The length-weight relationships, condition factors and gut contents of Syncrossus helodes (Sauvage, 1876) and Yasuhikotakia modesta (Bleeker, 1864) from the Mekong River, Muang District, Nong Khai Province, Northeastern Thailand

The length-weight relationships (LWR), condition factors (K) and gut contents of Syncrossus helodes (Sauvage, 1876) and Yasuhikotakia modesta (Bleeker, 1864) from the Mekong River, Muang District, Nong Khai Province, Northeastern Thailand were investigated in the dry-hot and dry-cool season 2010.  The LWR results indicated that S. helodes exhibited negative allometric growth in both seasons, whereas Y. modesta exhibited positive and negative allometric growth in the dry-hot and the dry-cool season, respectively.  The K values for S. helodes in the dry-hot season and Y. modesta in both seasons were greater than one, showing that they were in above good condition, whereas the K for S. helodes in the dry-cool season was lower than one, which indicated that the fish was in poor condition.  The percentage of point method (%P) results indicated that S. helodes fed on the same 7 categories of food items in both seasons, which were dipteran larvae (64.48%), trichopteran larvae (17.82%), ephemeropteran larvae (6.73%), Hemiptera (2.95%), terrestrial dipteran adults (2.31%), green algae and zooplankton (1.10% each).  Y. modesta also fed on the same 7 categories of food items in both season, corresponding to dipteran larvae (59.78%), trichopteran larvae (16.37%), zooplankton (12.87%), molluscs (5.51%), terrestrial dipteran adults (3.30%), plant materials (1.18%) and green algae (0.59%).  Plant parts and filamentous algae were also found in the analyzed gut contents; these materials may have been accidentally ingested along with the principal food item, Chironomid larvae.  The %P of the food items depended on both the season and the fish species (chi-squared test, p<0.05).  From the present study, it was concluded that S. helodes and Y. modesta are carnivores. Key words: Length-weight relationships, condition factors, gut contents, S. helodes, Y. modesta, the Mekong River.