Fish Active day and night

Delta Smelt

Hypomesus transpacificus

iNaturalist Wikipedia

The Delta Smelt is the 'canary in the coal mine' for California's water health. This tiny, translucent resident of the San Francisco Bay-Delta offers a rare glimpse into a fragile ecosystem found nowhere else on the planet.

0 Sightings

0 Habitats

Quick Identification

straighten

Size

5 to 7 cm (2 to 2.8 inches) in length; weighs approximately 1 to 4 grams.

palette

Colors

Translucent, pale silvery-blue body; faint iridescent lateral stripe with purple or pinkish hues; small dark spots on the head and back.

visibility

Key Features

Translucent, slender 'cigar-shaped' body
Small mouth that does not extend past the middle of the eye
Presence of a small adipose fin between the dorsal and tail fins
Emits a distinct, fresh cucumber-like scent when handled

When You’ll See Them

schedule

Activity pattern Active day and night

brightness_5

Peak hours 6-10 AM, 5-8 PM (dictated by tidal shifts)

calendar_month

Season December-May

restaurant

Diet Primary planktivores that feed on small crustaceans, specifically copepods and cladocerans, which they filter from the water column.

park

Habitat Estuarine mixing zones, brackish marshes, and freshwater sloughs with slow-moving water.

public Geographic range

Where Does the Delta Smelt Live?

The Delta Smelt is native exclusively to the North American continent, found only in a tiny corner of the United States. It is endemic to the Sacramento-San Joaquin River Delta and the Suisun Bay in California. This highly restricted range is limited to the upper San Francisco Estuary, making it one of the most geographically constrained fish species in the world.

Basemap © OpenStreetMap contributors

1 Countries

1.2K km² Range

Critically Endangered Conservation

US United States

eco

iNaturalist / Verified observation data

View on iNaturalist open_in_new

Explore more Fish arrow_forward

Behavior

The Delta Smelt is a short-lived species, typically living only one year. They are semi-anadromous, spending the majority of their lives in the brackish 'mixing zone' where fresh and salt water meet, but migrating upstream to freshwater sloughs and channels to spawn during the late winter and spring. They are weak swimmers compared to other salmonids and largely rely on tidal currents to move through the estuary.

Socially, Delta Smelt are schooling fish that move in loose groups to feed and avoid predators. They are extremely sensitive to environmental changes, particularly water temperature and salinity, making them a critical indicator species. When the Delta Smelt population thrives, it suggests the entire San Francisco Bay-Delta ecosystem is healthy; conversely, their decline signals significant environmental stress.

photo_camera EverydayEarth exclusive

Camera Tips

Capturing the elusive Delta Smelt requires an underwater setup, such as a specialized aquatic trail camera or a high-end action camera in a waterproof housing. Because these fish are nearly translucent, you need a camera with excellent contrast settings and high-resolution capabilities. To increase your chances, mount your camera to a dock piling or a weighted tripod in shallow, brackish water where the current is slow and steady.

Lighting is the most critical factor for these small, silvery fish. Use off-camera LED lights positioned at a 45-degree angle to the lens to prevent backscatter from silt and to catch the iridescent sheen on the fish's sides. Avoid direct flash, which can wash out their translucent bodies and make them appear like featureless white streaks in the water.

Set your camera to record at a high frame rate (60fps or higher) to capture their darting movements as they feed on plankton. Since these fish are highly endangered, do not use invasive baits; instead, rely on placing the camera near submerged vegetation or known spawning paths during the 'first flush' of winter rains. Use a wide-angle lens to maximize the field of view in the often murky waters of the Delta.

Similar Species

Species that look similar or are commonly confused with Delta Smelt.

Longfin Smelt

Has much longer pectoral fins that reach past the base of the pelvic fins and more prominent spotting on the body.

Browse all Fish arrow_forward

Frequently Asked Questions

Delta Smelt are active throughout the day and night, but their peak activity is often driven by tidal cycles rather than the sun. They are most frequently observed feeding near the surface during dawn and dusk when light levels are low but visible.

Unless your backyard includes a private dock on the Sacramento-San Joaquin Delta, you cannot attract them. The best way to support them is by advocating for water conservation and reducing pesticide runoff, which helps maintain the water quality they need to survive.

Delta Smelt are planktivores. They exclusively eat tiny organisms drifting in the water, with their favorite meal being a type of small crustacean called a copepod.

No, they are extremely rare and only found in the specific brackish and freshwater channels of the California Delta. They are never found in suburban ponds or inland lakes.

Look at the pectoral fins; on a Delta Smelt, they are short and do not reach the pelvic fins. Additionally, Delta Smelt have a translucent appearance and a smaller mouth compared to the more heavily spotted Longfin Smelt.

Record Delta Smelt at your habitat

Connect a camera to start building your own species record — AI identifies every visitor automatically.

Join free Identify a photo