By Rosemary Hartman
Delta Smelt
Picture of Delta Smelt, photo by Rene Reyes, US Bureau of Reclamation
Wakasagi
Picture of a Wakasagi, photo by Rene Reyes, US Bureau of Reclamation
You’ve probably heard of Delta Smelt (Hypomesus transpacificus), and you may have heard of their cousin, the Longfin Smelt (Spirinchus thaleichthys), but there is a third osmerid in the estuary. The Wakasagi (Hypomesus nipponensis), also known as Japanese Smelt, is in the same genus as Delta Smelt, and was once thought to be the same species. It is native to Japan, but was introduced to reservoirs in California by the California Department of Fish and Game in the 1950s, and now it is established throughout the watershed, including the Delta.
But what does this brother of the Delta Smelt do? Is there sibling rivalry? A group of IEP scientists was curious, so they decided to look at all of our existing data to see when, where, how big, and how many Wakasagi are in the Delta and how their environmental tolerances and diet compares to Delta Smelt. A paper about their analysis recently came out in the Journal San Francisco Estuary and Watershed Sciences.
In order to compare Delta Smelt and Wakasagi, they looked at all the data from thirty different fish datasets from San Francisco Bay, Suisun Marsh/Suisun Bay, the Delta, and the watershed (see map below). This resulted in a dataset with over 250,000 individual Wakasagi! They also looked at data from special studies of Wakasagi and Delta Smelt growth and diets in the Yolo Bypass.
Maps of the San Francisco Bay-Delta Estuary. (A) Four long-term CDFW monitoring surveys and region assignments used for the comparative Delta Smelt and Wakasagi analysis. (B) Sampling locations for a subset of additional surveys used to assess Wakasagi catch, as well as Yolo Bypass surveys used to assess life-history traits including growth, phenology, and diet. Map reproduced from Davis et al, 2022, with permission.
They found some similarities between delta smelt and Wakasagi – both fish really like hanging out in the Sacramento Deep Water Ship Channel and both like eating calanoid copepods (a particularly tasty variety of zooplankton). They spawn at about the same time, but Wakasagi are usually a little earlier (though this varies from year to year), and Wakasagi usually grow a little faster. They are similar enough that they sometimes interbreed and produce hybrid offspring.
Wakasagi aren’t the same as Delta Smelt though. Wakasagi aren’t actually very common in the Delta, instead finding their homes further upstream in reservoirs (they especially seem to like the Feather River, the screw trap there catches tens to hundreds of thousands of Wakasagi per year!). In the Delta they are mostly in the northern region, which might be just them washing in from upstream. Though they were mostly found in freshwater reaches of the Delta, Wakasagi can actually tolerate a wider range of salinity and temperatures than Delta Smelt, but they seem to prefer cooler temperatures.
So, are Wakasagi competing with Delta Smelt for limited food resources? Maybe a bit, but while they play a similar ecological role when they do overlap, they don’t overlap spatially very often, and both Delta Smelt and Wakasagi are rare in the Delta. However, they overlap enough that areas that are good for Wakasagi are probably good for Delta Smelt too. Delta Smelt are becoming more and more endangered, so we can use Wakasagi as indicators of good Delta Smelt conditions and as substitutes for smelt in some laboratory experiments.
Major similarities and differences between Delta Smelt and Wakasagi
| Delta Smelt |
Wakasagi |
Comparison |
| Annual life span |
Annual life span |
 |
| Spawn later |
Spawn earlier |
 |
| Eat calanoid copepods |
Eat calanoid copepods |
|
| Grow slower |
Grow faster |
|
| Narrower tolerances |
Wider tolerances |
|
| Endangered |
More common |
|
| Native |
Non native |
|
| Mostly semi-anadromous |
Mostly freshwater |
|
| Small and silver |
Small and silver |
|
| Loves the North Delta |
Loves the North Delta |
|
| Smells like cucumber |
Smells like fish |
|
Further reading
- Davis BE, Adams JB, Lewis LS, Hobbs JA, Ikemiyagi N, Johnston C, Mitchell L, Shakya A, Schreier B, Mahardja B. 2022. Wakasagi in the San Francisco Bay–Delta Watershed: Comparative Trends in Distribution and Life-History Traits with Native Delta Smelt. San Francisco Estuary and Watershed Science. 20(3).
- Calfish Website (UC Davis)
- Swanson C, Reid T, Young PS, Cech JJ. 2000. Comparative environmental tolerances of threatened delta smelt (Hypomesus transpacificus) and introduced wakasagi (H. nipponensis) in an altered California estuary. Oecologia. 123:384-390
- Fisch KM, Mahardja B, Burton RS, May B. 2014. Hybridization between delta smelt and two other species within the family Osmeridae in the San Francisco Bay-Delta. Conservation Genetics. 15(2):489-494