Single-cell protein meal may support production, survival of rainbow trout

By Aerin Einstein-Curtis contact

- Last updated on GMT

© GettyImages/KaraGrubis
© GettyImages/KaraGrubis

Related tags: Single cell protein, Protein, Fishmeal

Diets supplemented with single-cell protein meal may support farmed trout production, feed efficiency and boost survival rates, says researcher.

A study carried out at the University of Idaho assessed the effects of feeding juvenile rainbow trout KnipBio's single-cell protein (SCP) produced from Methylobacterium extorquens.​ The focus was on fish growth and survival.

The study on the KnipBio meal (KBM) was published in Aquaculture Research​.          

“Results showed that KBM can be included up to 10% replacing soybean meal in a rainbow trout diet without significantly affecting growth performance, feed utilization, nutrient retention or fish health,” ​the researchers said. “A small but statistically significant increase in survival under benign rearing conditions warrants further investigation to assess if KBM increases fish survival under production conditions.”

They said that slightly lower weight gain in fish fed the 10% SCP diet was largely due to lower feed intake, suggesting that adding palatability-enhancing ingredients to feeds may allow higher levels of KBM to be used without compromising fish growth.

KnipBio said it provided the single-cell protein meal used and funding to conduct the study at the University of Idaho. It said the company was involved in designing the study and in manuscript preparation but that it had no role in conducting the study or in data collection, analyses or interpretation. 

The aquaculture industry has responded to the shortage of fishmeal by substituting soybean meal, but has found that a diet based on plant proteins can lead to gut inflammation, disease, and other adverse side effects in numerous commercially important        species, said KnipBio CEO Larry Feinberg.

The industry clearly needs new sources of dietary proteins, he added.

Initial studies with KBM explored its use in fish and shrimp feeds and found that it supports production of Pacific white shrimp and smallmouth grunts, they said. It also has been found to be digestible by both Atlantic salmon and rainbow trout.

“It’s important to publish to communicate the general science and have a level of transparency – there is where we were in 2015, 2017, 2020,” ​said Feinberg. “We’ll continue to publish, but we want to be contributors and give back to the intellectual community.” 


The authors of the KBM paper said bacterial SCPs produced using methane as a feedstock was first evaluated in rainbow trout feeds by Kaushik and Luquet (1980), who reported that up to 80% of fishmeal could be replaced without compromising fish performance. 

They noted, though, that Skrede et al. (1998) and Storebakken et al. (1998) examined digestibility of meal produced from methanotrophic bacteria and reported values slightly lower than those for high-quality fishmeal.

Perera, Carter, and Houlihan (1995) found that bacterial SCP could be included in rainbow trout diets at 17.4%, replacing 25% of the fishmeal, without adverse effects on feed consumption, feed efficiency or growth rate. However, these authors reported that 17.4% dietary bacterial SCP reduced protein efficiency ratio and increased nitrogen excretion, and lowered protein digestibility (85% vs. 79.9%). These effects were attributed, in part, to the nucleic acid content of bacterial SCP. Nucleic acids contribute to dietary nitrogen content but are non-protein nitrogen compounds, they explained.

“It is difficult to draw generic conclusions from these studies, however, as different animals have distinct nutritional and immune responses, and the distinct formulations utilized represent the varied goals of these studies,” ​said the researchers. “Furthermore, there can be substantial differences in the composition of a bacterial SCP depending upon the species being utilized.”

Methylobacterium extorquens​ is a plant epiphyte with the ability to consume methanol and other multi-carbon substrates and organic acids, they said. Additionally, new SCP products may be altered to improve amounts of essential amino acids or levels of biomass constituents.

Trial methodology 

In the KBM feeding trials, the researchers said they gave three diets to 315 juvenile rainbow trout for a period of 12 weeks. All diets included the same amount of fishmeal and similar levels of fish oil.

The diets were formulated using feed software to contain 45% crude protein, 18% lipid, 3.1% lysine and 1% methionine, they said. The diets included a control, that diet with 5% KBM replacing soybean meal (SBM) on a crude protein basis, and a diet with 10% KBM in place of SBM.

Fish were bulk-weighed and counted every three weeks and fish mortality was recorded, they said. At the end of the trial, sample fish were collected at the start and end of the trial for whole-body analysis.

Weight gain, average weight gain, specific growth rate, feed consumption, feed conversion ratio (FCR), survival rate, protein efficiency ratio (PER), nutrient retention and energy retention were calculated, the researchers said.


Overall, fish weights were similar regardless of diet as were feed conversion ratios, found the team. However, feed intake decreased as more KBM was added to the diet.

“Results demonstrate that SCP from M. extorquens is a safe and effective alternative protein for rainbow trout diets at the low inclusion levels tested,”​ they said.

The protein efficiency ratio was slightly smaller (2.42) for the control than the diet with 5% KBM (2.49), they said. No significant differences were found regarding the feed utilization indices, whole-body proximate composition, energy content or nutrient retention values.

Survival was high for fish on all diets, but it was highest for those getting the diet with 10% KBM, they said.

Originally, there was not an expectation that fish receiving a diet with the Knipbio meal would have improved survival rates, said Feinberg. However, that element of the alternative proteins use has now been noted several times.

“We’re seeing that being statistically significant in every trial,”​ he told us. “Taken in the broader context of the knowledge base that we’re gathering, it becomes part of the overall source of information for us.”

“We will be continuing to look at the functional ingredient piece to the protein,” ​added Feinberg. “Not just the protein, but what makes the protein interesting.”

Source: Aquaculture Research


Title: Partial replacement of soybean meal with Methylobacterium extorquens single‐cell protein in feeds for rainbow trout (Oncorhynchus mykiss Walbaum)

Authors: R Hardy, B Patro, C Pujol‐Baxley, C Marx, L Feinberg

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