## 1 Introduction  
According to the Food and Agriculture Organization (FAO), total fish production reached up to 171 million tonnes in 2016 [1]. It has been estimated that \(\approx\)20-80% of fish weight are side streams (i.e., head, skin, bones, viscera, scales, and tails), which have been traditionally considered as a waste with low added-value, thus representing a potential negative environmental impact [2]. However, they are a great source of nutrients such as high quality protein, fat, and antioxidants, which can protect the human body from free radicals, thus delaying the development of many noncommunicable diseases [3]. For instance, some previous studies have evaluated the use of fish side streams from Sardine [4], tuna [5, 6], salmon [7], mackerel [8], seabass [9, 10], among others, as a source of protein hydrolyses and antioxidant peptides using conventional recovery strategies. However, there is a lack of information regarding the use of innovative approaches to recover proteins from sea bass side streams, and about their impact on protein molecular size distribution and the antioxidant yield. Ultrasound-assisted extraction (UAE) is a nonconventional technology that has emerged over the last few decades. UAE utilizes acoustic cavitation that promotes molecular move ment of solvent and sample, showing some advantages such as efficiency, reduced extraction time, low solvent consumption, and high level of automation. UAE has been reported as an interesting tool for the extraction of protein from the whole fish [11]. It has also been shown as a useful strategy to extract collagen and gelatin from different fish side streams (i.e., skin and scales) [12]. In this line, UAE has been used with different methods, including the green, environmentally friendly solvents, such as the deep eutectic solvents (DES) and their natural equivalents, the natural deep eutectic solvents (NADES) to improve the efficiency of the extraction process, and the tailored recovery of target compounds [13]. Moreover, UAE combined with other techniques can be an efficient tool for mycotoxin extraction from fish [14]. Mycotoxins are toxic chemical compounds resulting from the secondary metabolism of fungi, which can occur on different substrates under certain environmental conditions. They are natural micropollutants present in food and can affect consumers and animals health at subtoxic doses, due to their simultaneous presence in food and their continued ingestion throughout life. Mycotoxins are related with adverse effects such as hepatotoxicity, nephrotoxicity, estrogenicity, immunotoxicity, mutagenicity, teratogenicity, carcinogenicity, and diabetic action [15]. The toxigenic fungal species most frequently found in food belong to the genera _Aspergillus_, _Fusarium_, and _Penicillium_. Aflatoxins (AFs) are produced by _Aspergillus_ species, and Ochratoxin A (OTA) and Patulin (PAT) by both _Aspergillus_ and _Penicillium_. _Fusarium_ species produce trichothecenes (HT2, T2, Deoxynivalenol (DON), and Nivalenol (NIV)), Zearalenoene (ZEA), Fumonisins (FB1 and FB2) and emerging mycotoxins (Fusaproliferin (FUS), Moniliformin (MON), Beauvericin (BEA) and Ennatiins (ENNs)) [16]. Maximum concentrations have been established for some mycotoxins in different raw materials and processed foods based on their toxicity and consumption habits [17], however in fish products maximum levels have not been legislated yet. Mycotoxin carryover from feed to edible fish tissue has been previously reported in bibliography. [PERSON] et al. [18] and [PERSON] et al. [19] reported AFB1 contents in muscle and hepatopancreas of gibel carp and in edible muscle of rainbow trout. Moreover, they also found higher contents of AFB1 metabolites (aflatoxicol (AFL) and aflatoxin M1 (AFM1)) after dietary exposure. On the other hand, ENNs were reported in fish species and FUS-X and ENN B in gula substitute samples [20; 21].