Sustainable feed for Nile tilapia
Despite significant progress over the years, aquaculture still poses concerns, such as over-reliance on wild fish and terrestrial plants such as soy. Now a US team of scientists has created an alternative feed for Nile tilapia using a marine microalga co-product, reports Bonnie Waycott.
In 2014, aquaculture production in the total food supply overtook global capture fisheries production for the first time. Having surpassed wild fishing as the primary source of protein from the ocean, hopes are high for the world's fastest growing food sector. But although more and more fish are being raised on farms, their feed consists of species such as anchovies or sardines, which applies pressure to stocks in the wild, leading to volatile supplies while competition for such species is making fishmeal and fish oil increasingly expensive.
To address this, considerable efforts are being made in aquaculture to reduce the industry's dependence on wild-caught feed. One example can be found in the US, where scientists at Dartmouth College are working with algae meal to replace fishmeal and fish oil in Nile tilapia diets.
"If feed is going to become more sustainable, we must find alternatives to fishmeal and fish oil because of the environmental, food security and financial drawbacks of these ingredients," said Professor Pallab Sarker, a research assistant professor at Dartmouth's Environmental Studies Program. "Aquaculture is the largest consumer of fishmeal and fish oil, using 70% of global supplies. Current levels of exploitation, and the diversion of forage fisheries to produce fishmeal and fish oil undermine human food security and marine biodiversity. Terrestrial plant ingredients are useful but highly unsustainable and insufficient to achieve nutritionally complete diet formulations. This is why we are working with marine microalgae."
Essential for growth and quality
Professor Sarker and his team focused on Nannochloropsis oculata (N.oculata), a type of marine microalgae that is currently grown commercially. Oils extracted from it are used in products such as nutraceuticals and biofuels. After extraction, what remains is algal meal (co-product) that's rich in protein and omega-3 fatty acids. One of those, eicosapentaenoic acid (EPA), is said to be essential to fish growth and quality.
"Marine microalgae offers several advantages over terrestrial crops, such as improved land and water-use efficiency, better environmental sustainability and it can produce many times more biomass per unit area than terrestrial crop plants. It has tremendous potential for the lowest combined carbon, freshwater and arable land footprints," said Professor Sarker. "The large amount of DHA or EPA (omega-3 fatty acids) in microalgae can prevent a range of human diseases, while microalgae may soon become a cost competitive substitute for fishmeal and fish oil because of technological improvements that lower microalgae production costs."
To conduct their experiments, the scientists determined the nutrient digestibility and levels of nutrients and anti-nutrients present in dried whole cells of N.oculata and co-product. Then, standard tilapia feeds were formulated where co-product replaced 0%, 33%, 66% and 100% of fishmeal respectively. For the first three weeks of the experiment, the diets were administered at a rate of 10% of body weight each day, followed by 8% of body weight until the 6th week, 6% until the 9th week and 4% until the 12th week. The effects of replacing varying percentages of fishmeal with the N.oculata co-product were determined by quantifying the final weight of the fish, weight gain, weight gain percentage, feed conversion ratio (FCR), specific growth rate (SGR), protein efficiency ratio (PER) and survival rate.
Results showed that the N.oculata co-product had higher protein content than the whole cells. The co-product was also highly nutrient-dense, offering the highest digestibility of EPA and lysine. However, digestibility values for crude protein in the N.oculata co-product were significantly lower than the whole cells. The results also showed a similar growth, feed conversion and survival rate between tilapia fed a diet replacing 33% of fishmeal with Nannochloropsis co-product and tilapia fed a conventional diet. Increasing the level of N.oculata co-product lowered these fish performance levels, but did not depress other factors such as feed intake and palatability.
"We detected good growth, feed conversion and survival of tilapia that were given a diet replacing 33% of fishmeal with N.oculata co-product. But fish performance was less effective the higher the level of co-product," said Professor Sarker. "This means that we may need to improve nutrient digestibility by enhancing the co-product with one or several enzymes."
The Dartmouth study is the first report into the effects of a microalga co-product in tilapia feed. It suggests that lower trophic level aquaculture species such as tilapia, carp or shrimp may perform better with higher levels of co-product in their feed compared to higher trophic species such as salmon or red drum, and emphasizes the need for further information on anti-nutrients in microalga co-products, their impacts on aquaculture feed and what needs to be done to improve nutrient digestibility. Professor Sarker and his team believe that developing algal ingredients that can be digested easily will have a positive impact on feed conversion ratios.
"The possibilities are exciting, as we now have an opportunity to move aquaculture feed away from fish-based ingredients and towards fish-free feeds based on marine microalgae and co-products. Our findings provide new insight into how we might be able to achieve this," Professor Sarker commented.
As part of the team's aim to eliminate the reliance of aquaculture on wild fish and terrestrial crops, a new project is underway to combine N.oculata co-product with other types of marine microalgae. The team is also trying to find out whether adding one or more non-starch polysaccharide and protease enzymes to N.oculata co-product diets will enhance nutrient digestibility, retention and growth in Nile tilapia.
"In future, microalga co-products will need to become cost-competitive with conventional protein sources for aquaculture feed. Only then will the feed industry use them," said Professor Sarker. "Fortunately, there are some leading microalgae companies that have innovative business models to meet this challenge, such as multi-product bio refinery approaches. I believe there is a lot of opportunity in microalgae, and it will take us far."
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