New case study: understanding growth failure in Rainbow trout

Identifying the cause of failed fish syndrome in Rainbow trout reared in seawater

In aquaculture, the term ‘failed fish’ refers to fish that grow much more slowly than expected. This issue affects around 10% of farmed salmon, but in seawater-reared Rainbow trout, failure rates can exceed 30%. For farmers, this is a serious challenge but despite its impact, the reasons behind it are not well understood.

Rainbow trout naturally thrive in cooler, oxygen-rich waters, but in Scotland, limited sites mean many farms operate in full seawater. This environment appears to increase failure rates.

This project aimed to identify the causes of failed fish syndrome and explore nutritional solutions. Valued at almost £220k the partners were Kames Fish Farming Ltd and the University of the West of Scotland (UWS), with supported from DawnFresh.

The research involved two experiments. First, 500 trout were tagged and monitored during their transfer from freshwater to seawater. Blood and tissue samples revealed that failed fish showed signs of liver damage and metabolic stress, as well as eye tissue erosion, possibly affecting feeding ability. These findings suggest that seawater transfer can trigger physiological problems that hinder growth.

The second experiment tested whether diet could reduce failure rates. Fish were fed different marine-based diets before and after seawater transfer. Recovery diets enriched with fish oil and mussel extract significantly reduced failure rates, from over 40% to below 20% in some cases. However, initial fish size did not predict failure, and while some diets performed slightly better, differences were small.

Key insights:

• Failure is linked to stress during seawater transfer.
• Blood biomarkers show promise for early detection.
• Nutrition can help reduce failure rates, but further research is needed.

This study highlights the complexity of fish failure and the need for tailored husbandry and feeding strategies. Follow-on research is underway to explore genetic and proteomic factors, aiming to improve fish health, reduce waste, and boost productivity in trout farming.

Read the case study