Brachydanio rosea (Rosy Danio) Spawning Report
Submitted by Patchin Crandall Curtis
Background: Occurring in the Mekong River basin in northern Thailand, northern Cambodia and Myanmar, Rosy Danios inhabit small, well-oxygenated, running streams under forest cover, backwaters of larger rivers, and the main Mekong channel itself. Such habitats are seasonal in nature and water depth, flow, temperature and chemistry. In Myanmar, the fish are found in shallow, turbid, moderately fast-flowing water over a substrate composed of variably-sized boulders with some relatively little streamside vegetation. These fish are active, strong swimmers and require an aquarium at least 3’ long. Their stated temperature range is 67 – 77oF. For long-term care in a typical house, they should be kept in an unheated tank. The fish are morning spawning egg scatterers exhibiting no parental care.
Breeding Colony: I purchased 6 fish in the PVAS mini-auction from Marty Verdery on 1/4/2020. They were quarantined for 2 weeks in a 10g tank with no décor, a sandy substrate, and a sponge filter. There appear to be 2 males and 4 females. The males are slenderer and more colorful.
Spawning Set-Up #1: On 1/18/2020 I set up a 10g tank with a sand substrate (CaribSea Sunset Gold), a large mound of susswassertang on one side, a sponge filter in the middle, and several green yarn spawning mops tied to ceramic filter media as weights on the other side along with a brown tunnel. There is no light over the tank. It is exposed to ambient overhead light and indirect natural daylight. The water is soft, neutral tap water initially at 69-71oF.
Spawning Attempt #1: The colony was fed up on frozen bloodworms and cyclops, Golden Pearls, Repashy Soilent Green, and live BBS and white worms. The females very quickly came into breeding condition. Spawning behavior began immediately upon introduction into the spawning set-up and continued on and off throughout the day, although the morning hours after the lights came on were the most frenetic. The females used the catfish tunnel to hide in and rest periodically. The adults were removed after 48 hours. Upon close inspection with a chopstick and strong reading glasses, I was able to confirm the presence of at least some eggs. Unfortunately, the few I could see were white and didn’t appear to be viable, but I hoped there were more that I couldn’t see. A second, small always-on heater was placed in the tank to get the temperature up to 71-73oF for hatching and monitoring for larval fish. No larval fish appeared after 2 days. The breeding colony was re-introduced to the spawning set-up on 1/23/2020 and the same behavior repeated itself. Getting the fish to spawn was no problem. As soon as the spawning mop was shaken gently with a chop stick and some eggs would float upwards, a fish would sweep by and gobble them up. Keeping the eggs from being devoured was going to require a more structural solution than relying on removing the adults from the spawning tank.
Spawning Set-Up #2: On the evening of 1/25/2020 a spawning box designed and made by Bob Bock (thank you Bob!) was placed in the tank. The spawning box is a plastic food storage container with the sides cut out and lined with needlepoint mesh to allow cross-ways water flow, and a piece of egg crate secured to a piece of needlepoint mesh placed on the bottom to prevent the fish from reaching the eggs. I placed 3 weighted spawning mops in the spawning box around the edges. The fish immediately started inspecting and twirling around the box. I fed the fish some bloodworms and newly hatched BBS.
The next morning, the fish were in frantic spawning mode. One male was guarding the spawning box, chasing other males away, and trying to lure females in. However, all the other fish would jump in at the first opportunity. Fish were spawning outside the box in the susswassertang as well, but they appeared to be eating the eggs as fast as they were producing them. After observing their behavior for an hour or so, I decided to pull the spawning box to check for eggs. The box was carefully lifted out so that water flowing out the sides wouldn’t sweep away eggs from the bottom. The box was then emptied into a 250 micron mesh sieve, and the contents transferred to a petri dish for viewing under a X25 magnifying glass and photographing. There were 80-100 eggs, but the most startling thing was that the petri dish had many newly hatched larval fish swimming about. Eggs that I had thought might be fungussed turned out to be already hatched eggs. Eggs in the process of hatching could be seen as well. The spawning box had been in the spawning tank for no more than 12 hours.
The adult fish were removed from the spawning tank, 50% of the water was changed, the temperature was adjusted to 74oF, and the eggs and larval fish were placed back in the tank to hatch and grow out. Infusoria were added right away given the rapid rate of hatching and growth. Within 6 hours of placing the eggs in the grow-out tank there were ~20 fry, and within 24 hours there were ~30 fry.
Raising Fry: The newly hatched larval fish were quite long reaching as much as 4-5 mm. However, they were extremely tiny in diameter with very small mouths and stomachs. They required feedings 3-5 times per day of live rotifers, euglena, and paramecium and a slurry of vinegar eels, microworms, and 5-50 micron Golden Pearls. They did not appear to be able to eat paramecium or microworms for at least several days. I believe they may have starved to death if it weren’t for a happy accident – another tank in the process of cycling had a green water bloom about day 3 so I added a liter of green water per day to the danio grow-out tank and they stabilized. Even on day 7, their stomachs were not much bigger than their eyeballs. At this stage they appeared to be big enough to eat paramecium and microworms.
In the future, I will try to have a mature piece of driftwood covered in java moss and perhaps some stem plants tall enough to reach the surface to provide plenty of biofilm for fry to graze during the first couple of weeks. I believe they may have grown faster with biofilm available all day long every day.