S moltification, is a transition stage where Atlantic salmon parr prepare for a life in seawater (SW) while still in freshwater (FW) . In SW, fish experience osmotic water loss, therefore the intestine is responsible for net water absorption through ion-driven fluid uptake to prevent dehydration. During smoltification, there is a gradual increase of fluid and nutrient transport capacity driven by increased Na+/K+ -ATPase (NKA) activity, stimulated by cortisol. This capacity is further enhanced when smolts are transferred to SW. Further, in the Atlantic salmon aquaculture , SW transfer remains a significant challenge that may result in poor growth, and high mortalities. At present, optimal time for SW transfer is commonly determined based on the NKA gene transcript in gill and a seawater challenge test. However, the readiness of the intestine is often overlooked. In addition, fish are fasted for 3-4 days before SW transfer to maintain the water quality during well-boat transportation. Yet, the consequences on intestinal osmoregulatory capacity have not been studied. The aim of the study was to investigate how fasting and seawater transfer time affect intestinal fluid transport capacity and thereby SW acclimation of Atlantic salmon smolts.
Atlantic salmon smolts were produced at the Lerang Research station, Norway (Skretting Aquaculture Innovation) by use of a winter light regime (12 hours light and 12 hours darkness, 12:12) for 6 weeks, before the onset of continuous light (24:0) at 12 °C in FW. Smolts were transferred to SW at three timepoints as defined by the degree days (dd); (Early 168 dd, Normal 348 dd, and Late 516 dd). On each day of transfer, fish had been fed or fasted for 4 days. Sampling occurred; (1) in FW (right before SW transfer) and (2) in SW (6 weeks after SW transfer) where fish body weight (g) and intestinal mucosal wet weight (g) were measured. Gut sac method and Ussing chamber technique were used to measure the intestinal fluid transport and nutrient transport capacity respectively. NKA activity was also determined.
Mucosal weight and fluid transport capacity significantly increased with transfer time. S molts transferred at 516 dd had higher mucosal weight and fluid transport capacity after 6 weeks in SW compared to 348 dd (Fig1). Interestingly, four days of fasting significantly reduced mucosal weight and fluid transport capacity in FW smolts even though NKA activity increased by fasting. The reduced transport tended to be sustained even 6 weeks after SW transfer and normal feeding (Fig1). Our results show that smolts transferred later (516 dd) than the normal (348 dd) have increased fluid transport capacity due to higher intestinal mass and perform better the first month after SW transfer. Further, fasting reduce the mucosal tissue and the intestinal fluid transport capacity regardless of the transfer time.