The team ran select fish through this experiment on the second, fourth, sixth, eighth and 10th days after they'd been moved to the drug-free tank they also ran drug-free fish through the same experiment, as a point of comparison. When previously exposed fish showed a preference for the meth-tainted water in the experiment, that was taken as a sign of addiction to the drug, Horký told Live Science. To test for these withdrawal symptoms, the team devised an experiment where fish could choose between swimming in clean water or water with trace amounts of meth the tank is designed such that the two streams of water don't mix but the fish can still swim between them. The researchers then transferred the drugged fish into a clean tank for 10 days if the trout had grown dependent on meth, they would begin to show symptoms of withdrawal after losing access to the drug, the team theorized. The researchers had the latter group of fish soak in the meth-tainted water for two months - a step meant to simulate the effects of persistent drug exposure that might occur in a polluted river. The researchers placed 60 trout in a drug-free holding tank and another 60 in a tank laced with 1 microgram of meth per liter of water. In the new study, the team specifically focused on brown trout ( Salmo trutta), which are native to Europe, western Asia and northern Africa and have been introduced to every continent except Antarctica, Horký said. 'Whether you call it addiction or not, you can argue, but it's clear that methamphetamine changed how these animals behave,' and those effects could potentially hinder their ability to find food, avoid predators and reproduce in the wild, Bossé noted. In regards to the new research, 'it seems that the preference for meth dies down after just a few days,' whereas if the fish were truly addicted, he'd expect that preference to persist over a longer period of time, he said. 'I'm not sure you can truly say these fish are addicted to methamphetamine, but they certainly show a preference for the compound … which they shouldn't, really,' said Gabriel Bossé, a postdoctoral research fellow at the University of Utah who was not involved in the study Bossé uses zebrafish as a model to study complex brain disorders and recently developed a technique to study opioid-seeking behaviors in the fish. That said, an expert told Live Science that, even though the fish in the study sought out meth-tainted water, that may not be enough evidence to say they are truly 'addicted.'

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