Smelt are small, silvery anadromous fish of the Osmeridaefamily that live primarily in the sea but make spawning runs into freshwater streams. A few smelt are strictly marine; others live only in large freshwater lakes and spawn in tributary brooks and streams.

Some are marine by origin but have adapted to a strictly freshwater environment; populations of some species live both in the sea and in freshwater. In all environments they are extremely important as forage for predators, including many game species.

All smelt inhabit the cool waters of the Northern Hemisphere in the Atlantic, the Arctic, and the Pacific Oceans and their drainages. The family is related to salmonids, contains 11 species in six genera, and is most generously represented in Pacific waters; many smelt species are so similar in appearance that they are difficult to distinguish.

Like the salmon and the trout, the smelt have a stubby adipose fin just in front of the tail. The lower jaw projects slightly beyond the tip of the snout. A lateral line is prominent, and there are no scales on the head. Smelt are generally small (most growing to no more than 8 inches) schooling fish, often found in enormous numbers; in the spring, great numbers move from their marine or freshwater habitats to tributary waters to spawn.

Only one species, the anadromous Pacific longfin smelt (Spirinchus thaleichthys), spawns in the late fall and the early winter. All species spawn at night. In North America, the pond smelt (Hypomesus olidus) and the rainbow smelt (Osmerus mordax) are considered excellent food fish. In quantity, freshly caught smelt have an odor more nearly like cucumbers than like fish.

The rainbow smelt, which is also commonly known as the American smelt, is the species most familiar to anglers and most common in North American fish markets. The European smelt (Osmerus eperlanus) is similar in size and habits to the rainbow smelt.