Gut Morphometry Represents Diet Preference to Indigestible Materials in the Largest Freshwater Fish, Mekong Giant Catfish ().

The Mekong giant catfish is one of the largest freshwater fish, measuring up to 3 m in total length. This study was designed to determine the feeding habits of to better understand how the fish achieve their large body size. We compared the relationship between gut length (GL) and total length (TL) among related species in superfamily Bagroidea, order Siluriformes ( and ) in the Kaeng Krachan reservoir, Thailand. The mean relative gut length (RGL = GL / TL) of was 3.50, showing that they have relatively long guts, with values more similar to those of omnivorous (RGL = 3.70) than to those of carnivorous (RGL = 0.92). In the allometric relationship (i.e., log GL = log + log TL), the slope close to 1 for ( = 1.07) has been widely observed in carnivores, while has a greater intercept of the linear equation than and , resulting in a similar GL of to omnivorous at approximately 35 cm in TL. Moreover, GL of at 150 to 250 cm in TL showed great variations (RGL = 1.35-6.32). The variation in RGL for seemed to result from a poor nutritional state. In conclusion, is suggested to feed on indigestible materials such as plants, algae, and sediments, and potentially experience fasting in a reservoir.