Illustration of Atlantic salmon (Salmo salar). Image: Timothy Knepp, US Fish and Wildlife Service.
Low concentrations of the drug clobazam in rivers encourage salmon to take greater risks, altering their migratory behaviour.
He migratory behaviour of wild salmon changes when the animals are exposed to clobazam, a drug used to treat anxiety and epilepsy. Behavioural changes occur even at low concentrations, according to laboratory and field work by a predominantly Swedish team of zoologists (Science, 11 April). It appears that salmon under the influence of clobazam take greater risks: they seek the safety of a shoal less often. Although this increases their chances of being caught by a predator, these salmon reach the sea more frequently than conspecifics in a control group.
Clobazam is a widely used drug for people suffering from anxiety, tension and epilepsy. Like roughly a thousand other pharmacological substances, traces of it regularly end up in rivers and other bodies of water, where they persist for long periods. Since a drug such as clobazam is designed to act on conserved neurobiological pathways, it is not in itself surprising that the substance influences animal behaviour. That this does indeed occur is demonstrated by a field experiment in which the Swedish researchers tracked 279 wild Atlantic salmon (Salmo salar) during their 28-kilometre journey along the Dal River. In the course of this migration towards the Baltic Sea, the salmon must pass through two dams.
INJECTION METHOD
Using a specialised injection method, the researchers administered clobazam, the painkiller tramadol, a clobazam–tramadol combination, or a placebo to four groups of salmon, with the substances released slowly at low concentrations. The number of salmon subsequently reaching the sea after around ten days was approximately twice as high in the clobazam group. These salmon also managed to pass through the Lanfors dam in 2.77 hours and the Älvkarleby dam in 8.14 hours – at least twice as fast as salmon from the other groups. Laboratory work confirmed that the Swedish team used drug concentrations that can occur in rivers. Their method of administration also ensured that the substance reached various tissues, including the brain.
RISK-TAKING BEHAVIOUR
To explain the altered migratory behaviour under the influence of clobazam, the Swedish researchers looked at risk-taking behaviour in the salmon. In a laboratory experiment, they filmed the swimming patterns of six salmon in the presence or absence of a predator – a northern pike (Esox lucius).
Individuals under the influence of clobazam displayed less shoaling behaviour – at least when a northern pike was swimming in the same space. Apparently these salmon were willing to take greater risks. Although this exposed them to a higher risk of predation, the researchers suggest that this risk-taking behaviour may nonetheless explain why the salmon achieved greater migratory success. Risky dam passages may be easier for them, and moving as a group is in any case often slower.
See also:
–Largest study to date shows pharmaceutical pollution alters migration in Atlantic salmon – website Umeå University, 10 April 2025
–Pharmaceutical pollution affects migration in Atlantic salmon – website Swedish University of Agricultural Sciences, 10 April 2025