Leeches may be gross, but leech flies catch fish. Here’s what you need to know

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Leeches are found across Canada, especially in lakes, ponds and slower river sections with soft or weedy bottoms. In stillwater, fish eat leeches year round, while in rivers, they’re most important in spring and fall, when there’s less insect activity. Leeches naturally cling to underwater structure, but they’re easily swept away by waves or current. That makes them particularly vulnerable when swimming back to cover or when—ugh—looking for a host. (That said, while some leeches are bloodsuckers, most of Canada’s freshwater leech species are actually non-parasitic, and instead feed on tiny invertebrates, roe and aquatic detritus. But still.)

North American leeches can be as small as a fingernail or as large as a hot dog wiener—double ugh—but they’re generally just two to 10 centimetres long. They’re most active in low light during the shoulder hours and overcast days, or in stained water. And the good news for those who like a more colourful fly box is that, as well as black, leeches can be brown, olive and dark purple, often with a slight iridescence. Burgundy and a mix of black and red are also common colours, especially in western Canada.