Use of microalgae in fish nutrition can relieve pressure on wild fish stocks, but there is no systematic quantitative evaluation of microalgae benefits. We conducted a metanalysis on the nutritional benefits of Spirulina and Schizochytrium as replacements of fishmeal and fish or plant oil, respectively. We reviewed 50 peer-reviewed studies involving 26 finfish species and 144 control vs microalgae replacement comparisons. Inclusion of Spirulina in the fish diet significantly improved growth compared to controls (SMD = 1.21; 95% CI 0.71–1.70), while inclusion of Schizochytrium maintained the content of omega-3 PUFA of the fish fillet compared to fish fed on fish or plant oils (SMD = 0.62; 95% CI − 0.51–1.76). Benefits were apparent at replacement levels as low as 0.025% in the case of Spirulina and 10% in the case of Schizochytrium oil. Dose-dependent effects were found for Spirulina replacement on growth, but not for Schizochytrium on omega-3 fillet content. Subgroup analysis and meta-regression revealed that ~ 24–27% of variation in effect sizes can be accounted by variation between fish families, the rest likely reflecting variation in experimental conditions. Overall, the evidence indicates that Spirulina and Schizochytrium replacement in aquafeeds can be used to improve fish growth and maintain fillet quality, respectively, but considerable uncertainty exists on the predicted responses. To reduce uncertainty and facilitate the transition towards more sustainable aquafeeds, we recommend that feeding trials using microalgae are conducted under commercially relevant conditions and that greater care is taken to report full results to account for sources of heterogeneity.

Microalgae offer a sustainable source of omega-3 fatty acids that can replace fish oil in aquafeeds, but the nutritional benefits are not always clear, particularly when microalgae are used as complete oil replacements in starter feeds. We compared the survival, growth, omega-3 deposition and composition of the gut microbiota of Nile tilapia fed with aquafeeds that differed in dietary oil, from plant, fish and microalgae (Schizochytrium) origins. Survival was not affected by diet, but fish fed a diet where the entire oil component (5%) was replaced by microalgae oil grew twice as fast than fish fed plant oil or a mixture of plant and fish oil. Dietary omega-3 content was strongly correlated with omega-3 deposition in the fish fillet. Complete replacement of fish oil by plant oil caused a significant increase in the abundance of Aeromonadaceae which is often associated with an inflammatory response in the fish gut. In contrast, when fish oil was replaced by microalgae oil an increase in Peptostreptococcaceae and Mycobacteriaceae was observed. Our study indicates that Schizochytrium oil can be used to improve the growth of Nile tilapia and increase its omega-3 content without any of the detrimental effects on the gut microbiome typically associated with some plant oil replacements.

Use of microalgae in fish nutrition can relieve pressure on wild fish stocks, but there is no systematic quantitative evaluation of microalgae benefits. We conducted a metanalysis on the nutritional benefits of Spirulina and Schizochytrium as replacements of fishmeal and fish or plant oil, respectively. We reviewed 50 peer-reviewed studies involving 26 finfish species and 144 control vs microalgae replacement comparisons. Inclusion of Spirulina in the fish diet significantly improved growth compared to controls (SMD = 1.21; 95%CI = 0.71–1.70), while inclusion of Schizochytrium maintained the content of omega-3 PUFA of the fish fillet compared to fish fed on fish or plant oils (SMD = 0.62; 95%CI = -0.51-1.76). Benefits were apparent at replacement levels as low as 0.025% in the case of Spirulina and 10% in the case of Schizochytrium oil. Dose-dependent effects were found for Spirulina replacement on growth, but not for Schizochytrium on omega-3 fillet content. Subgroup analysis and meta-regression revealed that ~ 24–27% of variation in effect sizes can be accounted by variation between fish families, the rest likely reflecting variation in experimental conditions. Overall, the evidence indicates that Spirulina and Schizochytrium replacement in aquafeeds can be used to improve fish growth and maintain fillet quality, respectively, but considerable uncertainty exists on the predicted responses. To reduce uncertainty and facilitate the transition towards more sustainable aquafeeds, we recommend that feeding trials using microalgae are conducted under commercially relevant conditions and that greater care is taken to report full results to account for sources of heterogeneity.