**5. Conclusions**

This work showed the presence of protease inhibitors in *Ulva* able to inhibit digestive proteases of commercial fish species. The inhibitory capacity was dose-dependent. From a physiological point of view, high dietary inclusion of crude *Ulva* would be requested to achieve higher inhibition values. The thermal treatment during feed processing is high enough to inactivate the inhibitors from *U. ohnoi*; hence, it can be used efficiently as potential sustainable ingredient for aquafeeds.

**Author Contributions:** A.J.V., M.I.S. and A.G. performed experiments and analyzed data. A.J.V. wrote the manuscript. F.J.A. and T.F.M. conceptualized, designed the research, revised and corrected the paper. All authors agreed with the final submitted version. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the projects SABANA (grant # 727874) from the European Union's Horizon 2020 Research and Innovation program, DORALGAE (RTI2018-096625-B-C31) from the Ministry of Sciences, Innovation and Universities (Spain) and the European Regional Development Fund (ERFD), and the knowledge transfer action gran<sup>t</sup> GREEN4FEED # 5917 from the Ministry of Economy, Knowledge, Business and University of the Regional Government of Andalusia (Spain) and ERFD.

**Acknowledgments:** Authors thank IFAPA El Toruño center (Cádiz, Spain) for providing Senegalese sole specimens and *Ulva ohnoi* biomass. Gilthead seabream and seabass juveniles were kindly provided by Predomar (Almería, Spain).

**Conflicts of Interest:** The authors declare no conflict of interest.
