The first-ever study to investigate the effects of cannabis on transported exotic fish has found that a specific dose of cannabidiol (CBD) can significantly reduce stress levels and stress-related behaviours in fish during transport.
Researchers from the School of Health & Life Sciences at the University of the West of Scotland, in partnership with Waltham Petcare Science Institute – part of the international food giant Mars – studied groups of exotic fish called variatus platys over 30 minutes in water containing different concentrations of CBD.
Results showed that adding CBD to transportation water produced a calming effect, with fish displaying fewer stress-related behaviours such as biting, chasing and erratic swimming when observed immediately after transport, 30 minutes later, and two hours post-transport.
The mid-range CBD dose proved most effective, with treated fish also showing calmer movement patterns, shorter travel distances and reduced immobility in individual testing.
“Improving the welfare of fish transported internationally is of importance both ethically and economically,” said Professor Katherine Sloman from the University of the West of Scotland. “Within the ornamental fish trade, we know that transport-induced stress can lead to erratic or aggressive behaviour, which can exacerbate levels of anxiety and potentially lead to physical damage.
“Our research shows that a relatively small change to the way ornamental fish are transported, such as adding CBD within commercial water conditioners, could transform the industry and help safeguard the wellbeing of countless fish.”
It is estimated that one billion exotic fish are transported across the world each year, with more than 125 countries involved in the multi-billion-pound ornamental fish trade. Transported fish often exhibit high levels of stress that manifest in behaviours such as biting other fish, swimming erratically, increased pulse rate, avoidance or hiding, and altered foraging and social behaviour.
The stress caused during transportation can weaken immune responses in fish, increasing the risk of disease. Anxiety-induced behaviours can also result in physical harm. Both of these outcomes can require fish to be quarantined and treated before they can be sold.
The team randomly selected groups of five fish from a resource of 221 tanks containing 20 fish in each. The selected groups were placed into polythene bags containing one litre of water, which contained a 50:50 ratio of new water and water taken from the tanks the fish had previously been housed in.
The bags were then assigned to one of five separate treatments – eight bags of fish were assigned to each treatment. The water conditioners used contained three different concentrations of CBD: 3.9mg/litre, 7.8mg/litre and 15.6mg/litre, alongside one control and another control with added solvent.
“Improving the welfare of fish during transport by minimising stress is important to ensure the health of fish within the trade,” said Dr Donna Snellgrove, fish research manager at Waltham Petcare Science Institute. “We are proud to be able to establish scientifically supported solutions that will make a difference to such a large number of fish.”
The study, published in the journal Applied Animal Behaviour Science, found that CBD potentially modulates neural pathways related to fear, with no detrimental impact on the protective layer of mucus which covers fish skin. Fish, like mammals, have an endocannabinoid system. Previous studies have found cannabis, and specifically CBD, to have anxiolytic qualities in humans and animals. A 2025 analysis of multi-year data from over 47,000 dogs found that those receiving CBD supplements consistently showed less intense aggressive behaviours as they aged.
