First Spotfin Hatchetfish Aquarium Breeding on Record
I work as an aquarist at Ostrava Zoo in the Czech Republic, where most of our efforts are focused on breeding endangered fish. Of course, we also keep several exhibition tanks with fish species that are common but interesting, either for their appearance, their behavior, or both. An example of one such oddity is the spotfin hatchetfish (Thoracocharax stellatus). With its simple silvery coloration and very prominent ventral keel, the species is a nice addition to our South American exhibition tank.
Keeping the Spotfin Hatchetfish
The spotfin is considered to be the largest of all species in the hatchetfish family (Gasteropelecidae), with a standard length of 2½ inches (6.5 cm), but it is more commonly found in the trade measuring 1½ to 2 inches (3.5 to 5 cm) total length. We keep several dozen of them in a 1,300-gallon (about 5,000-liter) tank, where they coexist peacefully with other South American fishes, including the marble hatchets (Carnegiella strigata).
While the smaller Amarble hatchets hang in the dark top corner of the tank most of the time, the large, silver spotfins swim right under the overhanging lights and sometimes descend deeper into the water column. They are very active and not at all skittish.
We noticed some interactions between our freshwater hatchetfish, certain periods when they appeared to be courting. However, we could never hope for a successful breeding in the large show tank. In such a deep tank with so many other fish present, it was impossible to collect the eggs. And we actually never saw them spawning, just endlessly courting.
So the next time we received a new batch of spotfin hatchetfish to add to our exhibition tank, after the fish quarantine we selected eight specimens of different sizes and kept them behind the scenes in a breeding tank. Our breeding group ideally was to consist of four pairs, so we selected the four largest individuals (supposedly females) and four smallest (supposedly males). We would eventually come to discover that we didn’t hit our target ratio, rather having selected five females and three males. But ultimately that didn’t matter, and we can now present the report of a successful breeding.
Hatchetfish Breeding Facts
What did we know about hatchet breeding before we tried our own attempt? Not a whole lot. First, we couldn’t find any record of anyone having bred the spotfin hatchetfish. Nevertheless, the biology of all species in the family Gasteropelecidae is so similar that we could use the knowledge of others in breeding different species, mainly two of the most common aquarium inhabitants among hatchets, Gasteropelecus sternicla and C. strigata.
But here’s another surprise: You won’t find many set-in-stone facts even for them aside from “they were bred.” They are egg scatterers. Their eggs sink. Their eggs float. But aren’t those contradictory statements? Yes, and since we couldn’t identify a single reliable breeding report with first-hand testimonials, we stayed open to both options. Surely fish with such a strong relationship to the water surface might indeed have floating eggs…but who knows?
Conditioning Our Fish
What we did know was that we needed well-conditioned fish of both sexes and a suitable breeding tank—and likely some sort of trigger, or maybe just some good luck, to get them to spawn. We solved the first problem by feeding them small crickets and fruit flies with the occasional addition of live Daphnia, Moina, and mosquito larvae. The sex of young hatchets cannot be determined, only guessed by their size and the more-rounded bellies females develop. However, if you closely observe the behavior of each fish and combine that with its physical appearance, you can be pretty sure. It gets easier over time as the females become significantly bulkier than the males.
We kept our small group of T. stellatus in the 32- x 14- x 16-inch (about 80- x 36- x 40-cm) grow-out basin. When we decided it was about the right time to start breeding them seriously, we removed all the other fish except two marble hatchets to keep the spotfin group company. The tank was left with its usual decoration, fine sand on the bottom and some tree roots with Anubias and ferns attached. We put a bunch of hornwort (Ceratophyllum demersum) under the surface to create hiding/spawning spots. However, we later found that hiding places were not appreciated. In fact, more open space was needed for the fish to do their business. The courting of this species is time and space consuming.
So we cleared most of the top layer of the water column, fed the fish well, and waited. We observed courting many times; this wasn’t the usual nudge or rivalry between individuals, but a clear interaction between male and female.
Unresponsive females would hide away in a corner and even descend close to the bottom to avoid being chased. Some females would also attack the male to drive him away. But very often there was one female with a nicely rounded belly who accepted the game and let the male(s) pursue her. Males swam around frenetically trying to find the right female, chasing one side by side and sometimes blocking her, with their belly right in front of her snout. Such events were energy consuming for all of the males, not to mention the females, who respirated heavily while resting or hiding.
But again, we saw no real spawning, just courting, and our daily surface-and-bottom check for eggs was a futile effort. But we all know what it takes to be an aquarist—patience! So we waited, played a bit with the water chemistry, and decided to wait a few weeks before moving the breeding group to a tank with a larger surface area. But it turns out that was not necessary.
Success!
One day I approached the tank as usual, focusing my eyes on the bottom and surface, looking for tiny transparent spheres. I suddenly realized that I should focus elsewhere. Many small larvae were hanging on the front glass. The hatchets got us—they spawned without our knowing, and we even missed the eggs! The fry were numerous; I siphoned them up one by one with some airline tubing. I counted to 222 and then stopped, leaving at least 20 more small larvae with their parents in the breeding tank.
Our 222 small freshwater hatchetfish were brought into a breeding tank with a size of 12 x 8 x 8 inches (30 x 10 x 10 cm). We used the water from the breeding tank, with a pH of 6.9 and conductivity of 140 µS/cm. Our tap water has a pH value of 7.8 and a conductivity of 220 µS/cm; we mixed it with pure RO water and added some alder cones to get a nice blackwater tint.
While these were suitable parameters for our first hatching, we were also successful in later experiments with a pH above 7. It is certainly not necessary to have very soft and acidic water. The temperature was kept around 80°F (27°C).
Raising the Fry
The fry hung on the glass on the first day, but very often changed position. They were 4 mm long. On the second day they began to swim freely, close to the surface but not strictly below it, moving around a lot and constantly looking for food. They were too small to accept freshly hatched brine shrimp, but they came after any kind of fine powdered food that stayed on the surface or floated in the water.
We started offering the smallest ones freshly hatched brine shrimp nauplii along with powdered food from day six, and some fry managed to eat it and started to grow more rapidly. However, it took at least two weeks before we could fully switch to brine shrimp only. As soon as we could, we used the second or third instar enriched with essential fatty acids, but could only do so about a week later, as this brine shrimp is considerably larger than the newly hatched variety. The fry still accepted powdered food or crushed flakes, and they later enjoyed hunting Moina and the smallest crickets.
There were no losses at all, but there were significant differences in size right from the beginning. Most of the juveniles developed well, though a very few remained smaller and dark in color. We would move those to another tank, which helped immediately. Even if we put them in the grow-out tank with slightly larger fry of a different species, those stunted freshwater hatchetfish would grow quickly. What we found to be a very important factor was light: The fry love to gather directly under the light. Fish grow slowly in dimly lit tanks and remain dark in color. As we added more light, they turned silver, became more active, ate more, and grew better.
Lessons from the Breeding Project
The transformation from the usual sliver-like fry into small hatchets with big bellies was the most rewarding moment of this adventure. We were curious to see whether this transition would present any complications in the otherwise straightforward rearing of the juveniles. But it went so quickly and smoothly that we could only admire the beauty and wisdom of nature. The fry began to transform as soon as they were 16 days old, and the first fully keeled hatchetfish was here when they were 35 days old and 10 mm (a little less than a half-inch) in size. It was a small, glorious miracle that filled our hearts with great joy.
The fry were not susceptible to ill effects from water changes, handling, etc. We moved the small hatchets to a larger tank as they grew, still keeping them in quite a dense group. Our breeding group spawned again after 10 days, and then again after a month.
It was only after this third spawning that a colleague of mine, Michal Ventruba, first spotted the eggs before they hatched. They were quite large, about 1 mm in diameter, laying freely on the bottom. The fry hatched in two days. So the mystery is solved, the eggs don’t float. In fact, nothing special is required to breed the spotfin hatchetfish. Make your fish happy, feed them insects, give them a free and large enough surface…and then just watch and wait.
As for the small group of fry that remained in the breeding tank, I found them swimming under the surface the next day, just like the fry in the rearing tank. Sometimes the tiny fry would get swept away by their parents’ unruly movements, and I could see them all over the tank. They swam right in front of the adults’ mouths and were ignored. I thought they might be too small to be considered interesting.
The two marble hatchets in the tank also ignored the fry, even if they got very close. But I saw on at least two occasions the adult spotfin hatchets grabbing a fry and immediately spitting it out. It was a striking moment. Did the fish realize it was their own progeny, or was the fry simply unpalatable?
The story of those 20 or so fry has no happy ending, however. Day after day I found fewer and fewer fry in the breeding tank until they had disappeared completely, either eaten or battered by the wild movements of the adult fish. In the end, hatchets are characins, and as such tend to be reckless parents.
Watch and Learn
If you keep freshwater hatchetfish, it is well worth dedicating a single-species aquarium to them. You may not see them spawning, you may even miss the eggs, but you will eventually be pleasantly surprised by the presence of small fry. Don’t let the fact that you don’t know exactly what to do discourage you. The fish know—watch them closely and let them teach you.
See the full article on TFH Digital
https://www.tfhdigital.com/tfh/mar_apr_2021/MobilePagedReplica.action?pm=2&folio=62#pg65
