###### Abstract  
Fisherly processing industries generate large amounts of by-products. These by-products come from fish heads, skin, bones, thorns, and viscera. The disposal of these wastes represents an increasing environmental and health problem. Nowadays, there is a growing interest in how to utilize fish materials that are not used for human consumption. Among the different solutions proposed, the use of proteolytic and lipolytic microorganisms represents a green solution for waste vaporization. In this work, first we screened several conventional and non-conventional microorganisms for their proteolytic and lipolytic functions. Then, the most promising strains (_Yarrovia lipolytica_ YL2, _Y. lipolytica_ YL4, _Bacillus amyloidefaciens_ B5M and _B. subtilis_ B5C) were tested on a fish waste-based solution. After 72 h incubation at room temperature, the supernatants obtained using the strains of _Y. lipolytica_ showed the highest degree of hydrolysis (10.03 and 11.80%, respectively, for YL2 and YL4), the strongest antioxidant activity (86.4% in DPPH assay for YL2) and the highest formation of aldehydes (above 50% of the total volatile compounds detected). Hydrolysates of fish waste obtained with _Y. lipolytica_ may be reused in feed and food formulations for their functional and flavoring characteristics.  
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Biotechnological approaches represent a promising tool for fish waste valorization [3]. In fact, the use of selected microorganisms that can perform enzymatic activities in situ represents a \"greener\" and more sustainable approach to obtain several functional compounds [4; 5; 8; 9]. Compared to the use of pure enzymes, fermentation processes may enrich the final functionality and sensorial characteristics of the product [10; 11]. For instance, microbial lipolysis and proteolysis can generate free fatty acids and amino acids, respectively, which are precursors of volatile compounds [12]. Therefore, other than the type of substrate (fish species and waste components) [13], the selection of microbial species and strains can impact the quality of the final product and its features [11; 14]. Among the most hydrolytic bacteria, the genus _Bacillus_ has been widely used for fish waste valorization through the production of protein hydrolysates [15; 16], enzymes [17; 18; 19], polyhydroxyalkanoate [20] and biosurfactant [21]. For instance, Alcalase, an endoproteace extracted from _B. subtilis_, is widely used to produce fish protein hydrolysates with high solubility and digestibility. Among the yeasts, _Yarrowia lipolytica_ has been proposed as a promising tool to valorize agro-food wastes due to its enzymatic functions [22]. Most of the studies performed with _Y. lipolytica_ and fish wastes regard the production of single cell oil, lipases, yeast biomasses, or lipid reduction [6; 23; 24; 25]. However, _Y. lipolytica_ also possess proteases that play an important role in food ripening, such as in cheese and fermented sausages [22]. The use of this yeast to obtain fish waste protein hydrolysates has never been explored.  
The present work was focused on the selection of proteolytic and lipolytic microorganisms for the valorization of fish waste into functional and flavoring hydrolysates, to be potentially re-used in the food sector. Together with the well-documented Bacilli, the unexplored yeast _Y. lipolytica_ was also considered. The most promising strains were then tested in a fish waste solution and the peptide content, functional activity (antioxidant) and volatile molecule compounds were evaluated on the hydrolysates.