ONE thing Canada is not short of is lakes. It has so many that it can afford to set some aside to experiment on. And that is what Karen Kidd, an ecotoxicologist at the University of New Brunswick, has just done to a small lake in north-west Ontario. She has poisoned it in the name of science.

Her chosen poison was oestrogen, one of the hormones that make women women and help to control the menstrual cycle. People flush a lot of oestrogen down the toilet. Some is natural. Some is the synthetic stuff used in oral contraceptives. There is a strong suspicion that if this oestrogen is not removed during sewage treatment (some works do, some do not), it causes serious damage to rivers and lakes. Until now, however, proof has been lacking. Dr Kidd wanted to find out if the suspicion was correct. As she told a session of the American Association for the Advancement of Science meeting in Boston, it is.

The lake's algae, bacteria and invertebrates appeared unfazed by the extra hormone which she dumped in the lake at regular intervals. Presumably, its chemistry was not similar enough to their own biochemicals for them to notice. But the population plunged in the smallest fish species, the fathead minnow. Male minnows became feminised: their sperm production was delayed and they started producing eggs. After two years of treatment, the fathead minnow population collapsed.

Other changes took longer. Pearl dace mature more slowly than minnows and can outlive them by several years. The dace therefore managed to hold out for three years before the lack of male potency brought about a population crash.

There was damage even to the lake's largest fish, its trout. Here the cause was less that boys were turning into girls than that the trout were on short rations—since the minnows had disappeared. But the upshot was the same: fewer trout, and confirmation that oestrogens are very bad news for fish, even at low concentrations.

The better news was that things quickly returned to normal once the hormone treatment stopped. Fixing sewage works to prevent them from spewing out oestrogen should therefore help fairly rapidly. Controlling the sale of contraceptive pills will not be necessary, and fishermen will once again be able to use their rods and tackle with impunity.