Those fish that manage to get back to their former cradle spawn; soon after, most make it their graveyard. Their decomposing bodies nourish water grasses and algae, which form a camouflaging habitat for future hatchlings until they themselves turn into smolts and, like their forebears, depart.

Today, another kind of migration—a pernicious, microscopic one that folds together the private lives of humans with those of riverine creatures—risks disrupting this cycle, even as it offers (a meager silver lining) insight into fish mentality and animal migration. Pharmaceuticals are emitted from our bodies, homes, and factories, entering waterways and accumulating in fish, bugs, mollusks, crustaceans, birds, and warm-blooded animals. Areas around drug-manufacturing plants are hot spots for this kind of pollution. So too are watercourses near hospitals and aging sewage infrastructure. But medicinal compounds have also been detected in remote environments, imbuing surface waters even in Antarctica.

Waterways can contain traces of many drugs—among them antifungals, antimicrobials, and antibacterials, as well as ones for pain, fertility, mood, sleeplessness, and neurodegenerative diseases. If current trends persist, scientists estimate, the volume of pharmaceuticals diffusing into fresh water could increase by two-thirds by 2050. Recent modeling shows that a platypus living in a contaminated stream in Melbourne is already likely to ingest more than half a recommended adult dose of antidepressants every day.

Tracking medicines’ impact in the wild is difficult, but toxicologists believe their influences on fauna can occur at low concentrations—and may be distinct from their effects on humans. Already a variety of symptoms has been observed in lab studies. Amphetamines change the timing of aquatic insect development. Antidepressants impede cuttlefish’s learning and memory, and cause marine and freshwater snails to peel off rocks. Drugs that affect serotonin levels in humans cause shore crabs to exhibit “risky behavior,” and female starlings to become less attractive to males (who in turn sing less). Dosed with Prozac, shrimp are more likely to swim toward a light source—a dangerous tendency, given that many predators hunt in sunlit zones.

And Atlantic salmon smolts exposed to benzodiazepines—medications, such as Valium and Xanax, that are frequently used to treat anxiety—migrate nearly twice as quickly as their unmedicated counterparts. Recklessly so, for the juvenile fish are likely to arrive at the sea in an underdeveloped state and before seasonal conditions are favorable. The smolts do not usually exhibit such gusto: In fact, they are frequently observed traveling tail-first, as if reluctant. It would seem, then, that they have a cognitive and perhaps emotional switch that, when flipped, prompts them to strike out for sea. This complicates the common understanding of migration, which holds that animals are puppetted by seasonal cues and physical readiness (here, the adaptation of scales and gills to briny water). Pharmaceutical pollution reveals that a psychobiological release may also be required—to set off, the smolts must first surmount their own feelings of stress.