Marine fish farming is an important sector of aquaculture with most of the operations located in open seas exposed to oscillations of environmental parameters, parasites and pathogens. Ensuring robustness and fast growth are the ultimate goals under these conditions. In pursue of these goals, the use of bioactive compounds with immunostimulant and growth enhancing action is gaining appreciation while screening for such compounds asks for new approaches.
In this study, the effect of bioactive compounds, derived from by-products of olive (Olea europea) and Spirulina processing, on the growth parameters of gilthead sea bream (SBG) and European sea bass (BSS) was explored. Product 1 originated from olive pulp of pressed olives with a polyphenol content of 40.000 mg/Kg of dry product; Product 2 derived from Product 1 after extraction with 70% ethanolic solution under ultrasound to reduce the polyphenolic content to 17.000 mg/Kg of dry product; Product 3 was an enzymatically modified pomace olive oil that contained 80 ppm hydroxytyrosol; Products 4, 5 and 6 were Spirulina protein fraction of 3-10kDa, 10-30 kDa and >30kDa, respectively. A multi-tier approach was followed to study the effect of the bioactive compounds.
Fifteen experimental feeds were formulated with the addition of each bioactive compound in a standard feed formulation. Each feed had a different concentration of bioactive compound. The content of Kunitz Trypsin Inhibitors and Birk-Bowman Inhibitors of chymotrypsin was determined in all feeds along with the rates of dietary protein autohydrolysis and enzymatic hydrolysis in vitro, using digestive enzymes isolated from SBG and BSS, respectively. The species-specific appropriate levels of bioactive compounds were determined according to the results in vitro before the formulation of the second generation of two experimental feeds per species that were tested in a feeding trial in vivo. SBG and BSS of 7.45±0.26g and 9.07±0.15g, respectively were arranged in triplicate tanks per feed. Fish were fed ad libidum every day throughout the trial that concluded to approximately 55g final body weight.
Feed supplemented with a combination of product 1 (0.05%), product 2 (0.02%), product 3 (1%) and product 4 (0.1%) resulted in improved growth rate and feed conversion rate (FCR) in SBG. Similar results were yielded in BSS with feed supplemented with a combination of product 1 (0.2%), product 2 (0.5%), product 3 (1%) and product 4 (0.1%). The results are indicative of how by-products available in big volumes can be valorized in fish farming.
Acknowledgement: This research has been co-financed by the European Union and Greek national funds through the Operational Program Competitiveness, Entrepreneurship, and Innovation, under the call RESEARCH-CREATE-INNOVATE, Project title "Development of new functional fish-superfood for a more efficient fish farming" MIS 5069987.