Life history strategies of Mekong pangasiid catfishes revealed by otolith microchemistry

Fourteen species of pangasiid catfish are found in the Mekong River. They are key components of both capture fisheries and aquaculture, although little is known about their migratory ecology, which is particularly important as some species are in decline. In order to examine the life-history strategies of key pangasiid catfishes in the Mekong River, we collected otoliths from 126 specimens belonging to four species in the Lower Mekong Basin. We analysed the otolith microchemical structure using two methods (Laser Ablation – Inductively Coupled Plasma Mass Spectrometry and Scanning X-ray Fluorescence Microscopy) to identify evidence of diadromy (movements between the ocean and upstream waters). We followed an established method of examining patterns of strontium to calcium (Sr:Ca) and barium to calcium (Ba:Ca) ratios along otolith transect profiles to interpret whether fish were in fresh or salt water at various stages of their life. We confirmed anadromy in two species (Pangasius krempfi and P. mekongensis; spawning in river but growing in marine waters) with at least three migratory tactics identified for each species, thus highlighting more diverse life history strategies than previously thought. Based on locations where we collected fish, we can infer these species migrate up to 700 km (P. mekongensis) and 1400 km (P. krempfi) from the sea into the Mekong River for spawning. The other two catfishes (P. elongatus and P. bocourti) appeared to be freshwater species that occasionally strayed into the estuary. Long-distance migratory species require free flowing rivers to connect critical habitats used at different life stages. Any activity disrupting these migratory pathways and disconnecting these habitats must be carefully considered. Conservation and management of transboundary species require international agreements to protect them from the impacts of river development, and the evidence presented here is pertinent for catfish in the Mekong River.