The rainbow smelt (Osmerus mordax) is a species of fish of the family Osmeridae. Its distribution is circumpolar, and it has been introduced to the Great Lakes, and from there has made its way to various other places. Walleye, trout, and other larger fish prey on these smelt. The rainbow smelt prefer juvenile ciscoes, zooplankton such as calanoid copepods (Leptodiaptomus ashlandi, L. minutus, L. sicilis), and other small organisms, but are aggressive and will eat almost any fish they find. They are anadromous spring spawners and prefer clean streams with light flow and light siltation. The rainbow smelt face several barriers. They are weak swimmers and cannot overcome most fish ladders. This prevents them from making it past the dams to the headwater streams where they spawn. The rise in erosion and dams help to decimate the smelt population in the 1980s. There are currently plans to try to reduce damming and to help control erosion. With current efforts to reduce the human impact on this and many other affected species the population is back on the rise.