The impact of fasting and refeeding on intestinal fluid transport capacity and cortisol levels in salmonids

Abstract

The intestine in fish, as in most animals, plays a central role in nutrient absorption and contributes significantly to osmoregulatory balance, particularly under fasting or salinity-related stress conditions. This thesis investigated the impact of fasting and subsequent refeeding on intestinal fluid transport capacity, empty gut mass, plasma ion concentrations, and cortisol levels in seawater-acclimated rainbow trout (Oncorhynchus mykiss). The primary aim was to determine how fasting influences fluid absorption in the proximal and distal intestine and how systemic responses such as ion balance, intestinal mass, and endocrine regulation are affected. Fish were fed on day 1, followed by fasting periods of 24 to 120 hours, and then refed for 48 hours. Fluid transport was assessed using the non-inverted gut sac method. Plasma concentrations of Na⁺, K⁺, Ca²⁺, and Cl⁻ were measured using a plasma ion analyser, and cortisol levels were quantified using radioimmunoassay (RIA). Fluid transport per intestine (μL/h) increased with fasting and remained elevated after refeeding, particularly in the distal intestine. However, when normalized to surface area (μL/h/cm²), no significant differences between fed and fasted were observed, suggesting that tissue-level transport efficiency remained stable. Empty gut mass increased during fasting, especially in the distal region, and in the refeeding group it remained reduced in both intestinal regions. Plasma sodium levels decreased significantly during early fasting (24-48h) but recovered by 72h, while potassium showed a transient increase at 48h. Calcium was highly variable, and chloride remained unaffected. Cortisol levels progressively increased with fasting duration, peaking at 120h, and declined after refeeding. Notably, the fed control group showed poor feed intake, which may have limited the reliability of comparisons against truly fed individuals. These findings may suggest that intestinal fluid transport remains functionally responsive during extended fasting, potentially supported by plasma cortisol levels. However, the absence of histological and molecular data limits clear interpretation. Future studies should incorporate freshwater comparisons to further elucidate fluid transport rates under fasting conditions in rainbow trout.