A team of US researchers working out of New Hampshire-based Dartmouth College explored the use of marine microalgae Schizochytrium (Sc) as a potential replacement for fish oil in diets for farmed tilapia.
The group was trying to find the levels at which fish oil could be substituted with a DHA-rich microalgae ingredient in Nile tilapia diets, said corresponding researcher Pallab Sarker, research assistant professor in the department of environmental studies at Dartmouth.
“We first discovered that it is a quality fish oil substitute in tilapia feed, and that it really [could] solve the question of overfishing of small fishes,” Sarker told FeedNavigator.
The team found that completely replacing dietary levels of fish oil with the marine microalgae improved feed efficiency, weight gain and feed intake and boosted levels of polyunsaturated fatty acids (PUFA) amounts in tilapia fillets.
“In our study, the complete substitution led to omega three [levels] being higher and, when compared to omega 6 [amounts,] the ratio is really good,” said Sarker. The amount of omega-3s in the tilapia fillets also could have benefits for human consumption, he added.
The group was looking at algae as a way to find a sustainable and non-environmentally damaging replacement to the use of fishmeal and fish oil, which is generated from a fish and has a finite supply.
Tilapia is one of the most commonly raised fish in the world, said Sarker. Finding a way to add levels of omega-3 and omega-6 fatty acids to the fish could have benefits in addressing nutritional needs in developing countries where it is raised, he added.
The group picked the strain of microalgae to study after a series of experiments on the digestibility of different algae types, said Sarker. “The digestibility is really basic information that we need to develop for a replacement study,” he added.
The microalgae was found to have better digestibility than several strains of freshwater microalgae and that the absorption of omega-3 fatty acids also was high, he said.
Long chain polyunsaturated fatty acids, like DHA and EPA, are important for fish health, the researchers said in the study. And, they also offer neurological and cardiovascular benefits for humans.
However, how fish would respond, in terms of growth and production, to the inclusion of the specific strain of microalgae in their diet was unknown, they said.
“We showed that DHA is highly digestible in tilapia,” said Sarker. “We were really excited that they can digest it pretty well – that led us to formulate fish oil replacement in tilapia [feed].”
In the experiment, 600 fish in 15 tanks were given one of five diets for a period of 84 days, said the researchers. Trial diets included a control with fish oil (Sc0) and four with varying levels of fish oil and dried, whole-cell microalgae.
The replacement amounts were 25% microalgae instead of fishmeal (Sc25), 50% (Sc50), 75% (Sc75) and 100% (Sc100), they said. The microalgae strain used was a commercial product.
Fish were weighed at the start of the trial, every three weeks and at the end, they said. Weight gain was noted and the feed conversion ratio (FCR) specific growth rate (SGR), protein efficiency ratio (PER) and percent survival were calculated.
Additionally, five fish per tank were sampled on days 42 and 84 and tested for fatty acid composition, said the researchers in the report. And, fish were collected at the end of the experiment for whole-body composition.
The group found that fish getting the Sc100 diet had the highest weight gain and PER, along with an improved FCR and feed intake, said the researchers in the report. There were no differences in SGR or survival rate for any of the diets.
There was a linear relationship between amount of microalgae in the diet and weight gain and FCR, they said. But, whole body proximate composition did not differ by diet.
Saturated fatty acid fractions were higher in the middle of the experiment than at the end and, most did not alter among diets, they said. But, total saturated fatty acids were highest in fish getting the Sc100 diet.
Fish getting the Sc0 diet had the highest levels of monounsaturated fatty acids, said researchers. PUFA levels varied for diets, with most n6 fatty acids higher at the end of the experiment and, the largest amount was found in the Sc0 diet.
But, the largest amount of n3 fatty acids, including DHA and EPA, was found at the end of the experiment in the Sc100 diet, they said. The amounts of omega-3 fatty acids transferred from the diet to the fish were a surprise, Sarker added.
The group now is looking at ways to scale up algae production using waste water and an examination of microalgae coproduct that could be mixed with the whole-cell plant, said Sarker. “To have it succeed, we have to cut the high production cost of microalgae,” he added.
“Our next goal is to replace all fish meal and fish oil in tilapia diet using a combination of the different microalgaes,” he said. “That’s the goal of improving the social sustainability of aquaculture.”
An upcoming project will look at the potential of microalgae to reduce or replace the amount of fish oil and fishmeal needed in the diet of rainbow trout.
Source: Plos One
Title: Towards Sustainable Aquafeeds: Complete Substitution of Fish Oil with Marine Microalga Schizochytrium sp. Improves Growth and Fatty Acid Deposition in Juvenile Nile Tilapia (Oreochromis niloticus)
Authors: Pallab Sarker , Anne Kapuscinski, Alison Lanois, Erin Livesey, Katie Bernhard, Mariah Coley