The effects of partial (75%; diet PP75) and total (100%; diet PP100) replacement of fish meal by plant protein sources on the intestinal nutrient absorption of gilthead sea bream and rainbow trout were examined over a 12-week growth trial. The diets comprised a mixture of plant ingredients (corn gluten meal, wheat gluten, extruded peas and rapeseed meal) and met the amino acid requirements of the fish. A third group of fish on a fish meal diet was used as a control for comparative purposes. Diets were tested in triplicate and fish were fed twice a day until visual satiation. At the end of the trial, we measured amino acid (l-leucine, l-lysine, l-phenylalanine, l-alanine and l-proline) and d-glucose absorption at short-term (6 h) and long-term (36 h and 48 h in rainbow trout and sea bream respectively) post-feeding by means of brush border membrane vesicles obtained from pyloric caeca, proximal intestine and distal intestine. The absorption pattern at 6 h post-feeding was modified in both species in response to fish meal replacement. In PP75-fed trout absorption was delayed from pyloric caeca to proximal and/or distal intestinal segments, thus total absorption capacity was maintained in this group. On the contrary, total uptake was significantly decreased in trout fed the PP100 diet and in sea bream on both PP75 and PP100 diets. Glucose transport capacity was increased in both experimental sea bream groups and in PP75 trout. Long-term transport capacity was up-regulated for both species. Our results show that intestinal nutrient absorption is modified in response to the use of high levels of plant protein sources and that these changes are species-specific.

Here, we performed an 11‐week trial to study the effects of four experimental diets on the digestion, nutrient absorption and intestinal histology of gilthead sea bream. The diets were formulated with a low fish meal content (25%) and were rich (75%) in plant proteins. Fish oil (FO) was replaced at 0%, 33%, 66% and 100% by graded levels of a blend of vegetable oils (VO) (diets FO, 33VO, 66VO and 100VO respectively). Protease activity increased in the pyloric caeca (PC) and decreased in the proximal intestine (PI) of the 66 and 100VO groups, while lipase and α‐amylase activities were not modified by VO. The capacity of brush‐border membrane vesicles obtained from PC, PI and distal intestine to absorb amino acids was not modified, but d‐glucose and linoleic acid absorption diminished when VO was included in the diets. In summary, these results show that when 75% of protein is provided by vegetable sources in the diet of gilthead sea bream, up to 66% of FO can be replaced by a blend of VO without compromising digestive processes. Only the total replacement of VO results in an impaired digestion, with reduced total protease activity and accumulation of lipid droplets in the enterocytes of the PI.

Overcoming a stress situation, such as hypoxia episodes, which involve an allostatic load, will depend on the ability of fish to modulate physiological and biochemical systems to maintain homeostasis. The aim of the study was to determine the integrated stress response after acute hypoxia of the rainbow trout considering the different elements and areas of the stress response: systemic and mucosal, local and global, and from the systemic hypothalamic–pituitary–interrenal axis to skin mucosa. For this purpose, trout were subjected to acute hypoxia (dissolved O₂ down to 2 mg/L) for 1 h and then recovered and sampled at 1, 6, and 24 h after reoxygenation. Physiological responses were significantly affected by hypoxic stress and their interaction with time after the challenge, being significant for plasma lactate and cortisol levels, in both plasma and skin mucus. At the central brain level, only trh expression was modulated 1 h after hypoxia which indicates that brain function is not heavily affected by this particular stress. Unlike the brain, the head kidney and skin were more affected by hypoxia and reoxygenation. In the head kidney, an upregulation in the expression of most of the genes studied (gr, il1β, il6, tgfβ1, lysozyme, caspase 3, enolase, hif-1, myoglobin, sod2, gpx, gst, and gsr) took place 6 h after recovery, whereas only hsp70 and il10 were upregulated after 1 h. On the contrary, in the skin, most of the analyzed genes showed a higher upregulation during 1 h after stress suggesting that, in the skin, a local response took place as soon as the stressor was detected, thus indicating the importance of the skin in the building of a stress response, whereas the interrenal tissue participated in a later time point to help prevent further alteration at the central level. The present results also show that, even though the stressor is a physical/environmental stressor, all components of the biological systems participate in the regulation of the response process and the recovery process, including neuroendocrine, metabolism, and immunity.