Delta Smelt
Hypomesus transpacificus
Type: freshwater/estuarine
Physical Characteristics
Slender, nearly translucent body with a steel-blue lateral stripe. They have an iridescent sheen and a small adipose fin. The body is elongated with large eyes and a relatively small mouth.
Habitat & Distribution
Endemic to the Sacramento-San Joaquin River Delta in California, typically found in low-salinity estuarine environments and fresh water.
Size & Weight
Usually grows to 5.0 to 7.0 cm (2 to 2.8 inches) in length; maximum size around 12 cm. They are very light, usually a few grams.
Similar Looking Species
Wakasagi (Hypomesus nipponensis) and Longfin Smelt (Spirinchus thaleichthys). They can be distinguished by the number of pectoral fin rays and the position of the dorsal fin.
Ecological Role
A key indicator species for the health of the Sacramento-San Joaquin Delta ecosystem; serves as a primary link in the food web between plankton and larger predators.
Diet
Primarily zooplankton, specifically small crustaceans like copepods and cladocerans.
Behavior
A semi-anadromous species that moves seasonally within the estuary to follow the salinity gradient. They are generally short-lived and school for protection.
Reproduction
Annual life cycle; most individuals live only one year. Spawning occurs in late winter and spring in freshwater sloughs and river channels.
Conservation Status
Critically Endangered (IUCN); listed as Endangered under the Federal Endangered Species Act and the California Endangered Species Act.
Identification Tips
Look for a translucent body, an adipose fin, and a cucumber-like scent when freshly handled. Its position in the water column and location in the Delta are also key.
Interesting Facts
Delta Smelt are famous for having a distinct smell similar to sliced cucumbers. Due to their one-year lifespan, their population is highly susceptible to environmental fluctuations and water export levels.
Identified on: 5/20/2026
Mode: Standard