1. Introduction

Fishmeal is considered an ideal ingredient in aquatic feed because of its high protein content, essential amino acid composition, and easy digestion [1]. However, due to finite and limited supply, the price of fishmeal has increased, which, in turn, has led to a rise in the cost of feed and aquaculture production [2]. Lipids (oils and fats) are the most effective ingredients for achieving the best balance between protein and energy contents in aquatic feeds [3]. In fish, an appropriate dietary lipid level can provide high energy density, improve the absorption of nutrients, and enhance feed conversion [4, 5]. Therefore, high‐fat diets (HFDs) are capable of effectively supplying adequate energy in feed and reserving protein for growth. In this way, HFD can enhance feeding efficiency and cut down farming expenses [6]. However, excessive fat intake in fish can lead to adverse effects, including increased lipid deposition in tissues and metabolic disorders, which may induce related diseases and, consequently, result in economic losses. For example, feeding HFD increased lipid droplet content and damaged the morphological structure of the hepatopancreas and reduced antioxidant capacity in tilapia (Oreochromis niloticus) [7]. Other studies in teleosts showed that excess dietary lipid can disturb the oxidative status and metabolic enzyme activities in largemouth bass (Micropterus salmoides) [4], while HFD was associated with increased lipid deposition and intestinal damage in juvenile Monopterus albus [8]. Therefore, identifying safe and effective ways to improve the utilization of feed while alleviating the negative effects of the use of HFD is an urgent issue in the field of aquatic animal nutrition.

Recently, there has been growing interest in feed supplements with the potential to reduce the risk of disorders of metabolism and to mitigate the development of fatty liver in cultured fish fed HFD. Olive (Olea Europaea L.) leaf extract (OLE) is a phytogenic feed additive containing a variety of bioactive phenolic compounds, including oleuropein, hydroxytyrosol, luteolin, and rutin, among others [9]. Previous studies indicated that the bioactives in OLE can exert a variety of physiological effects in both humans and animals [10], including antioxidant, anti‐inflammatory, antihypertensive, and metabolic regulatory activities, with potential benefits for obesity‐related metabolic disturbances and cardiometabolic risk factors [11–13]. Given the known functional properties of OLE bioactives in mammals, there has been increasing interest in the application of OLE in aquafeeds. It was shown that dietary inclusion of OLE or olive leaf powder can enhance the growth, survival rate, immunity, and health of fish [14]. A study conducted on common carp (Cyprinus carpio) also reported that the carcass lipid content was significantly reduced in fish fed a standard diet supplemented with 200 mg/kg OLE [15]. However, the specific mechanisms of OLE actions in mitigating metabolic disorders and hepatic lipid deposition induced by HFD in fish are still unclear.

Golden pompano (Trachinotus ovatus) is a carnivorous marine fish that can thrive on formulated feeds and is characterized by rapid growth, strong resistance to adverse conditions, and high survival rates in aquaculture conditions. These traits have made it an economically important fish species with high suitability for cage culture in the marine aquaculture industry in China. However, it is important to highlight a currently widespread phenomenon, which is that deep‐sea intensive culture and the long‐term use of HFDs likely cause oxidative and inflammatory stress, resulting in fatty liver and growth suppression in fish [16, 17]. Thus, it is of increasing importance to find functional feed additives, optimize formulations, and develop high‐efficiency, low‐cost, and environmentally friendly compound feeds for the cultivation of T. ovatus. Previous studies have shown that 42%–49% protein and 6.5%–12.5% lipid are suitable dietary ranges for the growth and health of T. ovatus [18]. When the dietary lipid level was increased to more than 14%, growth and feed intake of juvenile T. ovatus decreased, and lipid content of the liver increased and triggered oxidative stress [19–21]. In the present study, we utilized a dietary lipid level of 16% to establish an HFD model. Hence, the current study aimed to elucidate the impacts of dietary OLE (~40% oleuropein) supplementation on the negative effects induced by feeding HFD in T. ovatus, including growth performance suppression, lipid metabolism disorders, inflammation, oxidative damage, and intestinal health problems in T. ovatus. The results provide a basis for optimizing compound feed suitable for the growth of T. ovatus and suggest that utilization of production waste could be a new development opportunity for the olive industry.