PS 36-8 - Niche partitioning in sympatric freshwater megafishes (the genus Pangasianodon) in a tropical Thailand reservoir

Abstract: Freshwater megafishes inhabit the rivers and lakes sympatrically in tropical Southeast Asia. Trophic niche widths of megafish can be determined by bottom-up regulation and/or limitation of habitat accessibility with ignorable predation risk. However, the differences in trophic niche width in megafishes have not well been demonstrated by quantitative methods based on niche metrics. Our objective was to investigate interspecific resource competitions and the intensity of intraspecific competitions in sympatric megafishes, the genus Pangasianodon (i.e., Pangasianodon gigas and Pangasianodon hypophthalmus), using stable carbon and nitrogen isotopes (δ¹³C and δ¹⁵N, ‰). In the Kaeng Krachan Reservoir, western Thailand, we measured the stable isotopes of the muscle and liver samples (long-term and short-term indicators of diet information, respectively) in 16 fish of P. gigas (159–230 cm in TL) and 11 fish of P. hypophthalmus (79–106 cm in TL). We calculated the isotopic niche width in each species and the niche overlap rate between the two species. Using δ¹³C and δ¹⁵N values of muscle and liver tissues, we calculated the within-individual component of variance (WIC) and the between-individuals component of variance (BIC) for each species and then compared these indices between the two species.

Isotopic niche areas in muscle and liver tissues were 0.97‰² and 1.73‰² for P. gigas and 3.05‰² and 2.86‰² for P. hypophthalmus. Niche overlap was not found between the two species in each tissue. There were no significant differences in δ¹³C values between P. gigas and P. hypophthalmus in each tissue type, but δ¹⁵N value of P. gigas was significantly higher than that of P. hypophthalmus in each tissue type. These results suggested that resource partitioning occurred between the two species of the difference in trophic position. In niche width, WIC (= 0.96) of P. gigas was greater than BIC (= 0.68), whereas BIC (= 3.00) of P. hypophthalmus was greater than WIC (= 1.89). These patterns in niche variance likely reflected smaller niche differences within the population and stronger intraspecific resource competitions of P. gigas than that of P. hypophthalmus. Overall, the two megafishes in the reservoir are thought to coexist with the trophic niche partitioning.