Utilization of Nannochloropsis oceanica in plant-based feeds by Atlantic salmon (Salmo salar)

Abstract

The phototropic microalga Nannochloropsis oceanica is a promising feed ingredient with the potential to provide nutrients including polyunsaturated fatty acids in aquafeeds. Complex and rigid cell walls limit the nutrient utilization of microalgae by Atlantic salmon. Here, we report results from two studies-a laboratory study on post-smolts and a farm trial on consumer-ready fish-that were conducted to understand the efficacy of both treated and untreated whole biomass.In the laboratory study, we investigated if extrusion can be used as a feasible pre-treatment method to enhance digestibility, growth, feed utilization and health of the fish. Here, we employed post smolt Atlantic salmon with an initial average weight of 141.8 ± 28.2 g and they were fed one of the below mentioned experimental feeds in five replicate tanks for 84 days. Four low-fishmeal feeds were formulated; a plant-based control feed without the microalga (CTRL), two feeds containing 7.5 (NE7.5) and 15% (NE15) of the pre-extruded microalga and one feed containing 15% of the un-extruded microalga (NN15). In the farm study, fish of average weight 1.83 ± 0.01 kg were reared in pens for 197 days. In this trial, two experimental feeds were used, a control low-fishmeal feed (CT) and a feed containing 7.5% un-extruded microalga (NW).In the laboratory study, all alga-supplemented feeds lowered weight gain, SGR and TGC compared to the control feed. The FCR and FI did not differ between the CTRL and NE7.5 groups but the parameters were significantly poor in fish fed feeds with the highest incorporation of the alga. Likewise, in the farm study feed conversion was not significantly affected by the inclusion of the microalga. The retention of lipid in the post-smolts showed a linear decrease with the incorporation of the microalga in the feed, and the protein retention was significantly reduced only at the highest incorporation level. The content of sum PUFA and EPA + DHA in fish fed microalgae were numerically and significantly higher in the farm and laboratory studies, respectively. The apparent digestibility coefficient (ADC) of protein and lipid in the post-smolts showed an inverse relation to the incorporation level of the microalga. The digestible lipid and energy retention efficiencies improved (p < 0.05) in fish fed pre-extruded microalgae. Expression of antioxidant and immune genes suggested that the microalga did not impart any negative effects on the post-smolts. The farm study indicated the ability of the alga to plausibly shape the intestinal micromorphology and affect the nutrient absorption as well as peristalsis.Based on the laboratory study, we can state that the differences between the pre-treated and untreated microalga diets were only minor suggesting that inclusion level of the alga in plant-based diets had a more significant effect than the pre-treatment, at least at 15% incorporation level. The main conclusion from both the studies is that Nannochloropsis oceanica in the feeds of Atlantic salmon can facilitate a slight increase in deposition of n-3 fatty acids, EPA and DHA.