Yukiko Yamagishi

Horizontal and vertical movement of Mekong giant catfish Pangasianodon gigas measured using acoustic telemetry in Mae Peum Reservoir, Thailand

The horizontal and vertical movements of eight immature hatchery-reared Mekong giant catfish Pangasianodon gigas were monitored using acoustic telemetry in Mae Peum Reservoir, Thailand between 2003 and 2004. The fish were monitored for between eight days and more than 9 months. All of the fish moved over the entire reservoir within approximately 40 days after release. Subsequently, the home range became small approximately 40 days after release. The fish preferred deep areas in the reservoir. The fish displayed diel horizontal and vertical movement patterns. The fish showed larger-scale horizontal movement during the daytime than at night. The fish repeatedly showed active vertical movement during the day, whilst there was little variation in swimming depth at night. Our results imply that the vertical movements of the fish were related to the environmental condition of the reservoir.

Vertical Movements of a Mekong Giant Catfish (Pangasianodon gigas) in Mae Peum Reservoir, Northern Thailand, Monitored by a Multi-Sensor Micro Data Logger

Abstract The vertical movements of one Mekong giant catfish Pangasianodon gigas were monitored for 3 days in August 2004 using a depth-temperature micro data logger. The logger was recovered using an innovative time-scheduled release system and located by searching for VHF radio signals. The logger was found approximately 2.2 km away from the release point and provided (n=705,128) depth and temperature data collected over a period of 98 hours following the release. The fish spent more than 99% of its time at less than 3 m below the surface. The maximum swimming depth was 5.6 m. No sharp thermocline was present during the experiment. Temperature did not have any detectable effect on the pattern of vertical movement of the fish. The dissolved oxygen concentration (DO) was stratified, with a concentration of >60% saturation in the first 3 m below the surface falling to 10% saturation at depths lower than 4 m. This specific DO stratification was found to limit the vertical movement of the catfish.

Seasonal movement of one Mekong giant catfish, Pangasianodon gigas, in relation to the environmental conditions of Mae Peum Reservoir, northern Thailand

Greater knowledge of the relationship between movement patterns of the target speeies and the environmental eonditions (e.g., temperature and dissolved oxygen) is needed for effeetive fisheries management and eonservation (BRILL 1994, CARTAMIL & LOWE 2004). The Mekong giant eatfish Pangasianodon gigas is one of the largest freshwater fishes in the world, measuring up to 3 m in length and weighing in exeess of 300 kg. The eatfish is one o f the most important fisheries speeies ofthe Mekong River Basin in the Southeast Asia; however, the eateh number of wild eatfish in the Mekong River has deelined due to development o f the river and over-fishing (HOGAN 2004) and the eatfish eurrently is listed in CITES Appendix I and on the IUCN Red List of threatened speeies as a Critieally Endangered Speeies. Hatehery-reared juvenile and young immature eatfish were released into lakes and reservoirs as well as the Mekong River to enhanee the stoek (MEYNELL 2003). The behavior, movement patterns, and survival rate of these eatfish after release into natural eonditions remains largely unknown (MEYNELL 2003), although that information is quite useful for fishery management and eonservation deeision making (LOWE et al. 2003, HUMSTON et al. 2005). W e started to investiga te the movementpatterns ofthe eatfish using aeoustie telemetry in 2001 (ARAI et al. 2005). To assess the movement patterns ofthe eatfish during the rainy season in 2003, 8 fish tagged with aeoustie transmitters were released in Mae Peum reservoir, northern Thailand in 2003 (MITAMURA et al. 2008). Although 7 of 8 tagged fish had expelled the transmitters duringthe study period (8 -136 days ), the remaining one fish was monitored for more than 14 mo during both the dry and rainy seasons, whieh provided the movement patterns in relation to the seasonal environmental eondition. There are not so many studies that eontinuously monitored the same fish over a year due to the same diffieulties, sueh as fish mobility, vastness ofthe study si te, an d transmitter detaehment (LucAs 1989). The objeetive ofthis study was to report the seasonal movements of one eatfish in relation to environmental eonditions.