Abstract
A key factor in the production of fish in commercial aquaculture is the optimization of the artificial diets, not only to achieve optimal growth, but also to maximize fish health. Evidence is accumulating that dietary lipids, particularly the fatty acid composition, can have a direct effect on the fitness of the fish. This thesis aims to clarify the role played by the polyunsaturated fatty acid arachidonic acid (ArA, 20:4n-6) in fish, particularly in relation to the stress response and osmoregulation. A comparison was made between two species: Mozambique tilapia (Oreochromis mossambicus), a freshwater species that is able to fulfil its requirement for ArA by synthesising it from a precursor, and gilthead seabream (Sparus aurata), a seawater species that depends on the diet to supply this fatty acid. The observations presented in this thesis indicate that a short feeding period with a relatively minor dietary change in a single fatty acid, ArA, is sufficient to considerably alter several physiological functions in Mozambique tilapia and gilthead seabream. The two species showed completely opposite responses to a stressor after their diets were supplemented with ArA, while it had a positive effect on the osmoregulatory capacity of both species. In addition, the experiments with tilapia and seabream indicate that the COX-derived metabolites of ArA play a relatively minor role in the modulation of the stressor-induced release of cortisol, despite the general assumption that the main effects of ArA in fish are mediated by prostaglandins. Free ArA and possibly the lipoxygenase and epoxygenase metabolites should be considered relevant regulatory factors as well. From a practical point of view, the replacement of fish oil in commercial feeds by vegetable lipid sources with very low levels of ArA clearly involves potential health and welfare risks for farmed marine or carnivorous fish species
