Bloater (Coregonus hoyi)

Bloater in aquarium
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Bloater fact file

Bloater description

GenusCoregonus (1)

The fish (Coregonus hoyi) is a small freshwater fish named after its swollen appearance when brought up from deep water, which is caused by its gas bladder expanding. Its slender, silvery body has areas of pink or purple shimmering iridescence. The snout is pointed and the dorsal and caudal fins have dark edges (2) (3).

The fish is similar in appearance to other species of its same genus and can be difficult to distinguish (3). It is almost identical in appearance to the kiyi (Coregonus kiyi), but the bloater has a smaller eye than the kiyi and its upper lip is less dark (2).

Adult maximum length: 37 cm (2)
Average adult weight: 207 g (3)

Bloater biology

Unlike shallow-water fish inhabiting the Great Lakes, the bloater is well adapted to reaching great depths (6). This is due to its high bodily fat content, which aids buoyancy, as well as its ability to metabolise food at low temperatures (5).

During the day, the fish can be found near or on the bottom of the lake, but at night, some of the bloater population move upwards in the water column to feed (6). The bloater is considered to be an opportunistic feeder, mostly feeding on small aquatic invertebrates (5).

Being able to descend to such great depths allows the bloater to feed on zooplankton near the lake bottom, which other species are unable to reach. Because of this, the bloater is considered to be an integral part of food webs in the Great Lakes (5).

The fish spawns between September and March in various lakes and spawning grounds. A female bloater can produce up to 10,000 small yellow eggs, which may represent 30 percent of the female’s body weight when it begins to spawn (3). The bloater may live to around nine years of age, with the female generally living longer than males (7).


Bloater range

The bloater has a restricted range, formerly occurred in all the Great Lakes of North America; except for Lake Erie (1).

However, it is now considered extinct from lakes Ontario and Nipigon (1) (2) (4).


Bloater habitat

Inhabiting large freshwater lakes, the fish can generally be found at depths of between 18 and 165 metres (4) (5). The adult bloater occupies colder, deep waters, whereas juveniles are restricted to the warmer surface waters (5).


Bloater status

The bloater is classified as Vulnerable (VU) on the IUCN Red List (1).

IUCN Red List species status – Vulnerable


Bloater threats

Species of the genus Coregonus were heavily exploited in the first half of the 20th century by the commercial fishing industry in the Great Lakes (3). This caused numbers to decrease and also contributed to the invasion of non-native species, such as the sea lamprey (Petromyzon marinus), alewife (Alosa pseudoharengus) and rainbow smelt (Osmerus mordax) (3) (5). Competition with and predation by these invasive species caused even further decline in the fish and related species (5).

Being the smallest of the cisco (Coregonus) species, the fish was not targeted by the fishing industry, which helped to minimise its decline. However, in the 1950s the introduced sea lamprey destroyed lake trout (Salvelinus namaycush) stocks, other cisco populations collapsed, and the bloater population became targeted, suffering a major population collapse from 1965 to 1970 (7). It was feared that that the bloater could suffer the same fate as other cisco species, some of which are locally extinct in many of the Great Lakes (3). However, out of the six ciscoes, the bloater is the only one which remains fairly widespread in the Great Lakes (5).

The sex ratio of the bloater population has been skewed over the last century, and at times it has been reported to reach 97 percent female. The skew is thought to be due to higher male mortality and females having longer life spans, and may affect the number of offspring produced (3) (7) (8).


Bloater conservation

A number of suggestions have been made since 1999 to reintroduce the fish into Lake Ontario, using individuals from more intact populations inhabiting other Great Lakes (9) (10). However, there is little information available on whether these planned reintroductions occurred or whether they were successful. The bloater is being continually assessed to monitor its population dynamics (7) (8).

View information on this species at the UNEP World Conservation Monitoring Centre.

Find out more

Find out more about the bloater:



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This species information was authored as part of the Arkive and Universities Scheme.


Caudal fin
The tail fin of a fish.
Relating to the back or top side of an animal.
A category used in taxonomy, which is below ‘family’ and above ‘species’. A genus tends to contain species that have characteristics in common. The genus forms the first part of a ‘binomial’ Latin species name; the second part is the specific name.
Animals with no backbone, such as insects, crustaceans, worms, molluscs, spiders, cnidarians (jellyfish, corals, sea anemones) and echinoderms.
The production or depositing of eggs in water
Tiny aquatic animals that drift with currents or swim weakly in water.


  1. IUCN Red List (November, 2011)
  2. Page, L.M. and Burr, B.M. (1991) Field Guide to Freshwater Fishes. Second Edition. Houghton Mifflin, New York.
  3. Becker, G.C. (1983) Fishes of Wisconsin. The University of Wisconsin Press, Wisconsin.
  4. FishBase – Bloater, Coregonus hoyi (November, 2011)
  5. Clemens, B.J and Crawford, S.S. (2009) The ecology of body size and depth use by bloater (Coregonus hoyi Gill) in the Laurentian Great Lakes: patterns and hypotheses. Reviews in Fisheries Science, 17(3): 174-186.
  6. TeWinkel, L.M and Fleischer, G.W. (2011) Vertical migration of bloaters: response to comment. Transactions of the American Fisheries Society, 130(1): 166-167.
  7. Schaeffer, J.S. (2004) Population dynamics of bloaters (Coregonus hoyi) in Lake Huron, 1980-1998. Annales Zoologici Fennici, 41: 271-279.
  8. Bunnell, D.B., Madenjian, C.P. and Croley II, T.E. (2006) Long-term trends of bloater (Coregonus hoyi) recruitment in Lake Michigan: evidence for the effect of sex ratio. Canadian Journal of Fisheries and Aquatic Sciences, 63: 832-844.
  9. Favé, M.J. and Turgeon, J. (2008) Patterns of genetic diversity in Great Lakes bloaters (Coregonus hoyi) with a view to future reintroduction in Lake Ontario. Conservation Genetics, 9: 281-293.
  10. Baldwin, B. (1999) (Native Prey Fish Re-introduction into Lake Ontario: Bloater (Coregonus hoyi)). Discussion Paper for the Great Lakes Fisher Commission, Lake Ontario Committee.

Image credit

Bloater in aquarium  
Bloater in aquarium

© Shedd Aquarium

Shedd Aquarium
1200 South Lake Shore Drive
United States of America
Tel: +1 (312) 692 3222


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