Abstract
Aquaculture, the fastest growing food sector, is expected to expand to produce an additional 30 million metric tons of fish by 2030, thus filling the gap in supplies of seafood for humans. Salmonids aquaculture exploits the vast majority of fishmeal and fish oil rendered from ocean-dwelling forage fish. Most forage fish diverted to these commodities are human-food grade, and all are primary prey for marine predators. Rising costs, price volatility, and environmental sustainability concerns of using these commodities for aquaculture feed are driving the global search for alternatives, including marine microalgae originating from the base of marine food webs but produced in culture. We report the first evaluation of two marine microalgae, Nannochloropsis sp. and Isochrysis sp., for their potential to fully replace fishmeal and fish oil in diets of rainbow trout (Oncorhynchus mykiss), an important model for all salmonid aquaculture. We conducted a digestibility experiment with dried whole cells of Nannochloropsis sp. and Isochrysis sp., followed by a growth experiment using feeds with different combinations of Nannochloropsis sp., Isochrysis sp., and Schizochytrium sp. We found that digestibilities of crude protein, crude lipid, amino acids, fatty acids, omega 3 polyunsaturated fatty acids (n3 PUFA), docosahexaenoic acid (DHA), n6 (omega 6) PUFA in Isochrysis sp. were significantly higher than those in Nannochloropsis sp. Digestibility results suggest that for rainbow trout diets Isochrysis sp. is a better substitute for fishmeal and fish oil than Nannochloropsis sp. The lower feed intake by fish fed diets combining multiple microalgae, compared to fish fed the reference diet, was a primary cause of the growth retardation. In trout fillets, we detected an equal amount of DHA in fish fed fish-free diet and reference diet. This study suggests that Isochrysis sp. and Schizochytrium sp. are good candidates for DHA supplementation in trout diet formulations.
Highlights
Aquaculture is the world’s fastest growing food sector (FAO, 2018), and its global activities signal a major change in humanity’s relationship with the ocean (Duarte et al, 2009)
We found the highest apparent digestibility coefficients (ADCs) of energy in the reference diet (81.0%), which was not significantly different from that in the Isochrysis sp. diet (78.0%), and the lowest in the Nannochloropsis sp. diet (75.2%)
We found that the ADCs of crude protein (87%) and four essential amino acids, isoleucine (63%), threonine (67%), tryptophan (12%), and valine (59%), in the Nannochloropsis sp. were much lower (P < 0.01) than their ADCs in the Isochrysis sp. ingredient for rainbow trout
Summary
Aquaculture is the world’s fastest growing food sector (FAO, 2018), and its global activities signal a major change in humanity’s relationship with the ocean (Duarte et al, 2009). Aquaculture is expected to expand to fill this gap and supply two-thirds of global fish consumption by 2030 (World Bank, 2013; FAO, 2018; Plagányi, 2019). Mariculture production (in marine and coastal waters or saltwater tanks on land) is led by aquatic plants (macroalgae) and unfed filter-feeding molluscs, followed by finfish (mostly salmonids), and shrimp and prawns (FAO, 2016; Clavelle et al, 2019). Mariculture comprised 36% of aquaculture (26.7 million t) and 18.25% of all edible seafood in 2014 (FAO, 2016) and is projected to increase (FAO, 2018). Underpinning these trends is a historic shift in what farmed fish are fed, away from extensive aquaculture
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