Causes of fatty liver in farmed fish: a review and new perspectives

Fatty liver is a physiological or pathological phenomenon and is commonly seen in farmed fish at present. Severe fatty liver in fish results in reduced growth and feed efficiency,impaired immune response and decreased nutritional quality. However,so far,people still don't know exactly the causes of fatty liver in farmed fish,and also have not found good solutions to eliminate fatty liver. The present article systemically reviews the results of fish fatty liver studies and has put forward some new perspectives to discuss the origins of fish fatty liver in the current aquaculture system. In the present review,five main causes of fatty liver in farmed fish are summarized: nutrition and formulated diet,environmental pollution,physiological process,species difference and genetic mutation. Among them,the unbalanced nutrients composition and excess energy intake,such as overfeeding,excess dietary lipid and carbohydrate contents,deficiency of vitamins which are needed to maintain normal lipid metabolism,are regarded as the principal causes of fish fatty liver.Beside,in some aquaculture regions,high amount of water contaminants,especially heavy metals and persistent organic pollutants( POPs),have been recently reported to contribute to the prevalence of fatty liver in farmed fish. The occurrence of fatty liver is also related to the gender and development of fish. For example,the change of fat content in liver of young and /or female fishes normally is more sensitive to dietary energy content than that in liver of older and /or male fishes. It is also noted that some fish species specifically deposit fat in liver,thus fish species difference should be considered in preventing fatty liver in farmed fish. Through reviewing the above results,the present article brings some new perspectives: 1) Most of fatty liver could be classified asnutritional fatty liverandoxidative fatty liverdepending on the fatty liver symptom caused from nutrient unbalance or oxidative stress; 2) The common occurrence of fatty liver in farmed fish fundamentally correlates with the contradiction between the requirement of high aquaculture production of human and natural physiological features of fish. Therefore,as long as the present fish species have not been genetically modified to adapt to modern high-density aquaculture industry,the common occurrence of fatty liver in farmed fish would last in future for a long time; 3) Fatty liver could not be completely regarded as a pathological symptom. Actually,initial stage of increasing hepatic triglyceride synthesis is a protective mechanism to avoid the lipotoxicity induced by high free fatty acids in circulation. In conclusion,feeding fish with optimal nutrients-balanced diets,keeping freshness of feed and raw feed materials,and maintaining a sustainable healthy water environment are the ultimately right measures to prevent fish fatty liver.