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Issue: July 2013

The Perfect Surface Dweller: Hatchetfish (Full Article)

Author: Phil Purser


Photographer: Aquariumphoto.dk
Hatchetfish have been in the hobby for many years, and, as one veteran hobbyist explains, they are still the best surface-dwellers for a community aquarium.





 

Nostalgia is a funny thing. So often when we look ba
ck on the joys or memories of our youth, we do not see things as they truly were, but as our minds want them to be. Summer afternoons were sunnier and milder, boy or girlfriends were lovelier and kinder, television shows were cooler, and toys never broke. But seldom are these memories anywhere close to reality. The truth is that those summer afternoons were hot, muggy, and filled with mosquitoes; the fragile, summer love given us by that boy or girlfriend was short-lived and less fulfilling than we thought; the television shows of our youth were way beyond cheesy (we’d perhaps be embarrassed to go back and watch them now); and our childhood toys were constantly breaking, falling apart, or jabbing us with sharp edges in the days before safety recalls. But when I look back on the carefree days of my own youth in the Appalachian foothills of western Georgia, and I see the 15-gallon tank that my father and I kept and diligently maintained, I see one of the tank’s many inhabitants, the hatchetfish, as it truly was.

These wonderful fish were a staple in my aquarium; they wore quietly attractive coats of silvery-bronze. The hatchets were of a very hardy constitution, and their odd shape—a body that dips deeply, like a pelican’s throat pouch or the keel of a ship—is uniquely attractive. Thus, hatchetfish can be a very singular addition to the community tank. Let’s take a closer look at these demure little oddities, and perhaps you too will discover, as I did so many years ago, that a hatchetfish is just right for your own freshwater community aquarium.


Natural History


The hatchetfish comprise the family Gasteropelecidae. Containing three genera and nine species, this family is endemic to Central and South America; native habitats include rivers, swamps, and calm pools throughout the Amazon and its riverine tributaries and headwaters. Taking their common name from the extreme convexity of their ventral surface (early nomenclaturists believed that this deeply bowed body closely resembled the head of an axe or hatchet, hence, “hatchetfish”), these fish have a flat, straight dorsum and uniquely backward-sweeping pectoral fins. These fins are considerably more rigid than those of most tropical fish.

This fin-rigidity and keel-like body, when coupled with the hatchet’s highly developed pectoral musculature, makes for a very fast fish with absolutely superior maneuverability in the water. Capable of making very swift and acutely sharp turns, hatchetfish are adept predators of aquatic insects and fry in the wild. Moreover, the elongate pectoral fins have evolved into wing-like projections and have, interestingly, limited wing-like properties. Extending upward and back from the pectoral joint, these fins not only aid the hatchetfish in making extremely sharp turns in the water when pursuing prey or evading predators, but they also help the hatchetfish when it performs the curious behavior of leaping from the water to catch low-flying insects.

Wild specimens have long been known to stalk clouds of insects hovering low over the water. Zeroing in on the target with the aid of its enlarged eyes, the hatchetfish will leap impressive distances out of the water and, within a limited capacity, steer in the air with the aid of its wing-like pectoral fins. Thus, this fish is capable of taking its prey from the air simply by locating a target, leaping out of the water, and snapping the tiny invertebrate in its upturned jaws before splashing back down into its watery domain.

A Hatchetfish Aquarium


Of course, the hobbyist cannot reasonably expect to witness this behavior in the home aquarium. But you can see how a tight-fitting lid is absolutely paramount in preventing any unsanctioned jumps! This lid must be without large holes or filter access cut-aways, as the hatchets are capable of target jumping through these openings. All species of hatchetfish are top-level swimmers; an individual fish may live its entire life mere centimeters below the surface. Some species feed, sleep, and even breed just below the surface and may never dive to any notable depth.

While the family Gasteropelecidae is divided into numerous species, only really species appear in the pet trade with any notable degree of frequency. These are the silver hatchet, the black-winged hatchet, the common hatchet, and, my personal favorite, the marbled hatchet. This quartet of big-bellied community tropicals has a lot to offer, and each one varies enough from its kindred to make it unique and attractive in its own way. Each has its own set of considerations, as well, such that slight modifications to your tank, décor, or community-fish population might be in order before you leap right into your own hatchetfish endeavor.

Common Hatchet


The largest of the frequently kept hatchets is the common hatchetfish or river hatchetfish. The common hatchetfish (Gasteropelecus sternicla) thrives in the warm waters of the central Amazon Basin north through Venezuela and west through the lower elevations of Peru. This hatchet wears a muted, dashingly attractive coat of bonze-colored scales from the midline of the body down though the bulbous belly. The midline is marked with a handsome chocolate-brown to black line running from the gills to the caudal peduncle. This dark line is flanked in bands of bronze-gold. The upper dorsum is evenly earth-toned. The overall effect makes for a very flashy species.

Growing to slightly less than 3 inches, the common hatchetfish is one of the least active of the hatchets; this species seems to have keener eyesight than its kindred, and it prefers stalking its prey rather than actively pursuing fleeing fare. Thus, in the home aquarium, common hatchets glide gracefully along just beneath the surface and do not exhibit the incessant, jerky movements seen in some other hatchet species. In my personal experience, the common hatchetfish is also the longest lived of the four species described here. Longevity of over four years is common, with some specimens exceeding five years.

Common hatchetfish prefer soft water (as their native Amazonian waters have very low water hardness); maintain 2.0 to 14.0 dGH. Temperatures should range from 72⁰ to 81⁰F. As you might imagine, the water should be slightly acidic. A pH of 6.0 to 6.8 is best, though acclimated specimens may adapt to 7.0 to 7.2.

Common hatchetfish are devout carnivores; vegetative matter does not, to any appreciable measure, factor into their natural dietary regimen. Feed protein-rich flakes, brine shrimp, tubifex, bloodworms, or small, floating, carnivore pellets. Obviously, sinking fare will quickly drop out of this fish’s feeding range; no species of hatchet regularly feeds lower than just a few inches below the surface. To that point, I would also like to advise turning off all aeration and filtration during feedings, as water turbulence will cause otherwise floating fare to quickly sink out of the hatchetfish’s reach. Note also that this species is prone to gluttony and overeating in the home aquarium. Swiftly darting toward and gobbling up all surface-floating food items, the common hatchet will typically outcompete all its tankmates for food during feeding time. Compensate for this by dropping a tiny pinch of flakes or worms directly over the hatchet, then placing a normal amount in the rest of the tank; the first, small food drop should keep the hatchetfish busy while the real food drop will accommodate the rest of your fish community. Speaking of which, the common hatchetfish mixes well with virtually all other tropical species. Slight territoriality may occur between multiple common hatchets, but this aggression amounts to little more than bluff and bluster.

Marbled Hatchetfish


The second species of note is the marbled hatchetfish (Carnegiella strigata). This species is easily the most commonly encountered of all the hatchetfish in the pet industry. As such, it is also the least expensive; most chain pet shops sell marbled hatchets for less than $5 each. Growing to roughly half the size of the common hatchet, the marbled hatchet seldom exceeds 1¾ inches in length.
This demure little species is, however, the most prolific and accomplished jumper of all the hatchets. Actually capable of “flapping” its pectoral fins while airborne, this fish does not simply glide through the air in a leap, but can add to its forward propulsion though pectoral fin movement. The musculature responsible for this pectoral movement makes up a substantial portion of this fish’s total body weight. Evolutionarily, this fish is built around its pectoral muscles; those oversized muscles are responsible for this species’ survival and ecological niche. Capable of traveling more than 5 feet high or forward, the marbled hatchetfish is, indeed, one of the most accomplished jumping fish of its size in the world. Thus, keeping a lid on this species is absolutely paramount!

The marbled hatchet wears a coat of silvery scales with both black and mother-of-pearl streaks marbling its bulbous belly. The midline is adorned with bronze to gold striping, edged by black, and the dorsum is greenish-bronze. The eyes are slightly oversized, and the upturned mouth is perfectly shaped for nipping insects from the surface of the water.

While some hobbyists recommend keeping the marbled hatchetfish in schools, I have found that intra-species territoriality can spike when too many marbled hatchets are housed too closely together. I recommend keeping this species of hatchet singly, though a tank with plenty of surface area (a 20-gallon long for example) can accommodate a small shoal.

When it comes to water conditions, the marbled hatchet is slightly more demanding and sensitive than its common cousin. Maintain the pH between 5.6 and 6.5. While I’ve seen specimens acclimate to neutral pH of 7.0, these fish do not thrive in non-acidic waters. Low water hardness of 2.0 to 12.0 dGH is mandatory, and temperatures should be kept in the relatively narrow range of 74⁰ to 80⁰.

Décor in the marbled hatchet tank may or may not include floating vegetation. First and foremost, these fish must have vast stretches of open water. When frightened or sleeping, however, they relish taking cover in the dangling root systems of floating aquarium plants; water lettuce is particularly useful for this purpose. Thus, if you provide your marbled hatchets with a small, but secure, amount of surface cover, but plenty of open water in which to dart and roam, you will meet this species’ requirements for décor and physical habitat. Many hobbyists also report great success with very tall sword- and grass-like plants whose leaves reach the top of the water column; their marbled hatchets seem to find security when swimming and darting among the vertical leaves.

The dietary regimen for marbled hatchets should include a wide variety of proteins. I feed a rotation of bloodworms, tubifex worms, and high protein flakes. If you can get them, flightless fruit flies (Drosophila spp.) dropped into the tank will be absolutely relished by your marbled hatchet.

A peaceful community species, the marbled hatchet can mix with virtually all other species that share its water-condition requirements. Be aware, however, that the marbled hatchet does not like sharing the top of the water column with any other fish. I do not recommend mixing them with bettas, paradise fish, top-level killifish, or other hatchetfish of any species. High-energy fish like tiger barbs and danios may also stress a marbled hatchetfish.

One downside to this lively little fish is its relatively short lifespan; one of the shortest-lived of all the hatchets, the marbled hatchet seldom exceeds two years in the home aquarium. The iridescent and black marbling on this fish, however, makes it a very attractive, if short-lived addition to the freshwater tank.

Blackwing Hatchetfish

The third hatchet I’d like to discuss is the blackwing hatchetfish. Smaller at adulthood than even the demure marbled hatchet, the blackwing hatchetfish (Carnegiella marthae) grows to little more than 1¼ to 1½ inches in length. Coloration of this species is brightly silver at the lowest point of the belly, becoming darker and darker towards the midline of the body, which is marked in a darker, less defined variant of the bronze-gold midline bar that is typical of all hatchetfish species. The convex belly is often trimmed in a thin band of black, as well. The dorsum is almost totally black, as are the pectoral fins, hence the common name “blackwing” hatchetfish.
Native to calm, heavily vegetated waters and not as prone to jumping as its marbled cousin, the blackwing hatchet tends to prefer a heavier covering of floating aquatic plants under which it can take refuge. I recommend water lettuce or water hyacinth. Duckweed also works well but may clog aquarium filters. A tightly covered tank, however, is still necessary, as this fish will jump out of the tank if given a chance. Accommodate both floating plants and a lidded tank by dropping the water level in your aquarium several inches below the level of the lid and employing fully submersible water filters.

Water conditions for this species are basically as described for the common and marbled hatchetfishes: pH of 5.8 to 6.9, dGH of 2.0 to 12.0, and temperatures of 74⁰ to 83⁰. Strict insectivores, these fish may be fed tubifex, brine shrimp, or bloodworms, but they will thrive on high-protein flakes, as well. One major behavioral difference between the blackwing hatchet and the other hatchets is this species’ propensity to seek cover in the home aquarium. Hiding among the dangling root-tangles of floating plants, this fish is not as needful of open water as are its larger, riverine cousins. Another feature of note is that the blackwing hatchet seems more skittish than either the common or marbled hatchet; this fish may stress more easily with rowdy tankmates or if it’s tank is located in a high-traffic area.

Help counteract this species’ flightiness by outfitting its tank with dark (black or earth-toned) substrate, several gnarls of driftwood, and plenty of floating vegetation. Bright colors in the tank are counter to the dark lagoons in which this fish naturally occurs, and, owing to the keen eyesight common to all Gasteropelecidae species, bright, unnatural colors tend to stress this species. Subdued lighting (perhaps filtered through a canopy of floating vegetation) is also recommended. Also, unlike some other species of hatchet, these fish seem less prone to intra-species aggression when multiple hatchets are housed together. The blackwing hatchet is frequently kept in small shoals without any problems.

Silver Hatchetfish


The fourth and final hatchetfish that is common in the pet trade is the silver hatchetfish (Gasteropelecus levis). Endemic to the southernmost reaches of the Amazon Basin, the silver hatchet is one of the most shy of all the commonly kept species. This timid fish wears, as its name suggests, a metallic-silver mantle with gold-bronze and black longitudinal stripes along the midline. Larger than either the marbled or blackwing hatchet, but smaller than the common hatchet, the silver hatchet seldom exceeds 2 to 2¼ inches in the home aquarium.

This species requires a stable pH between 5.7 and 7.0. It is worth noting that these fish are susceptible to radical or rapid swings in pH levels; severe pH fluctuations may sicken or even kill these fish. Likewise, these hatchets are also more prone to bacterial infection and stress-related ailments during ammonia spikes than other species. My best advice is to maintain silver hatchetfish only in mature tanks in which the biological processes have been long stabilized. Water hardness levels are of slightly less importance to these fish; keep dGH in the range of 3.0 to 17.0. Acceptable temperatures are between 74⁰ and 83⁰.

The natural and captive diet of the silver hatchet is as described for the other species: bloodworms, tubifex, and flakes supplemented with flightless fruit flies (pet shops that sell small geckoes usually keep flightless fruit flies in stock as feeders for those minuscule lizards) or pinhead crickets. A large adult silver hatchet will take a tiny cricket with great enthusiasm! Large common hatchets will also accept pin-head crickets sprinkled atop the water.

The only real husbandry difference between the silver hatchet and the other species discussed here is the level of skittishness found in the silver hatchet; these fish are afraid of boisterous tankmates, and they also get scared when housed in high-traffic areas of the house. You can alleviate this nervousness by outfitting your tank with plenty of live, floating plants. In the wild, silver hatchets take refuge in the dangling root clusters of floating vegetation as well as in dense networks of submerged driftwood gnarls. While not as prone to jumping as their marbled cousins, these fish certainly require tightly lidded environs. Captive longevity may exceed three years.

Long Live the Hatchetfish


In looking back, I find that the hatchetfish of my youth are still as viable and attractive in the home aquarium now as they were so many years ago. My fiancée, Alexis, and I are building our first aquarium together. It’s a 45-gallon tank, and in it we will put many things, hatchetfish included. When we have children, I hope they take an interest in fish. I hope very much that they may enjoy the same aquarium species in the days of their youth—when life is still enchanted and the world still magical and wonderful—that I enjoyed in mine. While many things will fade through the years and many nostalgic memories will prove untrue when we recall them, the simple beauty and enchantment inspired by the dashing and acrobatic hatchetfish cannot be numbered among them. My wonderful memories of these fish are just as true today as they were 25 years ago. If you keep a tight lid on them and feed them plenty of protein-rich fare, the hatchetfish can bring years of unique charm and ecological diversity to your freshwater community tank.

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