Rainbow smelt (Osmerus mordax)

Synonyms: Atherina mordax, Osmerus spectrum, Osmerus mordax mordax, Osmerus mordax dentex

Common Names: Smelt, freshwater smelt, American smelt, Atlantic smelt, leefish

Description:

Rainbow smelt (Osmerus mordax) is a small, silvery schooling fish native to the Atlantic coast that now lives in all five Great Lakes [1][2]. It got here on purpose: eggs were stocked into Crystal Lake, Michigan in 1912 as food for game fish, and the fish escaped into Lake Michigan and spread across the basin within about a decade [1][2]. MISIN tracks it because it preys on the young of native fish and carries an enzyme that causes a fatal deficiency in lake trout that eat it [1][4], and because it keeps reaching new inland lakes in bait buckets.

Identification:

Rainbow smelt is a slender fish, usually about 4 to 14 inches long [1]. In the water it is strikingly iridescent ? purple, blue, and pink along the sides with a bright silver stripe, a pale green back, and a white belly [1]. Out of the water that shimmer fades fast. The scales are large for the fish's size, fewer than 75 along the side, and they rub off easily [1].

Two features settle the identification. First, look between the dorsal fin and the tail for a small fleshy adipose fin ? a little rayless nub that minnows do not have [1]. Second, open the mouth: rainbow smelt has large, canine-like teeth on its tongue and on the roof of its mouth, which the whitefish and ciscoes it resembles do not [1]. The snout is pointed and the lower jaw juts out past the upper.

There is also a famous shortcut. A fresh rainbow smelt smells strongly of cut cucumber [1].

The easiest time to encounter them is early spring. Right after ice-out, once the water reaches about 40 ?F (4.4 ?C), adults crowd into tributary streams and shorelines at night to spawn [1]. That spring run is when smelt are visible in numbers, and it is when dip-netters catch them.

At-a-glance cues:

1. A small fleshy adipose fin between the dorsal fin and the deeply forked tail [1]
2. Large canine-like teeth on the tongue and the roof of the mouth [1]
3. Iridescent purple, blue, and pink sides with a bright silver stripe; large scales that rub off easily [1]
4. A fresh fish smells strongly of cucumber [1]

Habitat:

Rainbow smelt is native to the Atlantic coast, from New Jersey north to Lake Melville on the Labrador coast, and it occurs naturally in lakes and ponds in New Hampshire, Maine, and Atlantic Canada [2]. In the United States its native range covers Connecticut, Maine, Massachusetts, New Hampshire, New Jersey, New York, Pennsylvania, Rhode Island, and Vermont [2]. It is not native to the Great Lakes.

The fish lives one of two ways. Along the coast it is anadromous ? it lives in salt water and runs up rivers to spawn. In the Great Lakes and inland lakes it is landlocked, spending its whole life in fresh water [1]. It does best in clear, cool, deep lakes.

Where a smelt sits in the water changes as it grows. Young-of-the-year stay along shorelines in warmer water 6 to 20 feet deep, yearlings move out into cooler open water, and adults move up and down through the water column daily ? they have been found between about 60 and 300 feet deep in Lake Superior and Lake Michigan depending on the season [1].

It tolerates a lot. Rainbow smelt handles water from below freezing to about 68 ?F (20 ?C), and in extreme cold it produces a natural antifreeze that keeps its muscles working [1]. Acidity is one real limit: a review of 8,842 Ontario lakes found smelt only in lakes with a pH above 6 [1].

Rainbow smelt occurs in all five Great Lakes and is established well beyond them ? down the Mississippi and Missouri rivers, and into the Hudson Bay drainage [1][2].

Reproduction:

The way rainbow smelt reaches a new inland lake is almost always a person. It does not walk overland. It arrives in a bait bucket, in livewell water, or through illegal stocking by someone who wants more forage for game fish ? which is exactly how it entered the Great Lakes in the first place, as a deliberate 1912 stocking that got loose [1][2]. Two of the biggest range expansions in the country, into Lake Sakakawea in North Dakota in 1971 and Lake Oahe in South Dakota in 1974, were also stockings [2].

Spawning happens fast and early. After ice-out in spring, once water hits about 40 ?F (4.4 ?C) and peaking near 50 ?F (10 ?C), adults run into tributary streams and sometimes onto shorelines, mostly at night [1]. The eggs are sticky and sink, gluing onto the bottom until they hatch two to three weeks later [1].

The life cycle is short and the population turns over quickly. Smelt mature somewhere between age 1 and age 4 depending on the lake, and can live up to about 8 years [1]. Females grow larger and live longer than males [1]. A population can build very quickly once established.

Diet shifts with size, and this matters for the damage the fish does. Small smelt under about 6 inches eat zooplankton ? the tiny animals that native young fish also depend on [1]. Larger smelt add bottom crustaceans and, critically, larval and small fish to the menu [1]. So one fish competes with native young when it is small and eats them once it is big.

Impact and Damage:

Rainbow smelt is safe to handle and safe to eat ? it is a popular food fish, and the spring dip-net run is a long-standing tradition around the Great Lakes [2]. There is no hazard to people here.

The harm is to other fish, and the federal assessment is blunt: the U.S. Fish and Wildlife Service classifies this species' history of invasiveness as High, with a High certainty of assessment, giving it an overall risk category of High for the contiguous United States [2].

The clearest documented mechanism is a chemical one. Rainbow smelt is loaded with thiaminase, an enzyme that destroys thiamine (vitamin B1). Lake trout that eat a diet heavy in smelt develop thiamine deficiency, and their offspring die of early mortality syndrome [1][4]. Salmon fed on smelt swim worse and carry poorer body condition [1]. A predator can eat well and still fail to reproduce.

The second mechanism is predation on the young. Rainbow smelt eats larval fish and is a documented predator of lake trout eggs, and it also feeds on young lake whitefish and burbot [1]. In a review of introductions into inland Ontario lakes, 13 of 24 lakes with smelt showed a decline in lake whitefish recruitment [1]. Smelt has been implicated in declines of cisco, in the troubles of several at-risk Canadian fish including blackfin cisco, shortnose cisco, and deepwater sculpin, and has been suggested as a factor in the extinction of the blue pike [1][2].

For walleye, the evidence is real but narrower than it is often stated. A study of Wisconsin inland lakes from 1985 to 2004 found young-of-year walleye density was significantly lower in lakes with rainbow smelt than in lakes without [3]. That is a genuine finding. But a separate study of two Wisconsin lakes found that when fishing restrictions let adult walleye recover, smelt declined and native cisco came back [1] ? which suggests the relationship can run both directions. Researchers describe the overall extent of smelt's impact on native cisco in the Great Lakes as still uncertain [1].

The honest picture includes real benefits, and they are why the fish was moved around on purpose. Rainbow smelt is now a dominant prey fish for salmon, lake trout, walleye, and burbot in the Great Lakes [1]. Stocking it increased the growth of landlocked Atlantic salmon in Maine [1]. It is a fish that feeds the fishery and damages it at the same time ? which is exactly why moving it to a new lake is a bad bet rather than an obvious one.

Similar species:

Three groups of fish get mistaken for rainbow smelt, and one feature separates it from all of them: teeth on the tongue and the roof of the mouth [1].

Minnows (family Cyprinidae) are the most common mix-up, because many are small, slender, and silvery at the size a young smelt is [1]. Check the back: minnows have no adipose fin, the small fleshy nub between the dorsal fin and the tail that every smelt has [1].

Cisco (Coregonus artedi) is easily confused because it is a silvery, adipose-finned open-water fish living in the same cold lakes [1]. It has a noticeably deeper, less pencil-shaped body than a smelt, and no teeth on the roof of its mouth or on its tongue [1].

Lake whitefish (Coregonus clupeaformis) shares the same silvery color and adipose fin and turns up in the same waters [1]. Like cisco it is deeper-bodied than a smelt and lacks the tongue and mouth-roof teeth [1].

Monitoring and rapid response:

Signs to watch for:

1. In early spring right after ice-out, small silvery fish running into tributary streams or crowding the shoreline at night to spawn ? this is when smelt are most visible [1]
2. Slender iridescent fish with a small adipose fin near the tail and large teeth on the tongue and roof of the mouth [1]
3. A fresh catch that smells strongly of cucumber [1]
4. Smelt turning up in an inland lake that did not have them before ? that is the report worth making [1]

What you can do:

1. Clean, drain, and dry everything before you leave a lake ? boat, trailer, waders, nets, and buckets. Drain all water from the livewell and bilge on land. This is the whole ballgame for a fish that cannot move overland by itself.
2. Never move live fish between waters, and never dump a bait bucket into a lake or down a storm drain. Dispose of unused bait in the trash, on land.
3. Do not stock it. Rainbow smelt entered the Great Lakes as a deliberate 1912 stocking meant to feed game fish, and the two largest western introductions were stockings too [1][2]. It is still moved illegally for the same reason. The catch is that smelt can hurt the very predators it is meant to feed ? lake trout raised on a smelt diet fail to reproduce [1][4].
4. If you dip-net or fish for smelt, keep your catch ? eat it. Just do not carry live fish or lake water to another lake.
5. For lake associations and stewardship groups: focus on prevention at inland lakes that do not have smelt yet, because that is where action still changes the outcome. Rainbow smelt is established and self-sustaining in all five Great Lakes and is not going to be removed from them [1]. Boat-launch outreach and bait-disposal stations are the practical tools.

When to call a professional:

1. A new population in an inland lake ? contact your state fisheries agency. Removing an established fish from a lake is not something a volunteer group can do, and any response needs the agency that manages the water.
2. Suspected illegal stocking, which is an enforcement matter, not a stewardship one.
3. For chemical control options, contact a licensed applicator or your local cooperative extension office.

Report sightings to MISIN. Help track this species' spread by submitting observations through the MISIN web site or the MISIN mobile app (Android, iOS, Windows, and macOS). Photos at multiple angles paired with a precise GPS location are the most valuable data for confirming new occurrences and triggering rapid-response review.

Credits:

1. Fuller P, Neilson M, Procopio J. Osmerus mordax (Mitchill, 1814) ? Species Profile. U.S. Geological Survey Nonindigenous Aquatic Species Database / NOAA Great Lakes Aquatic Nonindigenous Species Information System. nas.er.usgs.gov
2. U.S. Fish and Wildlife Service (2025). Rainbow Smelt (Osmerus mordax) Ecological Risk Screening Summary. November 2023, revised February 2024, web version 8/11/2025. fws.gov
3. Mercado-Silva N, Sass GG, Roth BM, Gilbert SJ, Vander Zanden MJ (2007). "Impact of rainbow smelt (Osmerus mordax) invasion on walleye (Sander vitreus) recruitment in Wisconsin lakes." Canadian Journal of Fisheries and Aquatic Sciences 64(11). semanticscholar.org
4. Honeyfield DC, Hinterkopf JP, Fitzsimons JD, Tillitt DE, Zajicek JL, Brown SB (2005). "Development of thiamine deficiencies and early mortality syndrome in lake trout by feeding experimental and feral fish diets containing thiaminase." Journal of Aquatic Animal Health 17(1). semanticscholar.org
5. NatureServe. Osmerus mordax ? Rainbow Smelt. NatureServe Explorer. explorer.natureserve.org

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