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Increased reliance on the use of alternative ingredients in the formulations of diets for farmed fish has long been a priority to sustain the unparalleled growth of aquaculture. This need which has led the feed industry to explore many alternative protein and oil sources, has also led to many physiological impacts on those species being fed those alternatives. The intensification of aquaculture under a global climate change scenario also contributes as another major challenge to feed the blue revolution, since there is a need to develop diets that enhance fish robustness. Recent research on farmed fish species has resulted in the development of a range of innovative novel ingredients that are able to sustain fish growth performance, improve fish welfare and resilience, and still assure a safe, nutritious and tasty product for human consumption. Since the suitability and sustainability of vegetable ingredients in diets for carnivorous fish has increasingly being questioned, other resources need to be explored and increasingly we need to identify and develop alternatives that do not compete directly with human food supply. Insects and agrofood byproducts have great nutritional value and may contribute towards a circular economy concept. Biotechnology also offers a wide range of possibilities, including microbial biomass and single cell products, but scale-up is still required for many of these initiatives to realize any serious benefit. Micro and macroalge are also receiving attention, not only as sources of macronutrients, but also for their richness in bioactive compounds. However, the potential of each new feed ingredient has to be thoroughly evaluated before their wide acceptance by the feed industry. There is little to gain by simply transferring risk from one ingredient to another, when ideally what the feed sector needs is greater security of supply, quality and on a cost-effective basis.

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. © 2011 Elsevier B.V.