Mesopelagic fish represent a huge b iomass that has been explored as a sustainable new source of marine protein and lipid in several large interdisciplinary projects in terms of resource mapping, capture technology, stabilization, and processing. In this project, fish meal and silage were produced in pilot scale from fresh frozen Mueller’s pearlside at Nofima’s Feed Technology Centre in Bergen, and the nutritional potential evaluated in a feeding trial with Atlantic salmon. Five diets were produced where the control diet was a standard formulated salmon feed that contained 30 % fish meal produced from b lue whiting Micromesistius poutassou . The mesopelagic fish meal protein replaced respectively 50 % (Fm 50) and 100 % (Fm 100) of the protein in the control meal , while the m esopelagic silage replaced 10 % (FPC 10) and 20 % (FPC 20) of the protein in the control meal .
An 11-week feeding trial was carried out with salmon ( initial weight: 93.6 ± 0.5 g) , showing quadrupled fish body weight (f inal weight: 382.5 ± 9.9 g) and efficient growth in all dietary groups (average SGR: 1.85 ± 0.03 % ). No significant differences in feed intake , growth or feed conversion were found in the initial 6 weeks of feeding, and in the total feeding period (ns) . In the first feeding period, fish fed m esopelagic Fm tended to improve the dietary feed intake slightly (3 - 6 %) as compared to fish fed the control meal (ns). Fish fed mesopelagic Fm showed significant reductions in protein digestibility : 91.2 % (control) , 89.3 % (Fm 50 %) and 87.7 % (Fm 100 %) , consistent with the difference in true protein digestibility of 90.6 % (control Fm) and 84.6 % (mesopelagic Fm) measured in mink . T he protein retention efficiency was high in all dietary groups, and no differences in PER, PPV or BV was detected (ns), possibly due to the initial increased feed intake in fish fed mesopelagic fish meal. Lipid digestibility was high in all dietary groups (ADC lipid: 96.0 ± 0.9 %) , and no significant differences in lipid digestibility or liver fatty acid composition was found (ns). Mesopelagic ingredients (Fm, FPC) slightly increased liver lipid (ns), and significantly increased HSI and DOP in fish fed Fm 100 as compared to fish fed the c ontrol diet (P < 0.05) . No significant dietary differences in plasma enzymes (ASAT, ALAT), lipid classes (TG, Pl, total Chol, HDL Chol) , immune parameters (lysozyme), and stress markers (FRAP, TAC) were detected (ns). The s lightly higher plasma FRAP levels found in fish fed the mesopelagic Fm (P = 0.09) was consistent with higher levels of natural tocopherols (Naturnox 15) added to stabilize the mesopelagic Fm. The effects of feed composition on intestinal tissue segments showed that mesopelagic Fm significantly reduced the signs of enterocyte steatosis in the mid-intestinal epithelium of control fish (P < 0.05) , a condition associated with cholin deficiency, although other explanations need to be considered.
Additional analyses to study dietary impacts on the digestive functions will be presented.
In conclusion, Fm and FPC produced from M. pearlside demonstrated efficient growth, feed intake and feed conversion despite lower protein digestibility, showed high lipid digestibility and significantly improved transport of lipids from the mid-intestinal epithelium into the body as compared to control fish, thereby reduc ing the signs of enterocyte steatosis in A. salmon.