A good point to discuss the biotope structures! Thanks emartin!
WHAT IS A SLOW OR A FAST CURRENT GENERALLY AND IN THE BIOTOPE OF KATRIA KATRIA?
Interesting and challenging topic and how to imitate natural biotopes with currents (when there are currents) or without currents is fascinating. In the nature in the riverine biotopes the currents can be and they are much faster than we have in our aquariums. Even slow moving water flow in the nature can be a faster current for aquaristics. It would be great to get more knowledge about natural biotopes. Authentic videos are one and sometimes a good way to realize biotope structures, including river streams and currents.
To make sure I’m not misunderstood, I do not have extreme currents in my Madagascan tank. Extreme biotope currents are only for some rapid riverine biotopes, like in Lower Congo. The water is moving fast and violently also between the huge and angry rapids. They are not for Katria katria. Its biotope is different (the river is different) in the Nosivolo River; there are calmer sections alternating with small stretches of smaller rapids. What actually means a calm section? So many aquaristics I have been met usually think water flow is almost stopping in the so called calm sections between rapids but generally the water velocity after rapids and between rapids can be still quite fast. Only the side-pools off the mainstream are totally or much calmer. What actually is a strong or a slow current for a fish species?
I agree based on my experience and others and the scientific knowledge that a fish species which can handle strong currents does not mean it is anatomically or physiologically adapted itself to currents or even likes them. But there are a lot of exceptions. A fish species whose physical build is not streamlined can play long times with water currents in aquariums. They must have fun and they enjoy that. And there is always a reason (to explain it scientifically) for a fish doing so. I wrote that I have ‘strong’ currents, but I wanted to emphasize that they are ‘strong’ currents in a closed space, in an aquarium but they are not fast or strong currents in the nature. I use (at this moment!) one powerhead with oxygen diffuser (OD) (its power can be changed from slow to fast) on the left side and one Hydor Koralia circulation pump on the right side up (plus one internal filter with OD and one canister filter). This means a lot of saturated oxygen, like in the Nosivolo River. More accurately the oxygenated water is moving about at the same velocity (only some distance) AND slower as in the first order rainforest stream and only at certain and different levels in front and back in the tank (In the first order rainforest stream the current velocity seldom exceeds 0.5 m/sec ). There are long and calm sections in the tank (water is moving slowly) for four katrias and their habitats, naturally. So I can study the behaviour of Katria katria and its likings and preferences for water movements, also by closing the powerhead for hours or days and closing the powerhead only the surface water is flowing. Then my tank might resemble a deeper pool section between rapids (see the next paragraph). This means study possibilities and the possibilities to change and develop the biotope.
As I wrote all the four katrias gather close together every evening underneath and also on the upper side the oxygenated powerhead current (depending on the position of the powerhead) and stay there for long periods. Also in the mornings when the tank room is still dark all katrias are swimming around and close to the powerhead current. They have left their calmer habitats. Perhaps this is what happens in the nature at the night time. And the dominating male is doing so sometimes most of the time. Are they sleeping at the same time? Diurnal and night time habits? It is known that for spawning Katria katria needs absolutely a calmer section but there are not enough scientific field studies about the habits of Katria katria. Having moderate and different and changeable currents and calm sections in a Mada tank is a nice study project. They are small pieces from katrias’s biotope structure. As you wrote the other possibility could be a tank with no currents at all which would resemble the most calmest sections (side-pools off the mainstream, etc.) but in the Nosivolo River, what is the water velocity between the stretches of rapids? Studying photos from Nosivolo and topographic researches and maps reveals something. Only the side-pools seem to be very calm with slow water movement. According to the collection records katrias have been found so far in the pools below rapids, between rapids in deeper pools and in the side-pools off the mainstream. Depending on many physical channel factors water can flow in a different way (turbulent currents, fast water flowing just on the upper side of the pools, slower water flowing in the pools, etc.) in the deeper pools between rapids and below rapids.
The other Nosivolo species Oxylapia polli seems to behave quite the contrary comparing to katrias. As Patrick de Rahm wrote 1996:
'In their natural habitat, Oxylapia polli appear to keep to rocky areas swept by a very strong current, which is why they are practically impossible to catch with a cast net. It is only at night that some individuals rest in shallow, quiet places. No young fishes were observed and the local fishermen were unable to tell me anything about the breeding habits of Songatana. Breeding probably takes place in the diurnal habitat of the fish, where it is practically impossible to observe. If this is so, the species must have developed special behavior in order to be able to raise its young in turbulent fast-flowing water.'
STEATOCRANUS AND OTHER SPECIES IN CURRENTS
My good friend has a big tank imitating rapids from the Lower Congo. He has Steatocranus species and catfishes from different genera like Synodontis. Steatocranus species like water is moving fast and they never stay in the calmer side of the tank but they also stay away from the extreme powerhead currents with OD, always underneath as you wrote. His Steatocranus spesies are always staying at the bottom between rocks and for this reason underneath powerhead currents. One and perhaps the only reason for this habit is their swim bladder is so underdeveloped; their biotope is the bottom and rocks. In this Lower Congo tank Synodontis pardalis, S. brichardi and S. notatus swim just in the middle of the most strongest and extreme currents by powerheads and Hydor Koralia circulation pumps. And they are doing so lot, it’s their habit. In his another African biotope tank he has Tilapia snyderae which is not a good swimmer at all (anatomically) but it likes to play hours in front of the strong outlet current of the inner filter. In my 212 gallon Upper Congo river biotope tank I have two different currents (powerhead with OD and a magnum circulation pump) in the back and a calmer section in front with slower and different water flows. This is so possible to make when the tank is wide enough, mine is 30 inch (75 cm). To change the direction and power of the currents in the back and also in front I can study the behaviour of the well known Phenacogrammus interruptus, the Congo tetra and the other riverine species (they all are wild caught) whether they like and how much they like (and when they do so) moderate or faster currents or whether they prefer the calmer sections OR BOTH (there are no scientific researches about the biotopes and habitats or habits of my species. It is known SO FAR that they occur in certain rivers and their drainages). I could say that the Congo tetras are dancing in the middle of the oxygenated currents for long periods and this can happen any time a day or night. It looks beautiful and almost incredible. And they can choose their habit. This is what my Synodontis camelopardalis is also doing a lot and for long times. These are some of my examples.
My Tanganyikan tank has a large bottom area 48 x 40 inch (120 x 100 cm) and the water is circulating around the tank for good oxygen levels which is important for Tanganyikan fish species. Water is moving also at the bottom. I have oxygenated surface currents at the opposite sides in the tank and most of my Tanganyikan fish species just love that. They swim eagerly for hours just in the middle of these oxygenated flows and always in the daytimes. They are ‘eating’ oxygen drops but they are doing so in the middle of the current. Is it just the oxygen (hemoglobin adapted to high oxygen conditions) or is there another physiological reason for this habit? The night has again a story of its own.
But back to Katria katria and its biotope and habits. We know something about its biotope structure and it gives some clues about katria’s habits in the nature, maybe, but I do not know exactly. In a tank conditions (but only in a tank conditions) I can get some information about katrias’s lifestyle.