The sprinter of the freshwater world. Growing up to 12 centimetres, New Zealand smelt are slender, streamlined and almost entirely silver-white. Large expressive eyes and a slightly upturned mouth are designed for snatching small insects and plankton from the surface or water column. The most distinctive feature is the adipose fin, a tiny fleshy fin near the tail that marks them as distant relatives of trout and salmon. In the water, they do not swim so much as vibrate with energy, moving in tight, synchronised schools that make a pool look like it is filled with flickering diamonds. A life of constant movement. Most smelt are diadromous, moving between the sea and the river. They spend their youth in the ocean, growing fast on marine plankton, before returning to freshwater in massive runs during spring and summer. Unlike whitebait which lay eggs in grass, smelt are broadcast spawners. They find sandy or gravelly reaches in lower rivers and release eggs directly into the current, where they sink and stick to grains of sand. A high-volume, high-risk strategy. Millions of eggs laid to ensure at least a few thousand survive. The famous cucumber smell is not a gimmick. It is caused by a chemical called trans-2-cis-6-nonadienal, the same compound found in cucumber skins. A bucket of fresh smelt can scent an entire room. Smelt are also highly sensitive to turbidity. Because they hunt by sight, they are the first fish to leave a river when it gets choked with silt. Protecting smelt is about protecting the clarity of our rivers. They are the canaries in the water column, requiring clean, clear and fast-moving water to survive. To see a school of smelt flashing in a sunlit rapid is to see a river that is still breathing, a place where the water is clear enough for a tiny, cucumber-scented fish to find its way home.