Essential oils shown to improve trout performance

New research has demonstrated that rainbow trout fed a diet supplemented with thyme essential oil had improved growth, weight gain, immune response and disease resistance.

Essential oil supplementation could increase the non-medical treatment options available in aquaculture. Photo © Emma Macaskill, Kames Fish Farming

According to a new publication in Aquaculture Reserach, researchers have found that essential oil extracted from thyme (Thymus vulgaris) could be a beneficial dietary supplement for rainbow trout (Oncorhynchus mykiss). The researchers conducted a feed trial over two months with fish that initially weighed 10g. The fish were fed diets containing different concentrations of the essential oil: 0.5ml/kg feed, 1ml/kg feed and 2ml/kg feed.

Rainbow trout fry

Results showed that fish fed the 0.5ml/kg diet had a better specific growth rate and weight gain when compared to other treatments. Fish on the 1ml/kg diet showed higher levels of complement component 3 (C3), a protein that plays a vital role in the immune system. They also had more CD4 gene expression – meaning that their immune systems were able to fight infections more quickly than the control group.

Though these results were promising, researchers noted that during supplementation, fish fed the experimental diets showed fluctuating Alanine aminotransferase levels. This protein is used as an indicator of liver health. Fluctuations and increased levels of the compound are often associated with decreases in liver function. Though researchers monitored the trout’s liver function during the trial, they were unable to determine if the supplementation had a lasting negative impact.

As a final component of the trial, the researchers tested the survival rate of the fish after they were exposed to a common aquatic pathogen. The researchers challenged the trout with Aeromonas hydrophila, a bacterium that causes opportunistic infections and Aeromonas septicaemia in fish and amphibians. After exposure, fish fed the 0.5ml/kg diet showed the highest survival rate.

Overall, the researchers concluded that supplementing feed with thyme essential oil enhanced growth, immune responses and disease resistance in trout. They recommended using the 0.5ml/kg dosage to get the most benefits while reducing negative side-effects in liver function.