American and Canadian scientists have confirmed that some pangasiid catfishes inhabit coastal waters when adult and undertake long-distance migrations to spawn.
Publishing their results in the latest issue of the Journal of Fish Biology, Zeb Hogan, Ian Baird, Richard Radtke and Jake Vander Zanden studied catch data in southern Laos and analyzed bone and muscle tissue of Pangasius krempfi and found evidence that this species is anadromous.
The authors examined catch data for six consecutive years (1993"1998) in the Khone Falls area of the Mekong River in southern Laos, and found that Pangasius krempfi were only caught between May and November, with over 98% of the catch coming between mid-May and June.
This suggests that P. krempfi migrate pass the Khone Falls once per year, probably during spawning.
To test the theory that adult P. krempfi inhabit coastal waters, the authors measured carbon and nitrogen stable isotope ratios in the muscle tissue of the catfish.
Because animals incorporate isotopic signatures into their tissues that reflect their food source or the environment, the presence of elevated ratios in the tissue of fishes captured in fresh water (especially if the ratio is incongruent with the range of observed freshwater isotopic values) indicates that the fish presumably migrated recently from a marine habitat.
To this effect, the authors found that muscle tissue of P. krempfi indeed possessed elevated values of stable carbon and nitrogen isotopes typical of marine fishes, especially when compared against potadromous (pure fresh water) pangasiid species such as P. conchophilus, P. larnaudii, P. macronema, and P. pleurotaenia.
For further confirmation, the authors also analysed the strontium:calcium ratio in the otoliths of P. krempfi.
They found the mean strontium concentrations to be two to three times higher than in freshwater species, a result consistent with the hypothesis that P. krempfi spend part of their life in a marine environment.
The strong evidence for long-distance spawning migration in P. krempfi (the distance from Khone Falls to the Mekong delta is about 720 km) has important implications for the management and conservation of Mekong River fishes, given that the management of transboundary fish stocks is one of the primary biodiversity and food security issues in the Mekong River drainage and the potential for habitat fragmentation by hydroelectric dams is a very real threat.
For more information, see the paper: Hogan, Z, IG Baird, R Radtke and MJ Vander Zanden (2007) Long distance migration and marine habitation in the tropical Asian catfish, Pangasius krempfi. Journal of Fish Biology 71, 818"832.