SELECTION FOR ADAPTATION TO SALINE STRESS OF STRIPED CATFISH Pangasianodon hypophthalmus: TOWARDS SUSTAINABLE CATFISH FARMING IN MEKONG DELTA, VIET NAM UNDER EFFECT OF CLIMATE CHANGE
Introduction
Striped catfish (Pangasianodon hypophthalmus), a commercially important species cultured in Mekong Delta region in Southern Vietnam, is facing a significant challenge due to salinity intrusion as a result of climatic changes . Selection of new strains with high salinity tolerance allows economically feasible production of striped ca tfish in brackish environment. In this study, we demonstrated that survival rate and growth performance can be improved in striped catfish after a single generation of selection for ability to adapt to saline stress.
Materials and Methods
Production of base population : The fieldwork of the study has been conducted in College of Aquaculture and Fisheries, Can Tho University (CTU) , Viet Nam from July/2017 to April/2021. The analysis of the samples and of the genetic data is performed in Liege University. Catfish broodstock have been selected from 3 different sources (An Giang , Vinh Long and Can Tho province, with 10 males and 10 females in each province), with average weights ranging from 5 to 7 kg. Principally, the selected broodstock had to be healthy, without visible injury or abnormal signs. A piece of fin (~1 cm2) from each fish (i.e. 60 broodstock ) has been collected and preserved in 95% ethanol for genetic analyses.
Spawning has been induced by injection with human chorionic gonadotrophin . A dry fertilization process has been used, where eggs and milt have been mixed gently. We have crossed twenty-nine females (~ 3000 eggs of each female) with thirty males to create 870 families (one female from Can Tho province could not be used due to the low quality of her eggs). Fertilization solution (3 g urea and 4 g salt in 1 L of water) has been added to the mixture of eggs and milt to trigger fertilization after 4 min. The fertilized eggs have then been transferred into boxes (one box per family) for incubation. The fertilized eggs have started to hatch 24 hours after fertilization. Fertilization rates varied from 83-90 % and the hatching rate was 61-73 %. The larvae have been transferred to rearing earthen ponds within 15 hours after hatching and fed live feed. From each family, 2000 good quality larvae (with no abnormal signs, uniform size, swimming actively and responding to external stimuli quickly) have been selected for nursing in two earthen ponds (leading to 1.740.000 larvae in total). After 47 days post-hatching (dph ), 22.000 fries have been transferred from nursing ponds to CTU Recirculating Aquaculture System (RAS)
Selection principles : In RAS system, the fish were put progressively in the targeted saline conditions (10 ppt - parts per thousand ). Subsequent sequential massal upward selection of 50% of the survival and healthy fast-growing fish at 3 stages of weight (100 g, 300 g and 1000 g) were implemented (selected group) . In parallel, another set of fish was raised under similar conditions (pond, salinity, density, feed) and underwent a random selection process to serve as negative control (control group). At each time, approximately half of the fish were discarded. After 3 selection stages ( ~ one year), the average weight of selected group and random group were 1.380 ± 175 g and 793 ±230 g, respectively. In the third selection, all remaining fish were identified using Passive Integrated Transponders (PIT) tags. These PIT–tags were injected into muscle of fish and corresponding DNA samples were collected . The level of salinity was then decreased to 5 ppt and remained at this level until fish being mature.
Evaluation of response to selection: To assess the efficiency of selection, the offspring of the selected, control and freshwater ( fish matured in freshwater condition , representing normal broodstock in Mekong Delta) groups were challenged under different levels of salinity. To identify genetic relationships , a new algorithm using shallow sequencing information from the offspring was implemented, allowing parentage assignment in the selected and control groups based on genomic information. The rearing experiment was divided into four distinct periods according to each developmental stage of striped catfish: embryonic stage, larva to fry stage, fry to fingerling stage, fingerling to adult stage. For data collection, information on t wo major production traits (survival and growth) associated with the ability to adapt to salinity stress was recorded during the experiments.
Results
Preliminary results show that survival rate and growth performances of the offspring of the individuals from the selected group is better than for the two other groups, especially with freshwater group under saline stress . At 10 ppt level of salinity, in embryonic stage, hatching rate (%) of fertilized eggs is significantly higher in selected group (13.5%) compared to 10.4% in control group and 5% in freshwater group. For larva to fry stage, survival rate i s also significantly higher in selected group (30%) compared to only 19% and 18% for control group and freshwater group, respectively. At 15ppt, only selected group and control group survived with 3.6% and 1.1% survival rates, respectively. We observed a similar trend for fingerling to adult stage, with selected group tending to live better in saline condition (F igure 1). For growth performance, in all experiments, the growth rate of selected group was higher than for control and freshwater group. These results suggest that selection to improve ability of striped catfish to adapt to saline stress can be achieved, in terms of increased survival rate and growth rate of the selected group following saline exposure.
