Do Toxins Affect Aggression in Rainbow Trout?

A review of the paper: Bakke, M.J., Hustoft, H.K., Horsberg, T.E., (2010), Subclinical effects of saxitoxin and domoic acid on aggressive behaviour and monoaminergic turnover in rainbow trout (Oncorhynchus mykiss), Aquatic toxicology, 99, (1), 1-9

Algae in the Alexandium family produce saxitoxin and other analogues that are known as Paralytic Shellfish Posioning (PSPs) toxins, which affect the sodium channels in the nervous system. Another algal family that produce toxins are the Pseudonitzschia family, and these produce Domoic acid. This affects receptors in the brain, causing a suite of neurological symptoms. Both of these toxins have caused huge economic losses in the fish farming industry by causing mortalities and decreased growth via stress. They affect the brain of fish, so behavioural abnormalities should be expected. An important species of fish involved in farming is the Rainbow trout. This fish displays aggressive behaviour towards intruder fish, and protect their “home tank”. This aggressive behaviour is controlled by the serotonergic and domaminergic systems, which both of these toxins have been shown to affect in rats. The aim of this paper was to see whether these toxins affect the aggressive behaviour in Rainbow trout, and therefore if the toxins affect these systems.

4 groups of 8 Rainbow trout were placed into tanks and were exposed to intruder fish on 2 consecutive days. The intruder fish were always smaller than the resident fish to encourage aggressive behaviour. The first day was a control where no toxin was injected. On the second day, 4 different treatments were implicated: a control with no injection, a saline injection, a saxitoxin injection and a Domoic acid injection. Each fish was video-taped and the behaviour was analysed for: the time (latency) until the first attack, the number of attacks in the following 30 minutes and the type of attack (approach, chase or bite). If there was no attack in the first 15 minutes, the test was aborted. After the tests were complete, the resident fish were taken for tissue samples.

No significant differences were found from the behaviour of day one, however on day two there was a difference between the controls and the saline-injected fish, and the saline and Domoic acid-injected fish. The fish that were injected with the saline solution, showed a decrease in aggression. As there was no toxin in this injection, it can be deduced that this is down to the stress of handling. This suppression of aggression was absent in both the saxitoxin and Domoic acid groups of fish, despite the same handling stress showing that the toxins elevated activity. Seratonin levels were elevated all 3 groups of injected fish, showing elevated stress levels. The toxins did not affect the levels of monoamine or serotonin in comparison to the saline-injected group. However, this paper did conclude that the toxins masked the effect of the handling stress. To me, this implies that if the toxins were ingested naturally, there would be an elevated amount of aggressive behaviour but this paper has not been able to prove this.