Depth-Temperature data logger revealed the fine-scale vertical movement of Mekong giant catfish Pangasianodon gigas in the reservoir

Abstract

In order to better understand the moving behavior of Mekong giant catfish Pangasianodon gigas in a reservoir, Thailand, two catfish were monitored using Depth-Temperature data loggers (UME 190DT, Little Leonardo, Tokyo, Japan) which were attached with a brand-new time-scheduled release system. The release system automatically detached the loggers from the catfish 5 days after deployment. Then we searched for VHF radio signals and found one of the two loggers approximately 2.2 km away from the release point. The other was never found because it was hidden by a dense cover of floating waterweeds. The logger that was recovered provided depth and temperature data for approximately 98 hours. Results from the data analyses found that the fish spent more than 99 % of their time above the depth of 3 m. The fish showed active vertical movement during the daytime but remained inactive during the nighttime (especially midnight). The fish made only 8 deep vertical movements greater than 4.8 m and the maximum swimming depth was 5.6 m. The differences in the ambient temperatures during these movements were less than 2.0℃. The stable temperature might have little effect on the catfish behavior. During our experiment period, there was no thermocline although DO stratification was built up at the depth of 4 m, and the DO below 4 m deep was uniformly less than 10 %. These results indicated that the DO stratification might limit the vertical movement of the catfish. Furthermore, in 8 deep movements the descent rates were significantly lower than the ascent rate for each movement, respectively, which indicated that the fish might display the avoidance movement from the hypoxic waters.