Trophic niche partitioning and intraspecific variation in food resource use in the genus Pangasianodon in a reservoir revealed by stable isotope analysis of multiple tissues.

Understanding the mechanism by which non-native fish species integrate into native communities is crucial for evaluating the possibility of their establishment success. The genus Pangasianodon, comprising Pangasianodon gigas and Pangasianodon hypophthalmus, has been introduced into reservoirs, which are non-native habitats, for fishery stock enhancement. P. gigas and P. hypophthalmus often successfully establish and co-occur in several Thai reservoirs, but there is little information on differences in food resource use between the two species. To investigate the trophic niche width of P. gigas and P. hypophthalmus in a Thai reservoir, we conducted stable carbon and nitrogen ratio (δ13C and δ15N) analyses. We examined the degree of individual specialization in both species using the δ13C and δ15N values of muscle and liver tissues, which provides long- and short-term diet information. The isotopic niches did not overlap between P. gigas and P. hypophthalmus. The δ15N value of P. gigas was significantly higher than that of P. hypophthalmus, whereas the δ13C value did not significantly differ between the two species. The isotopic niche sizes were larger in P. hypophthalmus than in P. gigas. Individual specialization was observed in P. hypophthalmus but not in P. gigas, indicating that intraspecific variation in food resource use was larger in P. hypophthalmus compared to P. gigas. These findings suggest that trophic niche partitioning was one of the factors facilitating the establishment success of P. gigas and P. hypophthalmus in a reservoir, but the establishment process may differ between the two species.

Striped catfish (Pangasianodon hypophthalmus) exploit food sources across anaerobic decomposition- and primary photosynthetic production-based food chains.

Dietary information from aquatic organisms is instrumental in predicting biological interactions and understanding ecosystem functionality. In freshwater habitats, generalist fish species can access a diverse array of food sources from multiple food chains. These may include primary photosynthetic production and detritus derived from both oxic and anoxic decomposition. However, the exploitation of anoxic decomposition products by fish remains insufficiently explored. This study examines feeding habits of striped catfish (Pangasianodon hypophthalmus) at both adult and juvenile stages within a tropical reservoir, using stable carbon, nitrogen, and sulfur isotope ratios (δ13C, δ15N, and δ34S, respectively) and fatty acid (FA) analyses. The adult catfish exhibited higher δ15N values compared to primary consumers that feed on primary photosynthetic producers, which suggests ingestion of food sources originating from primary photosynthetic production-based food chains. On the other hand, juvenile catfish demonstrated lower δ15N values than primary consumers, correlating with low δ34S value and large proportions of bacterial FA but contained small proportions of polyunsaturated FA. This implies that juveniles utilize food sources from both anoxic decomposition and primary photosynthetic production-based food chains. Our results indicate that food chains based on anoxic decomposition can indeed contribute to the dietary sources of tropical fish species.

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

The Mekong giant catfish Pangasianodon gigas 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 P. gigas 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 (Pangasianodon hypophthalmus and Hemibagrus nemurus) in the Kaeng Krachan reservoir, Thailand. The mean relative gut length (RGL = GL / TL) of P. gigas was 3.50, showing that they have relatively long guts, with values more similar to those of omnivorous P. hypophthalmus (RGL = 3.70) than to those of carnivorous H. nemurus (RGL = 0.92). In the allometric relationship (i.e., log10 GL = log10 a + b log10 TL), the slope close to 1 for P. gigas (b = 1.07) has been widely observed in carnivores, while P. gigas has a greater intercept of the linear equation than P. hypophthalmus and H. nemurus, resulting in a similar GL of P. gigas to omnivorous P. hypophthalmus at approximately 35 cm in TL. Moreover, GL of P. gigas at 150 to 250 cm in TL showed great variations (RGL = 1.35-6.32). The variation in RGL for P. gigas seemed to result from a poor nutritional state. In conclusion, P. gigas is suggested to feed on indigestible materials such as plants, algae, and sediments, and potentially experience fasting in a reservoir.

Vertical movements of a Mekong giant catfish (Pangasianodon gigas) in Mae Peum Reservoir, Northern Thailand, monitored by a multi-sensor micro data logger.

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.

Comparison of two methods of attaching telemetry transmitters to the Mekong giant catfish, Pangasianodon gigas.

For conservation and successful stock enhancement of endangered species, an understanding of the movement patterns and behavior of the target species is indispensable. The Mekong giant catfish, Pangasianodon gigas, is endemic to the Mekong Basin and now is threatened with extinction. Although biotelemetry using acoustic transmitters and receivers is expected to reveal its movement patterns and behavior, the most suitable attachment method for the transmitter to the catfish is unknown. In this study we examined the effects of external attachment and surgical implantation on the survival and growth of the catfish, compared with those in control individuals, in an earthen fish pond for approximately 2 months. No fish died during the experimental period. Furthermore, we found no fungal infections in any fish and no significant differences in growth rate among treatment and control fish. However, all transmitters of externally tagged fish were lost during the experimental period. In contrast, transmitters of some surgically implanted fish remained in the peritoneal cavity. In conclusion, the surgical implantation technique is suitable for long-term monitoring of the catfish.