Article © Daniel Konn-Vetterlein, uploaded April 09, 2022.

The Rio Blanco is home to over 130 species of fish

It's been three years since my last expedition to Bolivia and my expectations were high when I returned in 2018. Together with three friends, it was our aim to operate a scientific study of a southern confluent of Río Iténez, the Río Blanco. We started our journey in Trinidad, in the north of Bolivia and headed towards Brazil - always on the waterway. After ten days of travelling and close to 130 collected species of several families, I can now present a small selection of some highly interesting encounters.

Immediately at the first collecting site we found an appealing knifefish in the overhanging roots and submerged branches near the riverbank: Platyurosternarchus cf. macrostomus. Besides this species there is only one more described (P. crypticus) and both are not really known among aquarists. Their snout is tubular and points downwards, which allows them to swim diagonal to pick invertebrates from the ground. All in all we collected six species of knifefishes, but the most fascinating was Gymnorhamphichthys cf. hypostomus. Currently there are six accepted species in that genus, but none of them is known very well. Exports are rare and there is a reason: Gymnorhamphichthys spp. hide all day long in the soil and are pretty much invisible at daytime. If you want to collect some, you need to do it at night, and even then they are hard to catch. We collected them near a river island in Río Iténez, the water was hardly 40 cm deep and the current was moderate to strong. At daytime no fish were visible, but at nighttime they showed up frequently and we could observe them from outside the water. Once we got into the water though, they immediately disappeared and there was no way to catch them, we couldn't even tell into which direction they went. So we started to work in a group: Two of us used their dip nets and one filmed everything underwater. That helped a lot! The knifes did not swim away at all, they simply dived into the fine sand, head first. Once we saw this on the camera we decided to scuff into the sand and thats how we finally succeeded. A beautiful adult specimen of Gymnorhamphichthys cf. hypostomus showed up in the net once the sand was removed carefully by us. The colouration was unremarkable, yellowish with black markings, but the appearance in general was breathtaking. The tubular snout is ideal to prey on insect larvae and likely also small fish fry. As fas as I am aware of, this genus has not been spawned in captivity yet.

In the middle of the day most semi-large fishes prefer to stay in shady areas near the shore or in small bays that are completely covered by trees. One of those species is Chalceus erythrurus, a well known and beautiful Characid for big tanks. Despite the information in literature that they live in groups, we only encountered single specimens. The intense red colouration of the caudal fin is absolutely smashing. On earlier trips we used to fry bigger specimens and they are quite delicious to be honest. Together with the red finned Chalceus we often found predatory species like Acestrorhynchus pantaneiro and A. cf. britskii. First is a widespreaded species and can be found southwards from its type locality in the Rio Madeira drainage: It is known from Argentina, Paraguay, Uruguay and Bolivia of course. The correct identification is difficult though if the catch location is not known. A. pantaneiro resembles A. abbreviatus, A. lacustris, and A. altus, forming the "lacustris-group". It was formerly often missidentified as A. altus, but latter one actually seems to be restricted to brazilian tributaries of the Amazonas river. A. britskii is described from the São Francisco basin, unfortunately yellowish Acestrorhynchus are really hard to identify and this is the closest I could get. Normally this phenotype is known from species ocurring in the Guyana shield and without knowing the location it would have been classified as one of them for sure. However, they are great to observe at home, but tend to do bad without life food and often deny frozen food in general. A group of them in a big tank is an amazing experience and I can only recommend to try it, especially because it is possible to breed them in captivity and only few people have achieved this until today.

Hoplias: omnipresent doom for smaller fishes

Still, even those predators have to fear others, for example Catoprion mento: The wimple Piranha feeds on scales of fishes up to three times it's own size. Their lower jaw is elongated and protruded, allowing the Piranha to collect loose scales from its prey. It doesn't bite deep enough to reach the meat of its prey, but hunts so precisely that single scales can be being picked off. They hunt on their own and very quickly. Wimple Piranhas join schoals of bigger fish, swim among with them and attack suddenly. The attack includes an impact with the other fish which loosens the scales in addition to the bite itself. Definitely a very interesting and highly specialized species to observe! Yet, there is another level of predator, and this one is absolutely omnipresent in Bolivian rivers: Hoplias malabaricus, no matter what type of water, there is always a Hoplias nearby. They are the fish you need to be afraid of, not Piranhas, it's Hoplias that bite you in your legs while fishing and they have truely harming teeth. Using a small dip net you might even find yourself attacked by jumping Hoplias, because thats what they do when they feel boxed in: they jump high out of the water. You don't want to have an arm long Hoplias in your face, trust me.

Swimming meadows are known to host many juvenile fishes, but this time we were able to find surprisingly many young Pimelodus as well. Even with our small nets we could catch some juveniles of P. cf. blochii in perfect condition. Apparently they use to feed on small tetras and cichlids like Apistogramma spp. and Mesonauta festivum. The specimens we collected measured between 60 and 80 mm. We never caught more than one, also it didn't seem that there were many more nearby, apparently they live a solitary life when young. Alongside the Pimelodus there was one truely stunning catch in the meadows. A species I tried to catch for years: Auchenepterichthys thoracatus. Sometimes you can see this name in the trade, but normally it's A. coracoideus hiding behind it. Both are fairly similar in appearance, and despite one characteristic the species determination is everything but easy. While the sternum is short and narrow in A. coracoideus, it is big in A. thoracatus, and hardly visble at all in A. longimanus, another similar species of the four described ones in the genus. But it was still getting better when a fishermen brought us a growling brownish something in the middle of the night. He was looking for "paiche" (Arapaima spp.), but of course he knew that some foreigners were in the area, so he wanted to meet us and brought quite a few nice fishes over the night. It started with a Tatia and it was only the first specimen of many to come during the trip. It resembled T. galaxias on first sight, but clearly had to be a different species since mentioned one is described from the Orinoco drainage. Even if we collected them throughout the whole trip, we could not identify them properly yet, no described species from the area matches the phenotype well enough. Hopefully they breed soon which might help us to determine this - possibly new - species.

L514

Among the most common Loricariidae in Bolivia, the genus Ancistrus probably holds most surprises for us. The first new species we found on this trip was a brownish Ancistrus that did not convince with it's colour, but with it's enormous broad head. The first specimen showed up on a trunk in a depth of more or less one meter. It was hiding in the same crevice as some Ancistrus sp. "L 463" and every single specimen needed to be collected by hand. In comparison to the other two species of Ancistrus, that inhabited the same biotope this species turned out to be far more delicate in terms of water parameters and was not as numerous as the others. It's head is covered in black lines, making it an attractive new discovery. However most aquarists will prefer to get their hands on another undescribed species we encountered, a red Ancistrus, likely the most attractive we got to know during the last years. Juveniles show an intense red colouration, and very few black markings. It resembles L 464, but the pattern is by far not as dense. Even adults look great and only loose a little bit of the high contrast and red colour. Both shown species were found together, but while the red Ancistrus mainly stayed on the outside of the trunk, the new, broad headed species seemed to prefer the most narrow crevices available.

Peckoltia sp. Río San Julian

Not a new discovery, but an attractive Loricariid that has never established itself in the hobby is Peckoltia sp. "Río San Julian". During our trip in September we encountered mainly juvenile specimens, ranging from 10 to 70 mm in size. This species grows up to 15 cm, back in 2007 I collected them in this size already. Apparently they spawn during the whole year, otherwise the huge difference in size couldn't be explained. It was named after a river in the department of Santa Cruz, but after this trip we know, that it's distribution is much larger than assumed before, it reaches up to the brazilian border and we hope to be able to breed and spread this species now, almost 20 years after it's discovery.

Breeding is a far off aim for another potentially new species of the genus Rhynchodoras, a dorad catfish only few specialists know and even fewer have ever kept. Two species are currently described: R. woodsi from Río Itaya (Peru) and R. xingui from Rio Xingu (Brazil). This is the very first time the genus has been recorded from Bolivian waters, and it is not unlikely that we discovered a new species. Unfortunately we only collected one specimen which was hiding in a submerged trunk from where I managed to pick it after several minutes of trying. Rhynchodoras don't grow very large with a standard length of 110 mm attained by R. woodsi, but our specimen was unlikely fully grown and did not exceed 40 mm. Using their unique mouth elongation they hunt on insect larvae and possibly also crustaceans. In Rio Xingu I observed them twitching at rough surfaces to look for food. Especially this finding showed us that there are many ways of fishing and that you need to focus on the fish and it's lifestyle to end up successful. We would have never found the Rhynchodoras using nets only and if we wouldn't have investigated that single trunk for at least one hour there would still be no record of the genus in Bolivia. I am already keen to go back to the places we worked in, because there were more tributaries, more silent bays and more sandbanks we passed by during our trip.

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