FINDING AN IDEAL FASTING PERIOD PRIOR TO THE LIVE FISH TRANSPORTATION: ASSESSMENT OF AMMONIA EXCRETION DYNAMICS AND AEROBIC METABOLISM TRADE-OFF

The ability to transport live fish to the market to be sold as fresh product, to re-stock in aquaculture facilities or to deliver to processing plants for slaughter, is a challenge for aquaculture producers and marketers. Fish are routinely fasted prior to live transport to reduce metabolic rate and excretion. Prolonged periods of fasting often diminish the performance of transported fish and limit the ability to recover in their new environment. In this scenario, an ideal fasting period would be determined when fish metabolisms become low enough to keep oxygen consumption as well as ammonia production minimal, while still being able to regulate ammonia-transport so that it enables them to cope with ammonia stress in aquaculture facilities.

Therefore, in this study we have focused on finding the ideal pre-transport fasting period by evaluating temporal ammonia dynamics and associated metabolic trade-off. The experimental fish species, common carp (Cyprinus carpio) were progressively fasted up to 14 days. Temporal effect of fasting on oxygen consumption rate (MO₂), ammonia  excretion rate (J_amm), plasma ammonia (T_amm), plasma ions, branchial Na⁺/K⁺-ATPase (NKA) and H⁺-ATPase activity, as well as branchial mRNA expression of NKA, H⁺-ATPase, Na⁺/H⁺ exchanger (NHE-3) and Rhesus (Rh) glycoproteins were assessed.  Results show that MO₂ and J_amm were considerably depressed from 4-6 days of fasting onwards. This coincided with the onset of higher resting plasma T_amm from day 6 onwards, possibly because in fasted fish the basal expression levels of Rhcg-a and Rhcg-b were only maintained up to 6 and 8 days respectively, after which a down-regulation was recorded. Plasma [Na⁺] and [Cl⁻] were temporarily reduced during 4-8 days of fasting, while an augmented [K⁺] was evident. The transcript level of NHE-3 was raised in 12-14 days fasted fish, which along with up-regulation in Na⁺/K⁺-ATPase activity and mRNA expression facilitated the recovery of [Na⁺] to control level. First signs of energy store depletion in liver, especially glycogen, were recorded from day 8 onwards. Overall, these data suggest that the beneficial reduction of metabolic rate (MO₂ and J_amm) started at 6 days of fasting. Interestingly, at this time fish were still able to regulate ammonia transport efficiently, and did not compromise their energy stores yet. Therefore, we propose 6 days as an ideal fasting period before transport.