Bagarius yarelli grows to more than 90 cm, and can attain a weight of around 50 kg. Rainboth (1996) indicated a maximum size of 200 cm. Interestingly, the largest individuals reported by Vietnamese fishermen were no bigger than 20 cm.

There are several possible explanations for this difference: (a) misidentification; (b) the larger individuals have habitat requirements that cannot be met in Viet Nam (see below); or (c) greater pressure from fishing activities in the lowermost part of the Mekong River.
Except for a few locations in the southern part of the Mekong delta, Bagarius yarelli was identified at all the stations where interviews were carried out. Although Rainboth (1996) stated that it is a relatively unimportant food fish, the species was recorded at five markets during the present survey.

This species appears to be more common in the river stretches from Kratie and upstream, probably because of a preference for rocky areas with a strong current, a fact that was mentioned by a number of fishermen, and concurred with by Rainboth (1996). One Thai fisherman in Chiang Saen stated that Bagarius yarelli never enters small streams, which is consistent with Rainboth (1996).

It was mentioned at least twice that the species migrates in schools, and one Thai fisherman in Nong Khai mentioned that it follows Catlocarpio siamensis during its upstream migration. Some fishermen reported that the fish migrates to follow their prey.
During the upstream migration, which occurs from March to August in the area upstream from the Khone Falls, the fish leave the rocky areas where they live during the rest of the year. The main migration apparently starts close to the peak flood when the current is very strong and the water is still turbid.

Migration patterns Bagarius yarelli is one of the few species for which several reports were made on downstream migration along the stretch from just above the Khone Falls to the northern station of Chiang Saen. The downstream migration in that area takes place from October to December. One fisherman in Chiang Saen reported that the fish are fatter during theirdownstream migration.

In the case of migrations below the Khone Falls, information is available only for localities in Kratie and Stung Treng provinces. The fish migrate upstream from October to February and downstream from April to September.

Fishermen at nine sites in Cambodia, the Lao PDR and Thailand reported seeing eggs in the abdomen of this species.

Apart from a report in December by one fisherman from Xayaboury in the Lao PDR, all the fishermen maintained that the eggs were observed between April and September, with a peak during June-July. The peak spawning period thus coincides with the upstream migration period above the Khone Falls and with the downstream period below the Khone Falls. Juveniles of only 2 cm in length were seen throughout the year, however, indicating that the spawning period is more protracted than demonstrated by the data on the occurrence of eggs.

None of the fish seen by the survey teams in markets in Kratie, Loei and Boulikhamxay from March to June had eggs or milt.

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