Self-selection of diets with different oil oxidation levels in gilthead seabream (Sparus aurata)

The present research looked at the ability of gilthead seabream to discriminate between two similar diets differing only in the extent of lipid oxidation. Six groups of 4 fish (254g initial wet weight) were allowed to select by means of self-feeders between two complete diets (34.5% casein + gelatin, 14.8% fish oil + soybean oil, 24.8% dextrin and 25.9% vitamins, minerals, filler and binder) which differed only in the oxidation level of the lipid source: 6.2 meq/kg for the control (D1) and 100.0 meq/kg for the oxidized diet (D2). During the first nine days of the trial, fish took similar amounts of D1 and D2, although there was a clear tendency to gradually avoid D2. Finally, on day 10 fish demanded a significantly higher percentage (82%) of D1. When the diets were intercharged to investigate feeder preferences, two selection patterns were observed: three groups resumed their selection for D1 from day 7 onwards, while the other three groups did not show a clear preference for any diet until they were subjected to a 3week fasting period (which boosts internal oxidation), after which they significantly selected D1. Taken together the results before and after fasting, gilthead seabream were able to discriminate and avoid a diet with oxidised lipids, and the physiological state of fish (oxidative stress caused by fasting) appeared to reinforce their selection/avoidance behaviour.