Temporal assessment of metabolic rate, ammonia dynamics and ion-status in common carp during fasting: A promising approach for optimizing fasting episode prior to fish transportation

In aquaculture practices, 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. Therefore, the main objective of this study was to assess the ideal fasting period prior to the transport. For this purpose, common carp (Cyprinus carpio) were progressively fasted for 2, 4, 6, 8, 10, 12 and 14 days. Temporal effects of fasting on oxygen consumption rate (MO2), ammonia excretion rate (J(amm)), plasma ammonia (T-amm), plasma ions (Na+, Cl- and K+), 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 Jamm were considerably depressed from 4-6 days of fasting onwards. Resting plasma Tamm were lowered during the first days of fasting preceding the reduced excretion rates, but returned to control levels at day 6 when excretion rates declined. In fasted fish, the basal expression levels of Rhcg-a and Rhcg-b were maintained up to 6 and 8 days respectively, thereafter down-regulation occurred. Plasma [Na+] and [Cl-] were temporarily reduced during 4-8 days of fasting followed by a recovery to control levels, while an augmented [K+] was evident from 6 days onward. The transcript level of NHE-3 was raised in 12-14 day fasted fish which, along with up-regulation in Na+/K+-ATPase activity and mRNA expression, most likely facilitated the recovery of [Na+] to control levels. Gill H+-ATPase activities and mRNA levels were not affected by feed deprivation. First signs of depletion of liver energy stores, especially glycogen mobilization, were apparent from day 8 onwards. Overall, these data suggest that the desired reduction of metabolic rate (MO2 and Jamm) was obvious from day 6 of fasting onwards, while at this time negative effects were not yet present: fish were still able to regulate ammonia and ion transport efficiently, and energy stores were not yet compromised.