, A., Bills, R., Getahun, A., Kazembe, J., Marshall, B. & Moelants, T. (2019). Mormyrus longirostris (amended version of 2018 assessment). The IUCN Red List of Threatened Species 2019. doi:10.2305/IUCN.UK.2018-2.RLTS.T182025A155052229.en

Saunders, L. (2014, July 23). The rare freshwater dolphin fish. Your Guide To Everything Animal. Retrieved February 1, 2022, from https://laurendeadly.wordpress.com/2014/07/23/the-rare-freshwater-dolphin-fish/

Skelton, P.H. 1993. A Complete Guide of the Freshwater Fishes of Southern Africa. Southern Book Publishers (Pty), Ltd, Halfway House.

Tweddle, D. 1983. The fish and fisheries of Lake Chiuta. J. Sci. Tech. (Malawi) 4(2): 55–83.

Tweddle, D. and Willoughby, N.G. 1982. The distribution and identification of mormyrid fishes in Malawi, with notes on the synonymy of Marcusenius nyasensis and M. livingstonii (Mormyriformes: Mormyridae). JLB Smith Institute of Ichthyology, Special Publication 24:

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