By: Pallab K. Sarker, Anne R. Kapuscinski, Brandi McKuin, Devin S. Fitzgerald, Hannah M. Nash and Connor Greenwood
Aquafeed manufacturers have reduced, but not fully eliminated fishmeal and fish oil and are seeking cost competitive replacements. This study developed by researchers at University of California Santa Cruz and University of Berkley, combined two commercially available microalgae, to produce a high-performing fish-free feed for Nile tilapia (Oreochromis niloticus). Researchers substituted protein-rich defatted biomass of Nannochloropsis oculata (leftover after oil extraction for nutraceuticals) for fishmeal and whole cells of docosahexaenoic acid (DHA)-rich Schizochytrium sp. as substitute for fish oil.
Feed inputs for aquaculture production represent 40– 75% of costs and are a key market driver for this sector.
The aquafeed market is expected to grow 8–10% per annum and its production of compound feeds is projected to reach 73.15 million MT in Ocean-derived fishmeal (FM) and fish oil (FO) in aqua feeds have raised sustainability concerns as the supply of wild marine forage fish will not meet growing demand and will constrain aquaculture growth.
Moreover, competition for FM and FO from pharmaceuticals, nutraceuticals, and feeds for other animals further exacerbates a supply–demand squeeze and also affects human food security. More than 90 percent of these fish are considered food grade and could be directly consumed by humans, especially food insecure people in developing countries.
Although more prevalent in aquafeeds for high-trophic finfish and crustaceans, FM and FO is also routinely incorporated (inclusion rates of 3–10%) in aquafeeds for low-trophic finfish like tilapia to enhance growth.
Tilapia (dominated by Oreochromis niloticus)—the world’s second top group of aquaculture organisms—is cultured in such large volumes and is such an integral part of human diets across the world, that even low inclusion rates of FMFO in aquafeeds for this species is a substantial portion of global demand of forage fish.
The aquafeed industry reduces reliance on FM and FO by using grain and oilseed crops (e.g., soy, corn, canola), however, terrestrial plant ingredients have low digestibility, anti-nutritional factors, and deficiencies in essential amino acids (lysine, methionine, threonine, and tryptophan).
Crop oils also lack long-chain omega-3s (n-3s), eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA), important for human health.
Elevated levels of n-6 (e.g. linoleic acid) fatty acids from crop oils changes the long-chain n-3/n-6 ratio in tilapia flesh that is passed on to human consumers, resulting in increased production of pro-inflammatory eicosanoids (via arachidonic acid), which has led nutritionists to doubt the health benefits of farmed tilapia.
Alternatives to terrestrial crops have been too costly for broad adoption by aquafeed manufacturers (Sarker et al.). However, nutritional disadvantages and poor fillet quality have prompted researchers to investigate marine microalgae as potential FMFO replacements in fish feeds due to balanced essential amino acids, minerals, vitamins, and long-chain n-3 fatty acids.
The peer-reviewed literature, however, lacks information on how using marine microalgae in fish-free diets affects growth, feed conversion and fillet quality of tilapia. There also are limited published data on the market price of fish-free diets made with alternative ingredients that show potential for economies of scale.
Materials and methods
This research was conducted to develop a new aquafeed formula by combining the protein-rich (50%) defatted marine microalgal co-products (under-utilized left-over biomass of Nannochloropsis oculata after EPA oil extraction for human supplement) with another DHA-rich (30% of total fatty acids) marine microalga (Schizochytrium sp.), increasingly available at commercial scale, to fully replace FMFO (fish-free) in tilapia aquafeeds.
“This study builds on recent microalgae aquafeeds research (Sarker et al.) where 33% of FM was replaced with under-utilized N. oculata defatted biomass in a tilapia diet that achieved final weight, weight gain, percent weight gain, specific growth rate, and protein efficiency ratio values comparable to the reference diet containing FM and FO.”
Furthermore, it was previously reported that Schizochytrium sp. is a highly digestible source of nutrients for tilapia and can fully replace FO in tilapia feed. To examine the commercial viability of using marine microalgae to replace both FM and FO, the researchers conducted a nutritional feeding experiment to compare three microalgal diets to a reference diet containing FM and FO levels found in commercial tilapia feed.
Microalgal diets included defatted N. oculata to replace 33%, 66% or 100% of FM, and whole cell Schizochytrium sp. to replace 100% of FO (33NS, 66NS, 100NS), see Table 1.
The effects of the four diets were measured on growth metrics, in vitro protein digestibility, feed conversion ratio (FCR), protein efficiency ratio (PER), and fillet deposition of n-3 long-chain polyunsaturated fatty acids (LC PUFAs) and minerals. Furthermore, a hedonic analysis was conducted to estimate the market price of defatted N. oculata meal and whole cell Schizochytrium sp., feed costs, and the economic feed conversion ratio (ECR).
Growth, nutrient utilization and proximate composition of tilapia carcass
Fish that were fed the fish free diet for 184 days displayed significantly better (p < 0.05) final weight, weight gain, percent weight gain and specific growth rate than the fish fed the reference diet, which contained FM and FO levels typically found in commercial tilapia diets (see Table 2).
Growth rates were linear throughout the experiment and weights measured for the fish-free diet diverged from those for the reference diet by day 128. Tilapia fed fish-free feed showed an improved food conversion ratio and protein use efficiency ratio though differences among diets and were not statistically significant.
We detected no difference in survival rate among all diets and all fish appeared healthy (no visual signs of illness or deformities) at the end of the experiment. The whole-body proximate composition did not significantly differ across the dietary treatments; lipid contents ranged from 2 to 5% and protein contents ranged from 13 to 17% across the four treatments.
These results demonstrate the feasibility of combining commercially available microalgal biomasses to formulate fish-free aquaculture feeds that are high-performing and show potential to become cost-competitive.
This is the first report of successfully combining protein-rich-defatted biomass of one microalgal species with DHArich whole-cell biomass of another microalgal species to achieve full replacement of FM and FO ingredients in a tilapia feed formulation.
This also is the first report of improved feed utilization metrics, including growth, weight gain, specific growth rate, and of beneficial DHA fatty acid profile in Nile tilapia fed a fish-free microalgal diet compared to a commercial feed formulation containing FM and FO.
Production is increasing for both types of microalgal biomass used in the fish-free diet, indicating good potential to achieve economies of scale. This estimate of the ECR for the fish-free diet supports the proposition that biomass from these microalgae will inevitably become cost competitive with FM and FO commodities.
Editor’s note: this is a summarized version; too see full results of all the metrics analyzed by this study, please access the full version of the article, link available at the end of this content.
Nutritional benefit of combining N. oculata defatted biomass and Schizochytrium in the fish-free diet
The combination of Schizochytrium sp. and defatted biomass of N. oculata in the fish-free feed exhibited two major benefits. First, fish fed the fish-free feed had improved growth consistent with prior observations that Schizochytrium sp. is a highly digestible ingredient for tilapia and that elevated levels of Schizochytirum sp. led to improved growth, FCR, and PER.
Second, researchers found the highest in-vitro protein digestibility in the fish-free feed, suggesting that protein originating from defatted N. oculata biomass was the most digestible when in the presence of highly digestible Schizochytrium sp., presumably due to the latter triggering certain digestive enzymes, release and activity.
Thus, the combination of defatted N. oculata biomass and Schizochytrium sp. appears to be better suited to the digestive enzymes present in tilapia digestive systems than conventional diets with FMFO; and the presence of Schizochytrium sp. may support more efficient digestion of the fish free-feed at the higher inclusion levels of N. oculata defatted biomass.
However, further research is necessary to elucidate the digestive enzyme profiles present under different dietary regimes and to assess the differences in the digestibility of microalgal fish-free feeds compared to conventional feed with FMFO.
Impacts of fish-free diet on macrominerals and trace elements
The literature has little data on the elemental composition of microalgae; and this study found that most of the essential macrominerals and trace elements were at higher levels in N. oculata defatted biomass and Scizochytrium sp whole cells (see Table 3) than in conventional terrestrial feed ingredients.
Feed conversion ratio (FCR) considerations FCR is a key driver of farming efficiency, economic and environmental performance. Improving the FCR of farmed tilapia through improved feed technology would help increase the cost effectiveness of fish-free diets.
Tilapia farming can further reduce the FCR close to 1:1 by a variety of means including better feed formulations using highly digestible feed ingredients, use of appropriate pellet size for each life stage, and better on-farm feed management practices (e.g., storage and feeding rates).
Extruded sinking pelleted feed could improve overall FCR; moreover, extrusion or enzymatic processing of under-utilized, defatted biomass of microalgae, such as N. oculata used in this study, could further improve the FCR of fish-free feed, and also help push feed formulated with microalgae towards being cost-competitive with conventional feed.
Economic analysis of fish-free feed formulated with microalgae blends
Authors of the study compared the estimated ingredient prices, the formulated feed prices and the ECR across experimental diets formulated with microalgae blends and the reference diet (see Table 4).
This estimate of the market price of defatted N. oculata meal is in good agreement with another study that used hedonic methods to estimate the of market price of defatted N. oculata meal.
The similar estimated costs of the fish-free feed (100NS) and reference diet suggest that using combinations of microalgal biomass, that are on track to achieve economies of scale, is a feasible strategy for achieving large-scale production of cost-competitive fish-free diets.
An emerging path to economies of scale for the two microalgae used in this study is a biorefinery business model whereby oil rich fractions of the microalgal biomass are marketed as high-value products, such as omega-3 rich human supplements, and other fractions as lower-priced feed ingredients. N. oculata contains an appreciable amount of the omega-3 fatty acid, EPA.
The projected global growth of over 14% in omega-3 fatty acids from microalgae in the near future will result in a large supply of defatted biomass. Furthermore, the production of Schizochytrium sp., already at commercial-scale, is also anticipated to grow, as the projected compound annual growth rate of DHA from microalgae sources is expected to exceed 10% in the near future.
In order for such high-performing fish-free feed for tilapia to succeed in the market, authors acknowledge that Schizochytrium sp. needs to become cost-competitive with FO sources for aquaculture feeds. Analysts predict ongoing technological improvements and R&D efforts to produce Schizochytrium sp. will quickly make it a cost competitive substitute for FO due to lower production costs and higher market availability.
These results provide a framework for the development of fish-free feeds and the first evidence of a high performing feed for tilapia that combines two different marine microalgae. Defatted marine microalgae, a protein-rich biomass left over after extracting oil for other products, is currently under-utilized (often creating disposal problems even though it is food-grade), and is increasingly available as the algal-oil nutraceutical market grows.
Advancing the use of microalgal defatted biomass in aquafeeds would improve the sustainability of aquaculture by reducing its reliance on FM extracted from forage fisheries.
Combining under-utilized defatted biomass protein with DHA-rich marine microalga in the fish-free feed
resulted in better tilapia growth compared with fish fed a conventional diet containing FMFO. Furthermore, tilapia fed the fish-free feed yielded the highest amount of DHA in the fillet, almost twice higher than in those fed conventional feed.
Thus, feeding a DH Arich, microalgae blended diet to farmed tilapia is a practical way to improve human health benefits of eating farmed tilapia. Moreover, these results suggest other kinds of microalgae combinations are possible and worthy of future investigation.
This fish-free formulation also shows potential cost-competitiveness, given that the ECR of the fish-free diet was slightly lower, though not significantly different, than the reference diet.
The microalgal ingredients in this fish-free feed, thus, show potential to supply the expanding aquaculture industry with a stable and affordable supply of healthy protein and oil for fish-free feed, doing so without causing harm to oceans or food security of resource-poor people.
*This is a summarized version developed by the editorial team of Aquaculture Magazine of the article “Microalgae-blend tilapia feed eliminates fishmeal and fish oil, improves growth, and is cost viable” written by: Pallab K. Sarker, Anne R. Kapuscinski, Brandi McKuin, Devin S. Fitzgerald, Hannah M. Nash and Connor Greenwood, that was originally published on the Nature Scientific Reports online journal (2020) 10:19328 and its full version can be found at: https://doi.org/10.1038/s41598-020-75289-x The article was published under a Creative Commons Attribution 4.0 International License. References cited by the authors within the text are available under previous request to our editorial area.