Towards sustainable and ocean-friendly aquafeeds: Evaluating a fish-free feed for rainbow trout (Oncorhynchus mykiss) using three marine microalgae species

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.

The Inclusion of the Microalga Scenedesmus sp. in Diets for Rainbow Trout, Onchorhynchus mykiss, Juveniles

“…replacing 15 and 30% of FM [19]. Although comparing feed efficiency parameters among different studies may lack in veracity due to differences in feed formulation, feeding practices and rearing conditions, FCR values recorded in the present trial were within the range of values found in other studies dealing with rainbow trout [71][72][73]. Under present experimental conditions, neither FCR nor FI were affected by the inclusion of Scenedesmus sp.…”

“…(2.4%), impaired growth performance in rainbow trout. The former results were associated to a reduction in feed intake due to changes in diet palatability [71], results that also contributed to poorer FCR values. These findings were similar to those found in Atlantic cod (Gadus morhua) fed a combination of Nannochloropsis and Isochrysis spp.…”

“…Under this context, the authors decided to test the suitably of Scenedesmus sp. in diets for juveniles of rainbow trout considering the promising results obtained in gilthead seabream (Sparus aurata) [18], Atlantic salmon [35], and several freshwater species [28], including rainbow trout [71].…”

The Application of Single-Cell Ingredients in Aquaculture Feeds—A Review

“…In many microalgal SCP resources, problems have been encountered with protein digestibility, with various studies focusing on different methods of processing the microalgae to improve the digestibility (Table 4). While protein digestibility values as high as 99% have been reported, most are usually somewhat lower (54-87%; mean ± standard deviation (SD) = 76 ± 14%, n = 18) [15,36,[99][100][101]. Significant differences have also been reported between different varieties of microalgal SCP, with an Isochrysis SCP (87%) having a much better protein digestibility than a Nannochloropsis SCP (69%/73%), which was better than a Desmodesmus SCP (54%) [99,101].…”

“…While protein digestibility values as high as 99% have been reported, most are usually somewhat lower (54-87%; mean ± standard deviation (SD) = 76 ± 14%, n = 18) [15,36,[99][100][101]. Significant differences have also been reported between different varieties of microalgal SCP, with an Isochrysis SCP (87%) having a much better protein digestibility than a Nannochloropsis SCP (69%/73%), which was better than a Desmodesmus SCP (54%) [99,101]. In a study examining the use of Nannochloropsis, Phaeodactylum and Isochrysis SCPs fed to Mink (Mustela vison), Skrede et al [35] found that inclusion of any of the three SCP (included at 0, 60, 120 or 240 g/kg) had a negative effect on protein digestibility, but the effect with Phaeodactylum was somewhat less than the other two SCPs.…”