Hypomesus nipponensis

Hypomesus nipponensis
Hypomesus nipponensis, wakasagi smelt
US Bureau of Reclamation photo by René Reyes.
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Actinopterygii
Order: Osmeriformes
Family: Osmeridae
Genus: Hypomesus
Species: H. nipponensis
Binomial name
Hypomesus nipponensis

Also called wakasagi, Hypomesus nipponensis is an important food fish native to the lakes and estuaries of Hokkaido, Japan, although it has been introduced to other locations, including the San Francisco Delta of the United States. Wakasagi are raised in fisheries, and are very similar in appearance to H. transpacificus.



A cartoon centipede reads books and types on a laptop.
Fried wakasagi in a Japanese dish, behind the green peppers.

Hypomesus olidus

Hypomesus nipponensis

Hypomesus japonicus

Hypomesus pretiosus

Hypomesus transpacificus

Phylogeny of the genus Hypomesus including H. chishimaensis under H. nipponensis.[1][2]

Hypomesus chishimaensis was described as being a new species in the lakes of Kunashir and Iturup in 1997 based on claimed morphological differences. Later studies failed to find these morphological differences, and was then regarded as an ecotype of H. nipponensis. A 2007 genetic analysis supported this classification, and recommended that H. chishimaensis not be given its own binomial designation.[1]

Introduction to the United States

Native to the lakes and estuaries of Hokkaido, Japan and introduced to the lakes on Honshu and Kyushu, the Japanese wakasagi (although at the time, both the California-native delta smelt and the Japanese smelt were both thought to be separated members of the same species, H. olidus[3]), was introduced to California water reservoirs by the California Department of Fish and Game to provide more prey for stocked rainbow trout after attempts to introduce native Delta smelt to three foothill reservoirs failed.[4][5] In 1959 the CDFG imported 3.6 million fertilized eggs attached to palm fiber mats from a population in Suwa Lake, located east of Tokyo; many of them dead on arrival. The fiber mats were placed in streams feeding into six lakes and reservoirs that appeared to be ecologically suitable for the smelt.[nb 1] It was thought at the time that these reservoirs could be chemically treated to eradicate the fish if they were found to be undesirable.[6] In 1972 and 1973 about 77,000 fish from the Shastina Reservoir were moved to the Almanor Reservoir in Plumas County. All attempts to introduce the fish were successful, except the Dodge and Big Bear Reservoir introductions,[4] the latter of which may have been partially attributable to chemical treatments meant to eradicate stunted crappie and goldfish.[6]

Progression into delta

Although a retrospective analysis of preserved Delta smelt samples caught in 1972 and 1982 from the Delta region has shown that wakasagi had been invading the estuaries in undetected quantities since at least the early 1970s, wakasagi expansion from these original introduction sites southward was not tracked until several years later.[4] In 1994 they were detected at the State Water Project pumping plant for the first time, and by 1998 the fish could be found throughout the estuary including the Suisun and San Pablo Bays.[7][3][4]


Because the two species are very similar in morphology and life history, wakasagi smelt present several potential threats to the endangered Delta smelt. Besides direct competition for nutritional resources and the possibility that wakasagi may prey on the eggs and larvae of Delta smelt,[4] hybridization could either dilute the species or cause population decline due to sterilizing effects. In fact, a few hybrids have been captured in the wild, although all of them were first generation crosses and no evidence of back-crossing has been found, which would suggest that the hybrids were not viable in some way. Misidentification of the species is an additional concern, which could lead to inaccurate assessments critical to making policy decisions; however this problem may be mitigated if genetic markers are used for identification.[3]

In addition to their negative effects on Delta smelt, the spread of wakasagi significantly reduced Kokanee fisheries, but helped increase growth rates of other salmons and trout fisheries.[8]


  1. ^ The fish were introduced into Freshwater Lagoon (Humboldt County) as well as the Dodge (Lassen County), Shastina (Siskiyou County), Spaulding (Nevada County), Jenkinson-Sly Park (El Dorado County) and Big Bear (San Bernardino County) Reservoirs.[6]


  1. ^ a b Ilves KL,Taylor EB. Are Hypomesus chishimaensis and H. nipponensis (Osmeridae) Distinct Species?. Copeia. 2007;1:180-185. doi:10.1643/0045-8511.
  2. ^ Ilves KL, Taylor EB. Evolutionary and biogeographical patterns within the smelt genus Hypomesus in the North Pacific Ocean. Journal of Biogeography. 2008;35:48–64. doi:10.1111/j.1365-2699.2007.01782.x.
  3. ^ a b c Trenham PC, Shaffer HB, Moyle PB. Biochemical identification and assessment of population subdivision in morphologically similar native and invading smelt species (Hypomesus) in the Sacramento-San Joaquin Estuary, California. Transactions of the American Fisheries Society. 1998;127:417-424. doi:10.1577/1548-8659.
  4. ^ a b c d e Moyle PB. Inland fishes of California. 2nd ed. California: University of California Press; 2002. ISBN 9780520227545. p. 232-234.
  5. ^ California Department of Fish and Game Biennial Report 1954-1956. Division of Fish and Game of the California Department of Natural Resources; 1956. p. 40.
  6. ^ a b c Wales JH. Introduction of pond smelt from Japan into California. California Fish and Game. 1962;48:141-142.
  7. ^ Sweetnam DA. Field identification of delta smelt and wakasagi. Interagency Ecological Program for the Sacramento-San Joaquin Estuary Newsletter. 1995;(Spring):1-3.
  8. ^ Dill WA, Cordone AJ. History and status of introduced fishes in California, 1871—1996. Fish Bulletin. 1997;178:83. doi:10.1111/j.1365-2699.2007.01782.x.