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Histological alterations in the liver of wild saddled bream, Oblada melanura (Sparidae): the effect of artificial feed

Floating fish farms attract a great number of wild fish species, changing their behaviour and physiology. Specifically, attracted fish change their diet using uneaten food pellets as the main resource. This study evaluated the effect of artificial feed on the liver morphology of the saddled bream, Oblada melanura, a common member of the aggregated assemblage at the Adriatic fish farms. We analysed and compared liver histomorphology of O. melanura from populations aggregated around the fish farm and from natural/control populations. Change in the feeding behaviour of investigated species sampled around the cages was evident because the majority of fish had pellets in their guts. Furthermore, necropsy of cage-associated fish revealed large amounts of visceral fat and yellowish liver while no macroscopic changes were found in control fishes. Microscopic analysis of the liver sections demonstrated that control saddled bream had no lipid accumulations and hepatocytes were characterized with granular cytoplasm and central and round nucleus. The main alterations found in liver sections of cage-associated saddled bream were: irregular shaped nuclei, cytoplasmatic and nuclear degeneration, and melanomacrophages which were identified as aggregates of cells containing dark-yellowish granules, normally close to the vessels. In general, hepatocytes of cage-associated fish contained large cytoplasmatic clear spaces which present excessive accumulation of fat in the hepatocyte cytoplasm. This observed morphological pattern is known as steatosis. When comparing sizes of hepatocytes and nuclei between cage-associated and control saddled bream, all sizes were significantly larger in cage-associated fish. Moreover, cage-associated saddled bream had significantly higher condition index than control fish, probably due to a diet based on the lost food pellets and a relative lack of exercise because of abundant, low-metabolic cost feeding opportunity. In conclusion, comparative histological studies of liver of the saddled bream from two sampled populations provided an index to their general condition. Observed liver modifications in cage-associated specimens confirmed an evident effect of artificial feed and are reaction to imbalanced feeding or a nutritional pathological process.

Oblada melanura; Aquaculture; Food source; Liver histology; Adriatic

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