Fish diurnal

Delta Smelt

Hypomesus transpacificus

A delicate indicator of California's water health, the Delta Smelt is a translucent marvel found nowhere else on Earth. Documenting this rare, cucumber-scented fish offers a front-row seat to the conservation of one of the world's most complex estuaries.

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Quick Identification

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Size

5 to 7 cm (2 to 2.8 inches) in length; weighing only a few grams.

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Colors

Translucent body with a faint silvery stripe along the sides; often exhibits a pale blue or steel sheen on the head and back.

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Key Features

Slender, nearly transparent body
Distinctive odor of sliced cucumber when handled
Small adipose fin and deeply forked tail
Relatively large eyes for its head size

When You’ll See Them

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Activity pattern diurnal

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Peak hours Daylight hours for feeding; dawn and dusk for migratory movement.

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Season December to May (during winter spawning migration)

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Diet A specialized carnivore that feeds on zooplankton, specifically small crustaceans like copepods and cladocerans.

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Habitat Open waters of the Sacramento-San Joaquin Delta, specifically the low-salinity zone and freshwater spawning grounds.

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Behavior

The Delta Smelt is a short-lived species, typically completing its entire life cycle in just one year. They are pelagic fish, meaning they spend most of their time in the open water column rather than hiding in vegetation or resting on the bottom. They are highly sensitive to environmental changes, functioning as a "canary in the coal mine" for the San Francisco Bay-Delta's water quality. Because they are euryhaline, they can handle a range of salt levels but prefer the "mixing zone" where freshwater and saltwater meet.

These fish are not aggressive and move in loose schools to forage. They are primarily driven by the "first flush" of winter rains, which triggers their migration from the brackish Suisun Bay upstream into the freshwater reaches of the Sacramento and San Joaquin rivers to spawn. Despite their environmental importance, they are incredibly fragile and have seen a massive population decline due to habitat loss and water diversions.

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Camera Tips

Capturing the Delta Smelt on a trail camera is a unique challenge that requires specialized underwater equipment and a location near the Sacramento-San Joaquin Delta. Use a waterproof action camera or a dedicated underwater trail cam with a high-resolution sensor. Because these fish live in turbid waters where visibility is often limited to a few feet, you should place your camera in shallower freshwater channels during the winter and early spring when the fish move upstream to spawn. Secure the camera to a weighted base or a pier piling at a depth of 3 to 6 feet.

Lighting is the most critical factor when filming in the Delta's murky environment. Avoid using high-intensity white LEDs, which reflect off suspended silt and create a "snowstorm" effect in your footage. Instead, try to use natural light by positioning the camera to face upward towards the surface, allowing the silvery, translucent bodies of the smelt to be silhouetted against the sky. If you must use artificial light, diffused infrared or low-intensity warm lights are best to avoid startling the fish.

Set your camera to record at a high frame rate, such as 60 or 120 frames per second. Delta Smelt are small and can dart through the field of view very quickly; high-speed recording allows you to slow the footage down for a positive identification. Since these are an endangered species, it is vital to avoid any bait or attractants that could disrupt their natural behavior or draw them toward dangerous water intake structures. Focus instead on narrow channels or areas with gentle tidal flow where zooplankton naturally concentrates.

Patience is key, as Delta Smelt are now incredibly rare. Check your camera housing for algae growth or silt buildup every few days. The best time to capture footage is during a high tide when the water is slightly clearer, or immediately after a light rain which stimulates their movement but before the water becomes too muddy from heavy runoff.

Similar Species

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

Wakasagi

The Wakasagi is less translucent and has more anal fin rays (12-13) compared to the Delta Smelt (8-10).

Longfin Smelt

Longfin Smelt have much longer pectoral fins that reach past the base of the pelvic fins, whereas Delta Smelt fins are much shorter.

Frequently Asked Questions

Delta Smelt are primarily diurnal, meaning they are active during the day. They rely on their sight to hunt tiny zooplankton in the water column, so their activity is highest when there is enough light to spot their microscopic prey.

Delta Smelt cannot be attracted to a traditional backyard because they are strictly aquatic and endemic to the San Francisco Bay-Delta. If you live along a Delta waterway, you can support them by reducing chemical runoff and maintaining natural shorelines, which helps preserve the water quality they need to survive.

The Delta Smelt's diet consists almost entirely of zooplankton. They are especially dependent on small crustaceans called copepods. The decline of native copepods in the Delta is one of the primary reasons the smelt population has struggled.

No, they are one of the rarest fish in North America and are currently critically endangered. While they may be present in the waterways near suburban communities in the Sacramento-San Joaquin Delta region, they are rarely seen without professional monitoring equipment.

The Delta Smelt is best identified by its translucent body and a faint silvery stripe. A unique (though not visually helpful) trait is that they smell like fresh cucumbers. They are often confused with the invasive Wakasagi, which has more rays in its anal fin and is less transparent.

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