In the process of olive oil production, of which Spain is by far the biggest producer in the world, large volumes of olive pomace “waste” are generated, which still contain important amounts of olive bioactive compounds, mainly triterpenoids. This side stream is a highly valuable resource that can be converted into feed additives to improve aquaculture productivity and animal well-being following sustainable circularity principles . The present study was developed to test the efficacy of an olive pomace extract (OPE) in common carp Cyprinus carpio L. farming.
A trial was performed at the Fisheries Research Station of the University of Agriculture, Krakow (Poland), in six flow-through earth ponds (3 per treatment) stocked with fifty 11-month-old sexually undifferentiated carps (start weight: 122.3±1.03 g ). Two diets were tested: a standard commercial pelleted feed (control diet) containing 34% CP and 7% CF, and a similar diet supplemented with 0.2% of an additive containing OPE (Luctactive® AQUA). Carps were fed at 2.5% to 3.5% body weight, depending on water temperature , from early May until the end of September (5 months duration). Every month, all surviving fish (>78% at 5 months) from each pond were weighed and counted. Additionally, blood and tissue samples were collected from 10 fish at the start of the trial and from 5 fish per pond (15 per treatment) at 3 and 5 months.
Growth was significantly improved (* P<0.05) by the additive, with 10-12% higher weight in fish fed the OPE treatment from 3 months onwards (Fig. 1). Several haematological parameters were also significantly affected at either 3 or 5 months (Fig. 2a): haemoglobin, mean corpuscular haemoglobin (MCH), MCH concentration and white blood cells count were all increased in the control, indicating a higher oxygen transport rate and energy expenditure , potentially associated to a higher stress response. Plasma biochemical parameters (Fig. 2b) revealed significantly higher levels of glucose, triglycerides, total cholesterol and HDL-cholesterol in the control. An improved liver function was indicated in the OPE treatment by significant reductions in plasma levels of alkaline phosphatase (ALP) and bilirubin (Fig. 2c). Finally, gene expression (qPCR) analysis at 5 months revealed a significantly elevated expression of il8 , nfkb and hsp70 in the liver of carp fed the OPE additive (Fig. 2d) . In head kidney, only hsp70 was significantly elevated in OPE, but a similar trend (P=0.065) w as observed for nfkb and infg. These results indicate a potential immunostimulatory and immunoprotective effect of the OPE additive, which has also been suggested by previous studies testing similar olive extracts in fish .