Pallab Sarker’s research team at Dartmouth University published the results of an initial digestibility and growth study examining the use of a coproduct generated from the microalgae Nannochloropsis oculata as a fishmeal substitute in tilapia feed.
“Previously, we discovered that fish oil can be replaced 100% by a microalgae,” he told FeedNavigator. “We achieved the success to replace fish oil, now we have to think about if fishmeal can be replaced with microalgae – we tried to replace fish oil by another microalga Schizochytrium sp. but in that case, we replaced not whole cells of microalgae but with a co-product – what is left after oil extraction.”
The initial study found that the coproduct was able to replace about 33% of the fishmeal while maintaining growth and fish performance, said Sarker. “But, if you want to [replace] higher level 80%-100%, then we need to treat it with enzymes to break down to the anti-nutritional elements,” he added.
Using microalgae in tilapia diets
The examination of microalgae use in aquaculture diets is part of Sarker’s ongoing interest in improving the sustainability of the aquaculture industry.
“This is my passion for aquaculture feed – to make it more sustainable,” he said. “To make the diet more sustainable and [we are] innovating high impact solutions to global challenges because overfishing of smaller fish to make fishmeal and fish oil cause depletion of limited resources, so it’s urgently needed to stop that activity.”
Using an alternative to fishmeal or fish oil in aquaculture feeds could relieve pressure on fish populations, he said, adding “this is the main driver of my research, and globally we’re playing a role in sustainable aquaculture feed research.”
The specific strain of microalgae explored for use in the study is used by companies that process microalgae to produce products like omega-3 nutraceuticals for humans, ink and oils for biofuel production, he said. The bio-refineries also generate large amounts of the microalgae coproduct that could be an inexpensive ingredient for feed use.
“Even though they extract the omega-3, still it’s EPA rich, and the coproduct is great for tilapia,” he said. Selling the coproduct provides an additional revenue source for the refineries and a less expensive algae ingredient supply for feed use, he added.
Additionally, as aquaculture production has increased and aquafeed producers have reduced the levels of fishmeal and fish oils used in diets, they have incorporated ingredients derived from terrestrial crops, the researchers said. However, ingredients like soybean meal or cornmeal may be missing some needed nutritional elements like lysine or methionine and have been linked to increased excretion of phosphorous.
Use of terrestrial crops also brings concerns about the diversion of products that humans could use directly to make animal feed, they said. Marine microalgae could be an alternative ingredient for aquafeed and play a role in environmental sustainability.
Exploring digestibility and fish growth
In the digestibility trial, the research team examined the use of three different diets. A control and that diet mixed with whole cells of the microalgae or the coproduct at a 7:3 ratio.
Tilapia were given one of the three diets for a 30-day period, they said. Fecal matter was collected twice a day during the trial and analyzed at the end of the time window.
In the feeding trial, four trial diets were developed by exchanging different levels of fishmeal with the microalgae coproduct at 0%, 33%, 66% and 100%, they said. The 600 tilapia received the diets for 12 weeks.
The group looked at performance indicators including weight, specific growth rate and weight gain, said Sarker.
Fish were weighed and sampled at the start and end of the feeding trial, the researchers said. The feed conversion ratio, protein efficiency ratio and survival rate were calculated and whole body proximate composition, amino acid and fatty acids in fish samples were determined.
Results and looking forward
The coproduct was found to have a higher percentage of protein, and lysine, said Sarker.
It also had a larger amount of the omega-3 fatty acid EPA, he said. “We thought it would be a good candidate for EPA supplementation in tilapia fish promotion,” he added.
However, the crude protein digestibility was higher in the whole cell microalgae than the coproduct, the researchers said. There were no differences in the lipid apparent digestibility coefficients (ADC) values, although some amino acid levels were lower in the coproduct, several including arginine, histidine, lysine, leucine and phenylalanine were higher.
In the feeding trial, final weight, weight gain, percent weight gain, specific growth rate and protein efficiency ratio were similar for the control diet and the diet with 33% of the coproduct, said the researchers. Feed conversion ratios, feed intake and whole body proximate composition of tilapia fillets were similar for all of the diets and survival rates were high for all groups.
The initial findings were exciting, said Sarker. However, it is thought that anti-nutritional elements in the coproduct meal may be preventing its use at higher levels.
The group posited in the study that use of enzymes could improve digestibility of the coproduct or that extruding the fish feed would help.
Some initial in vitro work looking at the use of enzymes with the coproduct has been promising, but no details of the project have been published at this time, added Sarker.
The work also builds on previous research published in 2016, looking at the use of microalgae as a possible replacement for fish oil in aquaculture diets, he said.
A future step for the development process could include combining the coproduct with other microalga whole cells for fishmeal and fish oil replacement, he said.
“This paper that was published, it is a stage for the next research,” he added.
Source: PLOS ONE
Title: Towards sustainable aquafeeds: Evaluating substitution of fishmeal with lipid-extracted microalgal co-product (Nannochloropsis oculata) in diets of juvenile Nile tilapia (Oreochromis niloticus)
Authors: Pallab Sarker, Anne Kapuscinski, Ashley Bae, Emily Donaldson, Alexander Sitek, Devin Fitzgerald, Oliver Edelson
DOI: doi.org/10.1371/journal.pone.0201315
