2004 Rio Ventuari expedition (28 March-28 April)

2004 Rio Ventuari expedition (28 March-28 April)

Nathan K. Lujan

Overview

Expedition Summary

The catfish fauna of the Rio Ventuari was found to be species-rich, particularly in Loricariidae (40 species) and Auchenipteridae (15 species). The total number of catfishes from the Ventuari is predicted to total over 100 species, out of nearly 400 fish species total. In comparison, the neighboring Caura drainage, which has been extensively collected by AquaRAP teams, contains 74 species of catfish out of a total of 278 fish species. This discrepancy exists in spite of the fact that the Caura and the Ventuari are of similar size and are comparable in their ecology and biogeography. Indeed, their headwaters in the Guyana Shield uplands interdigitate but the Caura flows north, away from the westward-flowing Ventuari, to contact the Orinoco in its mid-reaches. One difference between the two rivers that may have bearing on their differing levels of richness is the Caura’s lack of a waterfall of significant enough size to serve as a faunal barrier.

The expedition was conducted at the very end of the dry season during the month of April, causing water levels to be low and catch efficiency to be high. Productive catfish habitats in the Rio Ventuari included basalt riffles and outcroppings from which ancistrine loricariids were collected, coarse woody debris from which auchenipterids were collected, and open sand flats from which loricariins, doradids, and heptapterids, were collected. Local ornamental fish collectors encountered at several points in the lower Ventuari also constituted a major source of loricariid specimens. These collectors harvest loricariids in the dry season when the water is warm and the dissolved oxygen is low causing loricariids to concentrate in exposed rocky riffles. Loricariids are collected by hand by diving among the rocks and are then transported to a warehousing area typically located on a shallow sand bar in the mid-channel. The fish are housed in staked nets and are tended for one to two weeks by extended family groups. They are then sold to a traveling middle-man who packages them in oxygen-filled plastic bags for transport and sale across the border in Columbia, in contravention of Venezuelan law. The fish are presumed to then enter the global ornamental fish trade via Villavicencio and Bogota. This trade is of conservation concern due to its largely illegal nature and its negative impact on native loricariid stocks, reflected in our inability to collect loricariids with seines anywhere near a collecting station. Undescribed loricariid species reported herein are highly valued by ornamental fish collectors and only after we moved beyond their apparent sphere of influence were we able to collect these species with our seines.

The expedition sampled 36 sites from which 1111 lots have been accessioned. 12 collecting sites were located in the lower and middle Ventuari, and another five sites were sampled between the mouth of the Manapiare and Salto Tencua. Catfish habitat in the lower Ventuari was dominated by large, basalt outcrops spaced at regular intervals along the shore and in the mid-channel, but these grew increasingly rare in the mid-reaches, transitioning into long stretches of sandy beaches. The Rio Manapiare, a major tributary of the upper Ventuari, was also collected at 13 sites from its mouth up to Cano Yutaje and Cano Guaviarito. The Rio Manapiare was different in character from the lower and middle Ventuari and more closely resembled rivers of the llanos in terms of hydrology, geology, and fish fauna.

Travel up the Rio Ventuari was halted at Salto Tencua, a potentially major faunal barrier, due to lack of access and permission from local indigenous groups. These groups granted restricted access and guides to an area immediately above Salto Tencua and the fish fauna encountered was completely different from that below the falls. In a short period of collecting in torrential flow with seine and cast net, undescribed species of Rhamdia, Myoglanis, and Lithoxus were collected along with an ancistrine of unknown generic placement. This is the first collection of Lithoxus on the west slope of the Guyana Shield and it is hypothesized that upland regions such as this, protected by major barriers to fish migration, may isolate a distinct faunal assemblage with greater affinity to upper reaches of other, ecologically similar, Guyana Shield drainages than to fauna of the lowland reaches into which the rivers drain. Evidence in support of this hypothesis is also derived from the discovery above Salto Tencua of a new Geophagus species, strikingly different from anything yet collected from the upper Orinoco and most closely resembling a species (Geophagus harreri, H. Lopez pers. com) known only from the Guyanas. These are faunal elements missing from the ichthyofauna of the nearby Caura, a river that resembles the Ventuari in many ways but one: the Caura is studded by rapids and small falls but lacks a single waterfall high enough to function as a major faunal barrier. In contrast, the upper Ventuari features several large falls, beginning at Salto Tencua and extending upriver to Salto de Oso. The biogeographic significance of these geological features will be the target of future investigations and fieldwork in the upper Orinoco.

VEN04 specimens have been distributed among the Auburn University Museum Fish Collection (AUM), the Academy of Natural Science in Philadelphia (ANSP), and the Natural History Museum of Guanare (MCNG), and several specimens have been distributed to researchers working on those groups (Table 1). An image database featuring photos of all catfish species collected from the Ventuari is under construction and will be reachable from the ACSI website.

Expenses