Invasion History

First Non-native North American Tidal Record: 1959
First Non-native West Coast Tidal Record: 1959
First Non-native East/Gulf Coast Tidal Record:

General Invasion History:

Wakasagi or Japanese Smelt (Hypomesus nipponensis) is native to most of the Japanese islands (except the Ogusawara and Ryuku islands), the southern Kurile Islands, Sakhalin, and the Pacific Coast of Russia from Peter the Great Bay to the Serebryanka River, on the Sea of Japan. This fish has landlocked freshwater, estuarine, and anadromous populations (Saruwatari et al. 1997; Vu and Kartavtsev 2017). This fish was formerly considered a subspecies of the Delta Smelt (H. transpacificus). The Delta Smelt was a desirable forage fish for California reservoirs, but it was difficult to capture in large numbers, so in 1959 a decision was made to import and experimentally stock Wakasagi in six California lakes and reservoirs. One of these lakes was Freshwater Lagoon, a coastal lake in Humboldt County. A population of Wakasagi had established in this lake by 1961. Other stockings were in northern California (Siskiyou County) and southern California (Big Bear Lake, San Bernardino County), and two lakes in the upper San Francisco Bay watershed, Spaulding Lake (Nevada County), and Jenkinson Lake (El Dorado County), In 1972, Wakasagi were stocked in Lake Almanor, Plumas County, in the upper mountain reaches of the San Francisco Bay watershed and then spread further downstream to Lake Oroville in the foothills (Dill and Cordone 1997). The fish had mostly positive impacts on sport fisheries within the lakes.

Wakasagi were first collected in the Sacramento-San Joaquin Delta in 1974, probably transported by water releases from the Lake Oroville reservoir. The specimens were long unrecognized, due to their similarity to Delta Smelt. High river outflows in the 1990s may have increased the population in the estuary. Wakasagi have now been collected from Cache Slough (upstream) to Suisun Bay (downstream) at a salinity of 0-10 PSU. It is likely that some fish will be transported into San Pablo Bay during heavy river outflows. They have also been collected in reservoirs which feed into the Delta-Mendota Canal and the California aqueduct, so there is a potential for transport to southern California (Aasen et al. 1998).

North American Invasion History:

Invasion History on the West Coast:

Wakasagi was introduced to lakes in northeast China in the 1940s, and introduced more widely to central and northwestern China in the 1990s, as a forage fish for the native European Perch (Perca fluviatilis) (Zhou et al. 2014).


Wakasagi or Japanese Smelt (Hypomesus nipponensis) is a fish of the smelt family (Osmeridae). It is native to Japan and closely related to the very similar Delta Smelt (Hypomesus transpacificus), an endangered species native to the San Francisco Bay estuary. The two species were once considered conspecific subspecies (Page and Burr 1991; Froese and Pauly 2018).

Wakasagi has landlocked freshwater, estuarine, and anadromous populations (Eschmeyer and Herald 1983; Page and Burr 1991; Froese and Pauly 2018). This fish, like other smelt, is small (110-250 mm), with a streamlined body, a single, soft-rayed dorsal fin, abdominal pelvic fins, and a small adipose fin anterior to the forked tail. Their scales are small and silvery. Wakasagi have an upper jaw that ends before the middle of the eye. There are 7-9 dorsal rays, 13-15 anal rays, 12-14 pectoral rays, and 54-60 lateral scales. These ranges differ, but overlap, with those of the native endangered Delta Smelt (H. transpacificus). The two species differ in coloration. Wakasagi is golden-brown above, with silvery-white, nearly translucent sides, with black specks on the back and side. There is a line of ten or more black specks, on the isthmus, the ventral ridge between the mandibles. The native Delta Smelt is steel-blue above, with a silver stripe on the side, and few or no black specks between the mandibles. Wakasagi reaches a maximum size of 170 mm, while the Delta Smelt reaches 11 mm (Page and Burr 1991; Saruwatari et al. 1997). Although the species are very similar morphologically, allozyme analysis shows that they are genetically distinct. Molecular methods are useful for identification (Trenham et al. 1998).

The similarity of the Delta Smelt and the Wakasagi is the result of parallel evolution from closely related ancestors on opposite sides of the Pacific. Delta Smelt (H. transpacificus) are believed to have evolved from the marine, beach-spawning Surf Smelt (H. pretiosus) in the last 8,000 years, colonizing the San Francisco Delta. Wakasagi (H. nipponensis) similarly evolved from H. japonicus (Chika, Silver Smelt), also a beach-spawning marine species, becoming a smaller anadromous species, colonizing estuaries, rivers, and freshwaters (Stanley et al. 1995; Saruwatari et al. 1997; Bennett 2005).


Taxonomic Tree

Kingdom:   Animalia
Phylum:   Chordata
Subphylum:   Vertebrata
Superclass:   Osteichthyes
Class:   Actinopterygii
Subclass:   Neopterygii
Infraclass:   Teleostei
Superorder:   Protacanthopterygii
Order:   Osmeriformes
Suborder:   Osmeroidei
Family:   Osmeridae
Genus:   Hypomesus
Species:   nipponensis


Hypomesus transpacificus nipponensis (McAllister, 1963)
Hypomesus nipponensis (Klukanov, 1970)
Hypomesus chishimaensis (Saruwatari et al., 1997)

Potentially Misidentified Species

Hypomesus pretiosus
Hypomesus pretiosus (Surf Smelt) is a marine fish that sometimes enters brackish water. It is larger, reaching 25 cm, and ranges from Alaska to southern California (Eschmeyer and Herald 1983).

Hypomesus transpacificus
Hypomesus transpacificus (Delta Smelt) is endemic to the Sacramento-San Joaquin Delta and is listed as an endangered species. It differs from H. nipponensis in mean number of fin rays, gill rakers, pigmentation, and a smaller maximum size, but the species overlap in these features (Eschmeyer and Herald 1983; Page and Burr 1991; Saruwatari et al. 1997). Molecular identification is useful in separating populations (Trenham 1998).

Spirinchus thaleicthyes
(Spirinchus thaleicthyes (Longfin Smelt) is an anadromous fish, found in rivers and coastal waters from Alaska to Monterey Bay. Its dorsal, pectoral, pelvic, and anal fins are proportionately longer than those of other smelts (schmeyer and Herald 1983; Page and Burr 1991).



Wakasagi (Hypomesus nipponensis) is often regarded as a freshwater fish in its native Japan, and its introduced range in California. It needs freshwater or low-salinity brackish lakes to complete its life cycle, but anadromous populations occur and can coexist with resident lake populations (Aasen et al. 1998; Asami 2004; Arai et al. 2006). Sexes are separate. The fish mature in their second year, at a length of 60-70 mm. Most fish in Honshu die after their first spawning, but some fish in Hokkaido spawn in their second year. Spawning takes place in freshwater streams where they lay adhesive eggs on rocks and gravel. Eggs take about 12 days to hatch at 14 C (Aasen et al. 1998; Saruwatari et al. 1997; Froese and Paul 2018). Fecundity ranges from 1033 to 44,000 eggs per female (Froese and Paul 2018).

As noted above, Wakasagi, have a flexible life history, with anadromous migratory populations, landlocked lake populations, and estuarine resident populations (Aasen et al. 1998; Saruwatari et al. 1997; Arai et al. 2006). Eggs and early larvae are apparently restricted to freshwater.

Experiments indicate that adult Wakasagi can tolerate a wide range of temperature (2-29 C) and salinity (0-27 PSU) (Swanson et al. 2000). The salinity tests used NaCl, rather than seawater, so may have underestimated the upper salinity tolerance. We know some Japanese populations occur in coastal seawater (Arai et al. 2006). In the San Francisco estuary, most of the fish have been captured at 0-10 PSU (Aasen et al. 1998). We have not found reports of capture in the San Francisco Bay proper or in other coastal waters. Wakasagi are planktivores, feeding mostly on rotifers, copepods and cladocerans as larvae and adults (Aasen et al. 1998; Asami et al. 2004). Their competitors include the native, endangered Delta Smelt (H. transpacificus) and the introduced Threadfin Shad (Dorosoma petenense) and Mississippi Silverside (Menidia audens), all zooplanktivores (Feyrer et al. 2003). Predators in the San Francisco estuary include the introduced Striped Bass (Morone saxatilis), Largemouth Bass (Micropterus salmoides), and the native Sacramento Pikeminnow (Ptychocheilus grandis) (Nobriga and Feyrer 2007).


Copepods, cladocerans, mysids, rotifers


Fishes, birds


Hypomesus transpacificus, other planktivorous fish

Trophic Status:




General HabitatTidal Fresh MarshNone
General HabitatSalt-brackish marshNone
General HabitatNontidal FreshwaterNone
General HabitatUnstructured BottomNone
Salinity RangeLimnetic0-0.5 PSU
Salinity RangeOligohaline0.5-5 PSU
Salinity RangeMesohaline5-18 PSU
Salinity RangePolyhaline18-30 PSU
Vertical HabitatNektonicNone

Tolerances and Life History Parameters

Minimum Temperature (ºC)2.3CTM (mean critical temperature, temperature lowered 5-6 degrees C per hour, from acclimation temperature of 17 C) (Swanson et al. 2000
Maximum Temperature (ºC)29.1CTM (mean critical temperature, temperature raised 5-6 degrees C per hour, from acclimation temperature of 17 C) (Swanson et al. 2000
Minimum Salinity (‰)0This species can complete its life cycle in freshwater.
Maximum Salinity (‰)35This species has anadromous populations (Asami 2004; Arai et al. 2006). Swanson et al. (2000) reported a 'critical salinity' of 26.8 PSU by adding pure NaCl at 2 PSU per 12 hrs- this probably underestimated salinity tolerance.
Minimum Reproductive Salinity0This species spawns in freshwater.

General Impacts

Wakasagi (Hypomesus nipponensis) was introduced as a forage fish for California reservoirs in the 1950s. It was considered to be conspecific with the native Delta Smelt (H. transpacificus), which was difficult to obtain, while the Japanese form was widely reared in Japanese hatcheries. About 360,000 eggs were shipped from Japanese hatcheries and reared in California. It was considered to be a good link between the plankton community and the larger piscivorous gamefish (Dill and Cordone 1997). The native Delta Smelt, the only species endemic to the Delta estuary, was later recognized as a separate species from the Wakasagi. Delta Smelt are endangered, which gives them special significance and legal protection under the Endangered Species Act. In recent years, conservation of the Delta Smelt is a major goal of fisheries and water management of the San Francisco Estuary, together with native Chinook Salmon (Oncorhynchus tshawystscha), and the introduced Striped Bass (Morone saxatilis), the estuary's most important sportfish. All of these species are adversely effected by water diversions, canalization, and by the phenomenon called 'Pelagic Organism Decline' (Feyrer et al. 2007; Sommer et al. 2007; Hobbs et al. 2017; California Department of Fish and Wildlife 2018). A major factor in Pelagic Organism Decline was a shift of the food web from zooplankton grazers to the large biomasses of the introduced Asian Clam, Corbula murensis, reducing the food availability for planktivorous fishes, including the Delta Smelt, Longfin Smelt (Spirinchus thaelichthyes, locally threatened) and the planktivorous larvae and juveniles of larger predators (Sommer et al. 2007; Hobbs et al. 2017). The formerly abundant Delta and Longfin Smelt have shown dramatic declines from the 1970s to the 2000s. Drought, water diversions, and wetlands loss have also been major factors in the decline of the two smelt species (Bennett 2005; Hobbs et al. 2017). In addition to these challenges, Delta Smelt are also threatened by competition and possible hybridization with Wakasagi (Aasen et al.1998; Bennett 2005). The extent of these impacts is difficult to determine, however, Wakasagi is considered a high-risk introduction with a strong potential for establishment, especially in other parts of California (US Fish and Wildlife Service 2014).

Economic Impacts

Fisheries- Wakasagi (Hypomesus nipponensis) was considered a largely successful introduction as a forage fish in California reservoirs, at least in the short term. In Lake Almanor (Plumas County), the introduction of Wakasagi led to increased abundance and growth of Coho Salmon (Oncorhynchus kisutch), Rainbow Trout (O. mykiss), Brown Trout (Salmo trutta), and Smallmouth Bass (Micropterus dolomieu). However, it also led to decline of a planktivorous gamefish, landlocked Kokanee Salmon (O. nerka), and another forage fish, Threadfin Shad (Dorosoma petenense). In the reservoirs the benefits were considered to outweigh the losses (Dill and Cordone 1997).

Ecological Impacts

Competition- The impacts of Wakasagi on reservoir fish populations, described above, suggests potential impacts in the estuary. The impacts on the Delta Smelt are difficult to measure, based on the difficulty of separating the two species in catches. However, Wakasagi reach a larger size, have higher fecundity, have a wider temperature tolerance, a higher salinity tolerance, and a faster swimming speed than Delta Smelt, and thus, a potential competitive advantage (Swanson et al. 2000). In China, competition between introduced Wakasagi and larval Eurasian Perch (Perca fluviatilis, native) may have been responsible for the collapse of a perch population in Lake Ulungar (Zhou et al. 2013).

Hybridization- Hybridization of Wakasagi and Delta Smelt is a concern. However, in one sample of 280 Delta Smelt, only one F1 hybrid was found (Trenham et al. 1998). Despite their morphological similarity, the two species are genetically distinct, making introgression unlikely (Stanley et al. 1995). In a survey of 384 fish, 125 specimens morphologically identified as Delta Smelt, were genetically identified as Wakasagi. Three fish were F1 hybrids, and two were F2 backcrosses. All the hybrids had Wakasagi as the maternal parent, suggesting that the introgression was occurring from Delta Smelt to Wakasagi, but not in the other direction (Benjamin et al. 2018).

Regional Impacts

P090San Francisco BayEcological ImpactCompetition
The Wakasagi (Hypomesus nipponensis) is considered a probable competitor with the native Delta Smelt (H. transpacificus), an endangered species (Aasen et al. 1998; Swanson et al. 1990; Bennett 2005)
P090San Francisco BayEcological ImpactHybridization
Hybridization of the Wakasagi (Hypomesus nipponensis) with the native, endangered (Delta Smelt,H. transpacificus) is a concern, but Trenham et al. (1998) found only 2 F1 hybrids in a sample of 280 fish from the Delta. However, in one sample of 280 Delta Smelt, only one F1 hybrid was found (Trenham et al. 1998). Despite their morphological similarity, the two species are genetically distinct, making introgression unlikely (Stanley et al. 1995). In a later survey of 384 fish, 125 specimens that were morphologically identified as Delta Smelt, were genetically identified as Wakasagi. Three fish were F1 hybrids, and 2 were F2 backcrosses. All the hybrids had Wakasagi as the maternal parent, suggesting that the introgression was occurring from Delta Smelt to Wakasagi, but at very low rates, and not affecting the Delta Smelt genotype (Benjamin et al. 2018).

Regional Distribution Map

Bioregion Region Name Year Invasion Status Population Status
P090 San Francisco Bay 1974 Def Estab
P135 _CDA_P135 (Mad-Redwood) 1959 Def Estab
NWP-4b None 0 Native Estab
NWP-5 None 0 Native Estab
NWP-3b None 0 Native Estab
NWP-4a None 0 Native Estab

Occurrence Map

OCC_ID Author Year Date Locality Status Latitude Longitude


Aasen, Geir; Sweetnam, Dale A.; Lynch. Lisa M. (1998) Establishment of the wakasagi, Hypomesus nipponensis in the Sacramento-San Joaquin estuary., California Fish and Game 84(1): 31-35

Arai, T.; Yang, J.; Miyazaki, N. (2006) Migration flexibility between freshwater and marine habitats of the pond smelt Hypomesus nipponensis, Journal of Fish Biology 68: 1388-1398

Asami, Hiroki (2004) Early life ecology of Japanese smelt (Hypomesus nipponicus in Lake Abashirie, a brackish water, eastern Hokkaido, Japan., Scientific Report of the Hokkaido Fisheries Experiment Station 67: 1-79

Bick, Andreas; Bastrop, Ralf; Kotta, Jonne; Meißner, Karin; Meyer, Maria; Syomin, Vitaly (2018) Description of a new species of Sabellidae (Polychaeta, Annelida) from fresh and brackish waters in Europe, with some remarks on the branchial crown of Laonome, None <missing volume>: 349-364

Bilbao, Amaia Nunez, Jorge Viera, Beatriz Sosa Maria del Pino Fernandez-Palacios, Hipolito Hernavdez-Cruz, Carmen Maria (2011) Control of shell-boring polychaetes in (Reeve 1846) aquaculture: Species identification and effectiveness of Mebendazole, Journal of Shellfish Research 30: 331-336

Carlton, James T.; Blakeslee, April M. H.; Fowler, Amy E. (2022) Accidental associates are not symbionts: the absence of a non?parasitic endosymbiotic community inside the common periwinkle Littorina littorea (Mollusca: Gastropoda, None 167(97): Published online

Courtenay, Walter R., Jr., Hensley, Dannie A., Taylor, Jeffrey N., McCann, James A. (1986) Distribution of exotic fishes in North America, In: Hocutt, Charles H., and Wiley, E. O.(Eds.) The Zoogeography of North American Freshwater Fishes. , New York. Pp. 675-698

Dill, William A.; Cordone, Almo J. (1997) History and status of introduced fishes in California, 1871-1996, California Department of Fish and Game Fish Bulletin 178: 1-414

Eschmeyer, William N.; Herald, Earl S.; Hamman, Howard (1983) <missing title>, Houghton Mifflin, Boston. Pp. <missing location>

Feyrer, Frederick; Herbold, Bruce; Matern, Scott A.; Moyle, Peter (2003) Dietary shifts in a stressed fish assemblage: consequences of a bivalve invasion in the San Francisco estuary., Environmental Biology of Fishes 67: 277-288

2002-2014 FishBase.(World Wide Web electronic publication)..

Handley, S. J. (1996) Spionid polychaetes in Pacific oysters, Crassostrea gigas (Thunberg) from Admiralty Bay, Marlborough Sounds, New Zealand, New Zealand Journal of Marine and Freshwater Research <missing volume>: 305-309

Hobbs, J. A.; Bennett, W. A.; Burton, J. E. (2006) Assessing nursery habitat quality for native smelts (Osmeridae) in the low-salinity zone of the San Francisco estuary, Journal of Fish Biology 69: 907-922

Hogue, Charles L.; Miller, Scott E. (1981) Entomofauna of Cocos Island, Costa Rica , Atoll Research Bulletin 250: 1-29

Nobriga, Matthew L.; Frederick Feyrer (2007) Shallow-water piscivore-prey dynamics in California’s Sacramento-San Joaquin Delta., San Francisco Estuary and Watershed Science 5(2 (Art. 4)): 1-13

Page, Lawrence M.; Burr, Brooks M. (1991) Freshwater Fishes: North America North of Mexico, Houghton-Mifflin, Boston. Pp. <missing location>

Poursanidis, Dimitirs; Kougioumoutzis, Kostas; Minasidis, Vasileios, Chartosis, Nik; ,Kletou, Demetris; Kalogirou, Stefanos (2022) Uncertainty in marine species distribution modelling: trying to locate invasion hotspots for Pterois miles in the eastern Mediterranean Sea, Journal of Marine Science and Engineering 10(729): Published online

Ro, Hyejoo; Fowler, Amy E.; Wood, Chelsea L.; Blakeslee, April M. H. (2022) Trematode parasites have minimal effect on the behavior of invasive green crabs, Aquatic Invasions 17(2): 238–258

2014-2022 California Fish Website. Web database

Sato-Okoshi, Waka; Abe, Hirokazu; Nishitani, Goh; Simon, Carol A. (2018) And then there was one: Polydora uncinata and Polydora hoplura (Annelida: Spionidae), the problematic polydorid pest species represent a single species, Journal of Marine Biological Association of the United Kingdom 97(8): 1675–1684.

Sommer, Ted and 13 authors (2007) The collapse of pelagic fishes in the upper San Francisco estuary., Fisheries 32(6): 270-277

Swanson; Christina; Reid, Turid; Young, Paciencia S.; Cech Jr, Joseph J. (2000) Comparative environmental tolerances of threatened delta smelt (Hypomesus transpacificus) and introduced wakasagi (H. nipponensis) in an altered California estuary., Oecologia 123: 384-390

Teaca, Adrian; Begun, Tatiana; Menabit,Selma; Muresan,Mihaela (2022) The First Record of Marenzelleria neglecta and the Spread of Laonome xeprovala in the Danube Delta–Black Sea Ecosystem, Diversity 14(423): Published line

2003-2015 Nonindigenous Aquatic Species Database. Gainesville, FL.

Wangkulangkul, Kringpaka; Hayeewachi, Lutfee; Rodcharoen, Eknarin (2022) Changes in benthic macro-invertebrate assemblages in an estuary in southern Thailand after invasion by non-native bivalves Mytilopsis sallei and Mytella strigata, Plankton & Benthos Research 17(2): 137–146

Wilson, Douglas P. (1938) The larval development of Polydora ciliata Johnston and Polydora hoplura Claparede, Journal of Marine Biological Association of the United Kingdom 15: 567- 603

Wilson, Douglas P. (1938) The larval development of Polydora ciliata Johnston and Polydora hoplura Claparede, Journal of Marine Biological Association of the United Kingdom 15: 567- 603