Green Sturgeon

A sturgeon that lives at sea

Green sturgeon are the outlier of the North American sturgeon family. Where lake sturgeon and shovelnose live out their lives in rivers and lakes, and white sturgeon can complete their life cycle without ever reaching salt water, green sturgeon spend the majority of adult life in the ocean and in coastal estuaries, entering rivers principally to spawn. NOAA Fisheries describes a range from Alaska to Mexico, most common north of Point Conception, California.

The body plan is standard for the family and superbly adapted to what it does: a cartilaginous skeleton, an upturned heterocercal tail, five rows of bony scutes running the length of a scaleless olive-green body, four barbels beneath a long tapered snout, and a protrusible toothless mouth. As in all sturgeon, the snout carries dense fields of ampullary electroreceptors — the ampullae of Lorenzini, described morphologically for the family by Jørgensen — which detect the bioelectric fields of prey buried in soft sediment. In the turbid estuarine and marine bottoms where green sturgeon feed, that sense does most of the work.

Two population segments, one listed

NOAA Fisheries divides the species into two distinct population segments, and the division is geographic and reproductive rather than cosmetic. In the 2006 listing rule the agency defined the Northern DPS as populations in coastal watersheds northward of and including the Eel River, and the Southern DPS as coastal and Central Valley populations south of the Eel River.

The Southern DPS was listed as threatened under the Endangered Species Act on 7 April 2006. The Northern DPS is not federally listed. The listing decision turned on a single structural vulnerability, stated directly in the rule: the Sacramento River contains the only known green sturgeon spawning population in this DPS, and the concentration of spawning adults in one river places this DPS at risk.

That is the whole argument in one sentence. A population that reproduces in one river has no redundancy. A chemical spill, a barrier, a temperature excursion or a poor water year in that river is not a local setback; it is a population-level event. The Northern DPS, by contrast, spawns across the Klamath, Eel and Rogue rivers, and that distribution is what separates the two segments’ status.

Spawning

Green sturgeon spawn in deep, cool river pools over hard substrate. The listing rule reports that green sturgeon spawn in water temperatures ranging from 8° to 14° C — roughly 46 to 57°F — a distinctly cool window compared with the interior sturgeons. Spawning in the Southern DPS occurs in the Sacramento, with use of the lower Feather and lower Yuba rivers, and evidence of spawning in the upper Sacramento has been documented in the peer-reviewed literature by Brown.

Spawning is intermittent, as it is throughout the family. Work on the Rogue River population by Erickson and colleagues characterised spawning periodicity, migration and size at maturity, and Webb and colleagues described the reproductive structure of that adult population — the kind of demographic detail that is available for the unlisted northern fish and remains harder to obtain for the listed southern segment.

Fine-scale spawning habitat selection within the Sacramento has been mapped by Wyman and colleagues across three spawning locations, and adult migration through the system was described by Heublein and colleagues. More recently, Battaile and colleagues estimated spawning green sturgeon abundance in the Sacramento River using side-scan sonar and N-mixture models — an indication of how difficult it is to count a deep-holding, long-lived, infrequently spawning fish.

Longevity and late maturity

NOAA gives a lifespan of 60 to 70 years, with adults averaging 4.5 to 6.5 feet and reaching up to 350 pounds. As with every sturgeon, maturity arrives late and spawning is not annual. The result is the same demographic arithmetic that governs the family: adult survival dominates population trajectory, recruitment failures are invisible for well over a decade, and recovery from depletion runs on a multi-decade clock regardless of how favourable conditions become.

For a population that already spawns in a single river, that slowness compounds the risk. There is no fast-cycling portion of the life history that can rebuild numbers between disturbances.

Dams and the loss of spawning habitat

The threats section of the listing rule is largely a list of dams. Keswick Dam blocks access to historical spawning grounds on the upper Sacramento. Oroville Dam eliminated spawning habitat on the Feather River. The Red Bluff Diversion Dam and comparable structures were identified as potential adult migration barriers. Where the river remains accessible, water quality can still exclude the fish: NOAA noted that high water temperatures may pose a problem on the Feather River downstream of the Thermalito Afterbay outlet.

Beyond outright barriers, the rule identifies juvenile entrainment through the Central Valley’s water diversions and fishing mortality as ongoing concerns. The Sacramento–San Joaquin Delta, which every Southern DPS fish must transit twice, is among the most heavily plumbed estuaries in the world. The U.S. Geological Survey has tracked how green sturgeon actually use it, documenting adult movements in the Delta from December 2020 to January 2023 and juvenile movement in the lower Sacramento River from 2016 to 2020.

Critical habitat

NOAA designated critical habitat for the Southern DPS on 9 October 2009, effective 9 November 2009. The scale of the designation reflects how much of the species’ life is spent outside its spawning river. The rule designates approximately 515 kilometres (320 miles) of freshwater river habitat, 2,323 km² (897 mi²) of estuarine habitat, 29,581 km² (11,421 mi²) of marine habitat, 784 km (487 mi) of habitat in the Sacramento–San Joaquin Delta, and 350 km² (135 mi²) of habitat within the Yolo and Sutter bypasses.

Geographically that covers the Sacramento, lower Feather and lower Yuba rivers; the Delta and Suisun, San Pablo and San Francisco bays; the lower Columbia River estuary; Humboldt Bay in California; Coos, Winchester, Yaquina and Nehalem bays in Oregon; Willapa Bay and Grays Harbor in Washington; and coastal marine waters within 60 fathoms depth from Monterey Bay north to Cape Flattery, Washington. NOAA subsequently established protective regulations for the Southern DPS under section 4(d) of the Act in June 2010.

Estuaries, coastal movement and mixed aggregations

One of the more consequential findings of the last two decades is that green sturgeon from both DPSs mix in coastal estuaries far from their natal rivers. Schreier and colleagues investigated DPS composition in non-natal estuaries and reported preliminary evidence of Columbia River spawning — work that matters directly for management, because it means listed Southern DPS fish turn up in waters hundreds of miles north of the Sacramento, in estuaries where the local sturgeon are assumed to be northern.

That is why the critical habitat designation extends up the coast to Cape Flattery and into Washington and Oregon bays. The listed population is not confined to California, and a green sturgeon encountered in Willapa Bay or Grays Harbor cannot be assigned to a population segment on sight.

Identification and encounters

Green sturgeon are distinguished from white sturgeon by colour — olive to dark green with an olive-green ventral stripe, against the grey of a white sturgeon — by a longer, more tapered snout, by barbels set closer to the mouth than to the snout tip, and by generally fewer, larger lateral scutes. A single scute is typically present behind the dorsal and anal fins, which white sturgeon lack. Getting this right matters more than usual: one of these fish is federally threatened across the southern part of its range and the other is not.

Green sturgeon are a protected species rather than a target fishery across the Southern DPS, and the regulations governing any encounter are set by NOAA Fisheries and the states; this site links to them rather than restating them. What is worth carrying into the water is the identification, the knowledge that the Southern DPS reproduces in exactly one river, and the recognition that a large green sturgeon may be sixty years old and among a small number of adults sustaining a threatened population.