Acipenser medirostris

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General Information
Common Name: 
Northern green sturgeon
FID: 
AAM01
Status: 

Conservation Status in California: Class 3, Near threatened (Moyle et al. 2011).
The southern (Central Valley) green sturgeon Distinct Population Segment (DPS) is listed as a threatened species. However, very little is known about the North Coast green sturgeon (northern DPS), which remains subject to harvest.

Life History: 

Life History: The ecology and life history of green sturgeon have received little study until recently, because of their generally low abundance in most estuaries and their low commercial and sport-fishing value in the past. Adults are more marine than white sturgeon, but can spend up to six months in fresh water (Benson et al. 2007, Erickson et al. 2002).
Spawning populations of the northern DPS are limited to the Rogue and Klamath Rivers. Green sturgeon migrate up the Klamath River between late February and late July. The spawning period is March-July, with a peak from mid-April to mid-June (Emmett et al. 1991, Van Eenennamm et al. 2006, Benson et al. 2007). Although the spawning period is similar in the Rogue River, post-spawn adults are found in fresh water in both the spring and fall (Webb and Erickson 2007). In the fall, adults are most likely using the Rogue River for foraging. Spawning females are generally larger, heavier, older and in better condition than spawning males (Van Eenennamm et al. 2006, Benson et al. 2007, Erickson and Webb 2007). From 1999 to 2003, the length of spawning females in the Klamath River was 151-223 cm FL while males measured 139-199 cm FL. In the Rogue River, male and female green sturgeon become sexually mature at 145 cm TL and 166 cm TL, respectively (Erickson and Webb 2007). Most females were 19-34 years old, while males were 15-28 years old. Males are slightly more abundant than females in the spawning runs (female:male = 1:1.38). Adults in the Klamath River exhibit four distinct migration patterns characterized by varying lengths of freshwater residency of up to 199 days (Benson et al. 2007). Individuals migrate at rates of 1.18 to 2.15 km per day. Individuals do not appear to spawn in successive years but rather at two or more year intervals (Erickson and Webb 2007, Webb and Erickson 2007).
According to Moyle (2002, p. 110): “Spawning takes place in deep, fast water. In the Klamath River, a pool known as The Sturgeon Hole (1.5 km upstream from Orleans, Humboldt County) apparently is a major spawning site, because leaping and other behavior indicative of courtship and spawning are often observed there during spring and early summer.” Female green sturgeon produce 51,000 to 224,000 (Adams et al. 2002), with an average diameter of 4.33 mm (Van Eenennaam et al. 2006). Based on their presumed similarity to white sturgeon, green sturgeon eggs probably hatch around 196 hours (at 12.7 ?C) after spawning, and the larvae should be 8-19 mm long; juveniles likely range in size from 2.0 to 150 cm (Emmett et al. 1991). Morphological (large pectoral fins) and behavioral (rostral wedging) traits allow smaller green sturgeon to hold in rivers for extended periods of time (Allen et al. 2006). Juvenile green sturgeon appear to be largely nocturnal in their migratory, feeding, and rearing behavior during the first 10 months of life (Kynard et al. 2005). Green sturgeon retinas are dominated by rods, further supporting the idea that green sturgeon evolved to live in dim environments (Sillman et al. 2005).
Juveniles migrate out to sea before 2 years of age, primarily during summer-fall (Emmett et al. 1991). Length-frequency analyses of sturgeon caught in the Klamath Estuary by beach seine indicates that most green sturgeon leave the system at lengths of 30-60 cm, when they are 1 to 4 years old, although a majority apparently leave as yearlings (USFWS 1982). Although juvenile green sturgeon can withstand brackish (10 ppt) water at any age, the ability to osmoregulate in salt water develops around 1.5 years of age (Allen and Cech 2007). They apparently remain near estuaries at first, but can migrate considerable distances as they grow larger (Emmett et al. 1991). In the ocean, adults complete deliberate annual migrations northward in the fall and southward in the spring (Lindley et al. 2008). Important overwintering habitats have been identified between Cape Spencer, Alaska, and Vancouver Island. Adults can migrate more than 50 km per day during return spring migrations. Individuals from all spawning populations are known to congregate at Willapa Bay, Washington, in the summer (Moser and Lindley 2007).
Green sturgeon grow approximately 7 cm per year until they reach maturity at 130-140 cm, around age 15-20. Thereafter growth slows down. The maximum size seems to be around 230 cm (USFWS 1982). The oldest fish known are 42 years, based on annuli on fin rays, but the largest fish are probably much older (T. Kisanuki, pers. comm., 1995). Juveniles and adults are benthic feeders on both invertebrates and fish. Adult sturgeon caught in Washington had been feeding mainly on sand lances (Ammodytes hexapterus) and callianassid shrimp (P. Foley, pers. comm., 1992). In the Columbia River estuary, green sturgeon are known to feed on anchovies, and they perhaps also feed on clams (C. Tracy, minutes to USFWS meeting). Adults may optimize growth in the summer by feeding on burrowing shrimp in the relatively warmer waters of Washington estuaries (Moser and Lindley 2007).

Habitat Requirements: 

Habitat Requirements: The habitat requirements of green sturgeon are poorly known, but spawning and larval ecology probably are presumably similar to that of white sturgeon. Preferred spawning substrate likely is large cobble, but can range from clean sand to bedrock (Nguyen and Crocker 2007). Eggs are broadcast-spawned and externally fertilized in relatively fast water, apparently at depths >3 m (Emmett et al. 1991). The importance of water quality is uncertain, but silt can prevent the eggs from adhering to each other (Gisbert et al. 2001) Furthermore, sand can impair the growth and survival of larval green sturgeon by decreasing feeding effectiveness (Nguyen and Crocker 2007).
Temperature appears to be closely linked to migration timing. In the Rogue River, adults enter freshwater from March to May when water temperatures range from 9 to 16 °C (Erickson and Webb 2007). Adults may hold in deep (>5 m) pools with low velocities after spawning for up to six months (Erickson et al. 2002, Benson et al. 2007). Adult river outmigration initiates with low river temperatures ( 100 cms). Juveniles appear to prefer dark deep pools with large rock substrate during winter rearing (Kynard et al. 2005). Nocturnal downstream migration by juveniles continues until water temperatures decreased to about 8°C (Kynard et al. 2005).

Environmental Tolerances: Temperature has a major influence on green sturgeon physiology and survival. The upper thermal limit for developing embryos is 17- 18 °C (Van Eenennaam et al. 2005). Incubation temperatures above 22°C result in deformities (Mayfield and Cech 2004, Werner et al. 2007) and or mortality (Van Eenennaam et al. 2005) of developing embryos. Although age 1 to 3 year old green sturgeon appear to tolerate moderate changes in water temperatures (Kaufman et al. 2007), optimal temperatures for age 1 juvenile sturgeon range from 11 to 19 °C. In this same age group, temperatures between 19 and 24 °C increase metabolic costs, while temperatures above 24 °C cause severe stress (Mayfield and Cech 2004). However, the metabolic costs associated with temperatures between 19 and 24 °C may be offset when food and oxygen are abundantly available, resulting in unimpaired growth (Allen et al. 2006). Kaufman et al. (2006) determined that juvenile green sturgeon also are limited in their ability to handle increases in CO2. However, time of day, length of exposure and temperature can affect the ability of green sturgeon juveniles to respond to stress (Lankford et al. 2003, Werner et al. 2007).

Distribution: 

Distribution: In the ocean off North America, green sturgeon have been caught from the Bering Sea to Ensenada, Mexico, a range which includes the entire coast of California, but except for a few tagged fish, it is not known from which DPS ocean-caught sturgeon originate. Migrations generally follow northern routes along shallow waters within the 110 m contour, with individuals from all populations congregating in Willapa Bay (BRT 2005, Moser and Lindley 2007). There are records of green sturgeon from rivers in British Columbia south to the Sacramento River. There is no evidence of green sturgeon spawning in Canada or Alaska, although small numbers have been caught in the Fraser, Nass, Stikine, Skeena and Taku rivers, British Columbia (COSEWIC 2004). Green sturgeon are common in the Columbia River estuary and have been observed as much 225 km inland in the Columbia River, prior to the construction of Bonneville Dam (Wydoski and Whitney 1979). However, they apparently do not spawn in the Columbia River or other rivers in Washington. In Oregon, juvenile green sturgeon have been found in several coastal rivers (Emmett et al. 1991) but spawning is confirmed only in the Rogue River (Erickson et al. 2002, Erickson and Webb 2007). For the north coast green sturgeon DPS spawning has been confirmed in recent years only in the Klamath and Rogue rivers, although spawning probably once occurred in the Eel and Umpqua rivers as well (Moyle 2002, Adams et al. 2007). Overall, it is likely that sturgeon from this DPS once spawned in the larger rivers from the Eel River in California north to the Columbia River in Oregon/Washington. The Klamath River, however, has presumably always been the principal spawning river, based on size, flow/temperature regime, and habitat availability.

Abundance Trends: 

Trends in Abundance: In California, green sturgeon have been collected in small numbers in marine waters from the Mexican border to the Oregon border. They have been noted in a number of rivers, but spawning populations are known only in the Rogue, Sacramento and Klamath Rivers (see below). The following distributional information on green sturgeon in California waters was compiled by Patrick Foley (University of California, Davis, 1992) and updated from information in Adams et al. (2007)

North Coast. From the Eel River northward, it is likely that most records of sturgeon caught in rivers and estuaries refer to north coast green sturgeon. However, most early references regarding sturgeon from the north coast did not identify the species and some reports indicated white sturgeon to be more abundant (Fry 1979). While white sturgeon do occur on occasion in the Klamath and other rivers, it is highly likely that most historic records are for north coast green sturgeon. Accounts from 19th century newspapers (The Humboldt Times) report sturgeon from the mainstem Eel River, South Fork of Eel River and the Van Duzen River (Wainwright 1965). Length and weights given in these newspaper accounts are most consistent with those of adult green sturgeon.
In the middle part of this century, two young green sturgeon were collected in the mainstem Eel River and large sturgeon were observed jumping in tidewater (Murphy and DeWitt 1951). Two additional young green sturgeon were taken from the Eel River in 1967 and are in the fish collection at Humboldt State University. Substantial numbers of juveniles were caught by CDFG in the mainstem Eel River during trapping operations in 1967-1970 (O'Brien et al. 1976): 22 at Eel Rock in 1967, 53 at McCann in 1967 and 161 in 1969, 221 at Fort Seward in 1968, and smaller numbers at other localities. Green sturgeon have been included in lists of natural resources found in the Eel River Delta (Monroe and Reynolds 1974, Blunt 1980). However, CDFG biologists D. McCleod and L. Preston observed a 1+ m long sturgeon, most likely a green sturgeon, in a gravel extraction trench in the mainstem Eel upstream of the Blue Lake Bridge (river mile 16) on May 20, 1992. Green sturgeon are still occasionally seen in the Eel River (Adams et al. 2007). One green sturgeon was detected near Cock Robin Island in the Eel River estuary in 2008 (S. Lindley, pers. comm.).
Records of sturgeon in the Humboldt Bay system, comprising Arcata Bay to the north and Humboldt Bay to the south, are almost exclusively green sturgeon. Ten years of trawl investigations in South Humboldt Bay produced three green sturgeon (Samuelson 1973). Records from Arcata Bay are more numerous. On August 6 and 7, 1956, 50 green sturgeon were tagged in Arcata Bay by CDFG biologist Ed Best (D. Kohlhorst, pers. comm.). Total length ranged from 57.2 cm to 148.6 cm with a mean TL of 87.0 cm (± 20.6 cm SD). In 1974, nine green sturgeon were collected over a two-month period in Arcata Bay (Sopher 1974). Total length of these fish ranged between 73-112 cm. The Coast Oyster Company, Eureka, pulls an annual series of trawls in Arcata Bay in order to decrease the abundance of bat rays, Myliobatis californica. Green sturgeon are incidentally taken in this operation. Eight green sturgeon collected for parasite evaluation in 1988 and 1989 had total lengths ranging between 78-114 cm. One large individual, 178 cm TL and 18.2 kg, was returned to the bay.
Green sturgeon have been reported from the Mad River (Fry 1979), but recent evidence of their presence is scant (Bruce Barngrover, pers. comm. 1992).
An occasional green sturgeon is encountered in the coastal lagoons of Humboldt County (Terry Roelofs, pers. comm. 1992). Big Lagoon and Stone Lagoon are connected to the ocean during part of the year and migrating sturgeon may gain entry at this time. In June 1991, a 120-cm green sturgeon was gillnetted in Stone Lagoon (Terry Roelofs, pers. comm. 1992). In 2007, green sturgeon tagged with sonic tags were detected moving in and out of Humboldt Bay in array set up to study the movements of coho salmon (S. Lindley, USFWS, unpublished). Both the northern and southern DPS use Humboldt Bay during the spring and fall (S. Lindley, pers. comm.).

Klamath and Trinity Rivers. The largest spawning population of green sturgeon in California is in the Klamath River Basin. Both green sturgeon and white sturgeon have been found in the Klamath River estuary (Snyder 1908a, USFWS 1980-91) but white sturgeon are taken infrequently, in very low numbers, and are presumed to be coastal migrants (USFWS 1982). A sturgeon investigation program, initiated in 1979 by USFWS, found that almost all sturgeon occurring above the estuary were green sturgeon (USFWS 1980-83). The sturgeon primarily use the mainstem Klamath River and mainstem Trinity River, but have also been seen in the lower portion of the Salmon River as well (Adams et al. 2007).
Both adults and juveniles have been identified in the mainstem Klamath River. Adults are taken annually, spring and summer, by an in-river Native American gillnet fishery. The numbers average around 500 fish per year (see below). They have also been taken by sport fishermen as far inland as Happy Camp (river km 172) (unpubl. CDFG Tagging Data 1969-73, Fry 1979, USFWS 1981). However, the apparent limit for the spawning migration is Ishi Pishi Falls, upriver from Somes Bar, Siskiyou County (approximately river km 113). A few juveniles have been taken as high up as Big Bar (river km 81) (Tom Kisanuki, pers. comm.), but most have been recovered by seining operations directed at salmonids in the tidewater (USFWS, CDFG). Sampling by the USFWS captured 7 juveniles in (June) 1991 and 23 in (June-July) 1992 (T. Kisanuki, pers. comm.1995). Catch data from six outmigrant traps placed in the Klamath River reported that juvenile green sturgeon were caught every year (2000-2005) during trap operations (unpubl. Cunanan and Hines 2006, USFWS). The number of green sturgeon captured each year varied from one (2005) to 775 (2003). The total number of juvenile green sturgeon captured over the six years of operation was 1599. The size of captured green sturgeon varied from a minimum of 20 mm to a maximum of 252 mm and averaged 68.5 mm. Green sturgeon captured by the traps were most likely juveniles ranging in age from a couple of weeks to less than two years old, based on growth curves developed by Nakamato et al. (1995) and Van Eenennamm et al. (2001). However, the average size (68.5 mm) was similar to the size of artificially reared Klamath River green sturgeon at 35 days old (66.4 mm; Van Eenennaam et al. 2001).
The Trinity River enters the Klamath River at Weitchpec (river km 70). The earliest green sturgeon described from the Klamath Basin came from the Trinity River (Gilbert 1897). Both adults and juveniles have been identified; 211 sturgeon, between 7-29 cm TL, were captured near Willow Creek, Humboldt County, incidental to a salmonid migration study in July-September, 1968 (Healey 1970). The USFWS has collected juvenile green sturgeon in recent years from the Trinity River, as far up as Big Bar: 2 (in 1989), 0 (1990), 6 (1991) and 36 (1992) (T. Kisanuki, pers. comm.). Adults are caught yearly in a Native American gillnet fishery (USFWS 1980), a traditional fishery with a long history (Kroeber and Barrett 1960). Spawning migrants penetrate the mainstem Trinity River up to about Grays Falls, Burnt Ranch, Trinity County (river km 72).
Sturgeon have also been reported to use the South Fork Trinity River, a third-order stream entering above Willow Creek (river km 51) (USFWS 1981). Oral histories from old-time residents confirm this. However, a large flood in 1964 had devastating effects on anadromous fish habitat in this subbasin (U.S. Department of the Interior 1985). Millions of cubic yards of soil were moved into South Fork Trinity River and its tributaries. Channel widening and loss of depth resulted. This event, along with other changes in basin morphology, has apparently resulted in the loss of suitable sturgeon habitat. There are no recent sightings from this watershed.
The Salmon River is a fourth-order stream entering the Klamath River at Somes Bar (river km 106). The water in this river is generally clear and becomes turbid only during high run-off periods. Adult sturgeon have been seen swimming in this river by observers standing on bluffs overhead. The approximate limit to upriver migration is at the mouth of Wooley Creek (river km 8), a third-order stream.

Del Norte County. Green sturgeon have been taken during gillnet sampling in Lake Earl (D. McCloud, pers. comm.). Lake Earl is located along the coast of Del Norte County, 8 km north of Crescent City and 11 km south of the mouth of Smith River. It is connected by a narrow channel to Lake Talawa, a smaller lake directly to the west. A sand spit separates Lake Talawa from the ocean and is occasionally breached by winter storms or by human activities. Coastal migrant green sturgeon enter at this time and become trapped after the sand spit is rebuilt (Monroe et al. 1975).
The Smith River is the northernmost river along the California coast, entering the ocean approximately 5 km south of the Oregon border. Blunt (1980) included green sturgeon in an inventory of anadromous species found in the Smith River. They occasionally enter the estuary and have been observed in Patrick's Creek, an upstream tributary 53 km from the ocean (Monroe et al. 1975). Juveniles have not been found.
Overall, north coast green sturgeon are apparently occur in fewer streams than they did historically and presumably are also in reduced numbers, although evidence is limited. The only time series data available of green sturgeon abundance in the Klamath River comes from tribal catch data (see below). The number of females spawning in the Klamath River is estimated at 760-1500 per year. The population of subadults-adults is estimated at tens of thousands, with no clear evidence of population decline (Adams et al. 2002).

Description: 

Description: Sturgeons, with their large size, subterminal and barbeled mouths, lines of bony plates (scutes) on the sides, and heterocercal (shark-like) tail, are among the most distinctive of freshwater fishes. Green sturgeon have 8-11 scutes in the dorsal row, 23-30 in the lateral rows 23-30, and 7-10 in the bottom rows. The dorsal fin has 33-36 rays, and the anal fin 22-28. They as distinguished from white sturgeon, with which they co-occur, by (1) having one large scute behind the dorsal fin, as well as behind the anal fin, (2) having scutes that are sharp and pointed, and (3) having barbels that are closer to the mouth than to the tip of the long, narrow snout. Their color is olive-green to pale brown, with an olivaceous stripe on each side and scutes that are paler than the body. Individuals from the northern and southern Distinct Population Segments (see below) of green sturgeon cannot be distinguished based on morphology, only genetics.