Can Japanese Smelt Replace Delta Smelt?

by Peter Moyle

Wakasagi. Photo by Rene Reyes, USBR

A question I get asked on occasion is: Why all this fuss about endangered delta smelt when there is another smelt that looks just the same that can takes its place? The smelt being referenced is the wakasagi (Hypomesus nipponensis), which is indeed similar to the delta smelt (Hypomesus transpacificus). In fact, both species were once thought to be a single species (H. olidus), the pond smelt, with populations scattered along the Pacific Rim, from California to Japan. In 1963, Don McAllister, a Canadian ichthyologist and smelt expert, examined all populations and concluded that the populations in Japan and California were different from the intervening populations. But he also concluded that the two comprised just one species, with the scientific name noting their trans-Pacific distribution. Because having one freshwater species in two locations separated by thousands of miles made no sense from an evolutionary perspective, the species was later split into two species. This left the delta smelt stranded with the transpacificus epithet, following standard rules of zoological nomenclature.

The natural separation of the two species was broken by biologists from the California Department of Fish and Game (CDFG) in 1959. CDFG thought the pond smelt, then recognized as just one widespread species, would be the perfect forage fish for trout and salmon if planted in cold-water reservoirs. Because of the difficulty of collecting smelt in California, CDFG imported 3.6 million smelt eggs from Japan, where the smelt was cultured as a valuable food fish. The transplant was very successful and populations of wakasagi were soon widely established in reservoirs. The reservoirs included Folsom and Oroville reservoirs, whose water flows into the Sacramento River and Delta, via the American and Feather rivers, respectively. When the reservoirs spilled, smelt were spilled as well. Wakasagi probably reached the Delta by the 1970s but they went largely undetected until the early 1990s, presumably because of their similarity to the abundant delta smelt (Stanley et al, 1995, Trenham et al. 1998).

Once the presence of wakasagi in the Delta was realized, records of capture in sampling programs improved. Biologists trained in fish identification found they can identify wakasagi by sight, using the ‘gestalt’ of slightly narrower body, somewhat larger eye, and somewhat larger mouth; wakasagi also grow to larger sizes than delta smelt so any fish >90 mm is likely a wakasagi. Identifications can be supported by counting dark chromatophores on the ‘chin’ (isthmus; delta smelt has 0 or 1), although a few wakasagi (<5%) also have low numbers of chin chromatophores (Jenkins et al. 2020). Jenkins et al (2020) recommend that field identifications of wakasagi be confirmed with photographs and/or genetic tests to reduce uncertainty.

Wakasagi (top), delta smelt (middle), and wakasagi x delta smelt hybrid (bottom). From Jenkins et al.(2020)

But despite recognition, the wakasagi has remained scarce in the Delta and in rivers below the lowermost dams. In 149,455 trawl or seine pulls made during 1980-2017 in eight long-term surveys, only 364 contained wakasagi. Most of the catch was by the United States Fish and Wildlife Service Beach Seine Survey in the lower Sacramento River, below the mouth of the American River. There was a relatively high catch in 2011, a wet year, presumably of fish being blown out of the reservoirs (D. Stompe, UCD, unpublished analysis). This analysis indicates that most wakasagi in the Delta came from reservoirs.

Around 1997, a bogus scientific paper became widely circulated that claimed that the delta smelt, like the wakasagi, was a Japanese import. This false report gained some credence among water agencies, so Dr. Randy Brown of the Department of Water Resources asked if I would do genetic studies to determine if the claim was false. Not being a geneticist, I arranged with Dr. Bradley Shaffer, UCD herpetologist, to have his graduate students do the study (Stanley et al. 1995, Trenham et al. 1998). The results were clear. The delta smelt was indeed a distinct California species, whose closest relative was the surf smelt (Hypomesus pretiosus), a marine fish common along the California coast. The studies also showed that the closest relative of the wakasagi was also a marine smelt, found along the Japanese coast. The delta smelt and wakasagi were also shown hybridize (see above photo). Hybrid fish are rare and only maternal backcrosses are known, a genetic dead-end, so the rarity of both smelts combined with their distant genetic relationship has maintained the genetic integrity of wild delta smelt (Benjamin et al. 2018).

So, there is no question that the wakasagi is a species of smelt quite distinct from delta smelt, even if they look alike. But can the wakasagi substitute for delta smelt in the San Francisco Estuary? The answer is definitely ‘no’. They likely persist in the Delta mainly through continuous re-introductions from reservoirs. Presumably, the same poor environmental conditions that suppress delta smelt also suppress wakasagi. The failure of this Japanese smelt may also throw light on the potential for delta smelt to become re-established through repeated introduction of captive-bred fish. In contrast, the success of wakasagi in reservoirs suggests promise may also exist in some reservoirs for supporting delta smelt.

Further Reading

Benjamin, A., Sağlam, İ.K., Mahardja, B. et al. 2018. Use of single nucleotide polymorphisms identifies backcrossing and species misidentifications among three San Francisco estuary osmerids. Conservation Genetetics 19: 701–712.

Jenkins, J., N. Ikemiyagi, B. Schreier, B.E. Davis. 2020.  Exploring secondary field identification of delta smelt and wakasagi using image software. IEP Newsletter, October 2020.

Moyle, P. B. 2002. Inland Fishes of California, Revised and Expanded. Berkeley, University of California Press.

Moyle, P., K. Börk, J. Durand, T. Hung, A.L. Rypel. 2019. Futures for Delta Smelt,

Moyle, P., K. Börk, J. Durand, T. Hung, A.L. Rypel. 2021. 2021: Is this the year that wild delta smelt become extinct?

Stanley, S. E., P. B. Moyle, and H. B. Shaffer.  1995.  Allozyme analysis of delta smelt, Hypomesus transpacificus and longfin smelt, Spirinchus thalichthys, in the Sacramento-San Joaquin estuary, California. Copeia 1995: 390-39

Trenham, P. C., H. B. Shaffer, and P. B. Moyle.  1998.  Biochemical Identification and assessment of population subdivision in morphologically similar native and invading smelt species (Hypomesus) in the Sacramento-San Joaquin Estuary, California. Transactions, American Fisheries Society 127: 417-424.

About Andrew Rypel

Andrew L. Rypel is a Professor and the Peter B. Moyle and California Trout Chair of coldwater fish ecology at the University of California, Davis. He is a faculty member in the Department of Wildlife, Fish & Conservation Biology and Director of the Center for Watershed Sciences.
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