|Scientific Name:||Acipenser medirostris|
|Species Authority:||Ayres, 1854|
|Taxonomic Notes:||Synonyms = A. medirostris mikadoi Hilgendorf, 1892 (although this is now probably best treated as a distinct species); A. acutirostris Ayres, 1854.
The Green Sturgeon was once considered conspecific with the Sakhalin Sturgeon (A. mikadoi) and the two forms were synonymized (Berg 1948). However, recent molecular data on three mitochondrial genes and morphometric studies show large differences between North American and Asian forms (Birstein and DeSalle 1998, North et al. 2002). Sakhalin Sturgeon is now believed extinct in Japan, Korea and China and the species sustains only a remnant population in Russia's Tumnin River where there is a hatchery (Adams et al. 2002).
|Red List Category & Criteria:||Near Threatened ver 3.1|
|Assessor(s):||St. Pierre, R. (US Fish&Wildlife Service) & Campbell, R.R. (COSEWIC Freshwater Fishes SSC)|
|Reviewer(s):||St. Pierre, R. & Pourkazemi, M. (Sturgeon Red List Authority)|
Acipenser medirostris nearly meets Vulnerable under criterion A2c. Although the estimated population size is above the thresholds for any of the categories for threat, it is not large (34,000–166,000 adults and sub-adults) and the number of mature individuals is unknown. The estimated number of spawning females for the Klamath River (which supports most of the northern DPS) is only 760–1,500 per year. However, four out of seven (57%) of the known spawning locations have been lost within the last three generations for reasons, not all of which are clearly understood, and may not have ceased or reversible.
Green Sturgeon are not subjected to overfishing and habitat fragmentation that have affected other sturgeon species. There are active fisheries restrictions in place throughout the species’ range in the U.S. and Canada.
The current adult–subadult population size is estimated at least in the tens of thousands and there is no evidence of overall population decline, although four of the seven known spawning locations may have been lost. Considerable research is underway or planned to fill in information gaps on stock structure, abundance and migrations. Although Green Sturgeon currently does not warrant "threatened" status under U.S. Endangered Species Act, concerns remain about the limited number of spawning rivers, potential growth of fisheries, and persistent habitat threats from water withdrawals and agricultural pesticides. Given the species’ life history characteristics and limited number of spawning sites, it is considered likely that the species would quickly move into a threatened category if current conservation efforts were withdrawn. It is therefore assessed as Near Threatened.
|Range Description:||Green Sturgeon is an anadromous species that spend most of their life in near-shore marine and estuarine waters from Mexico to southeast Alaska (Houston 1988, Moyle et al. 1995, Davies 2004).
Large concentrations of Green Sturgeon are still found in coastal estuaries, but their range in freshwater has been restricted by damming in some rivers. For example, Green Sturgeon were historically observed hundreds of kilometers upstream in the Sacramento and Columbia rivers, but are currently restricted in the Columbia River to the lower 60 km downstream of the Bonneville Dam (Moyle 2002). Spawning is presently known to occur in only three rivers in North America, all of which are in the United States: the Rogue River in Oregon (Erickson et al. 2001, Rien et al. 2001), and the Klamath and Sacramento river systems in California (EPIC 2001, Adams et al. 2002, CDFG 2002, Moyle et al. 1992,1994). Spawning populations have been extirpated from the San Joaquin, Eel, South Fork, and Trinity rivers, and possibly the Umpqua River (EPIC 2001, NMFS 2002). Seasonal concentrations occur in several estuaries including the Sacramento-San Joaquin, Eel, Columbia, Willapa Bay, Grays Harbor and the Fraser River (Beamesderfer and Webb 2002).
Native:Canada (British Columbia); Mexico; United States (Alaska, California, Oregon)
|FAO Marine Fishing Areas:||
Pacific – eastern central; Pacific – northeast
|Range Map:||Click here to open the map viewer and explore range.|
Preliminary population genetics results suggest that Green Sturgeon from the Klamath and Rogue rivers are similar to each other but distinct from fish from San Pablo Bay. Green Sturgeon commonly occur in the lower Columbia River (Oregon–Washington) and genetic samples taken there appear to be a mixture of the other populations (Israel et al. 2002).
The National Marine Fisheries Service (NMFS 2003) has determined that, based on genetic evidence of discreteness, there are two Distinct Population Segments (DPS's) for Green Sturgeon. The northern population ranges from the Eel River, California, to at least the Rogue River, Oregon. The southern population is principally comprised of the Sacramento River spawning stock.
Their wide distribution, large numbers observed seasonally in some areas, and projections based on demographic rates suggest that total Green Sturgeon numbers are at least in the tens of thousands (Beamesderfer and Webb 2002). The largest known spawning stock of Green Sturgeon is that of the Klamath River and its tributary, the Trinity River. The Klamath River population is estimated to number up to 66,000 individuals of which 3,000 would be mature adults. Actual spawner numbers in the Klamath were estimated at 760–1,500 females per year based on average harvest and total mortality rates (Beamesderfer and Webb 2002).
Based on a review of recent tagging studies, harvest analyses, and stock assessments from many locations, Beamesderfer and Webb (2002) estimated that the total adult and subadult population size of Green Sturgeon is within the range of 34,000 to 160,000 fish. Of these, greatest abundance was recorded for the Columbia River estuary, and ocean and bay waters of Oregon and Washington.
Attempts to determine trends in sturgeon abundance in the northern DPS have been inconclusive regarding the status of Green Sturgeon, and the status of the northern Green Sturgeon population is unknown (NMFS 2002, Davies 2004).
Similar attempts to determine the status of Green Sturgeon in the southern DPS were also inconclusive, and with the exception of anecdotal evidence of a reduction of Green Sturgeon entrainment at water export facilities indicative of a population decline, the status of the southern DPS is also unknown (NMFS 2002, Davies 2004).
|Habitat and Ecology:||
The Green Sturgeon is among the most marine oriented and widespread of the sturgeons. Like most sturgeons, the Green Sturgeon is characterized by large size, longevity, delayed maturation, high fecundity, and far-ranging movements. Green Sturgeon reach sizes up to 2.7 m and have been aged to over 50 years (Nakamoto et al. 1995, Farr et al. 2002). Mortality rates of subadults and adults are low and probably do not exceed 10% per year from natural causes based on age frequency analysis. Males typically mature at about 15–17 years and 1.5–1.8 m, while females typically mature at 20–25 years and 1.8–2.1 m (Van Eenennaam and Doroshov 2001a). Similarly aged males are generally smaller than females with males from the northern DPS achieving a maximum size of 168 cm, while sexually mature males and females from the southern DPS range in size from 139 to199 cm and 157 to 223 cm, respectively (Adams et al. 2002). Spawning is believed to occur every 3-5 years and the generation time is 27–33 years (Davies 2004).
Green Sturgeon are oviparious broadcast spawners and adult Green Sturgeon typically migrate into freshwater beginning in late February and spawning occurs in April to June in deep, turbulent river mainstems (Moyle et al. 1995). Klamath and Rogue River populations appear to spawn within 160 km of the ocean while the Sacramento population may travel over 320 km upriver. Green Sturgeon eggs and larvae are comparatively larger than those of other sturgeon species. For example, Cech et al. (2000) reported a egg diameter of 4.34, 3.40 and 2.62 mm for green, white, and Atlantic Sturgeon (A. oxyrinchus) respectfully indicating that Green Sturgeon have eggs which have a volume twice that of white and over four times that of Atlantic Sturgeon. Consequently, Green Sturgeon have a relatively lower fecundity in comparison to other similar sized sturgeon species as reproductive energy is more heavily invested in egg size rather than egg number (Van Eenennaam et al. 2001b, Cech et al. 2000). Fecundity varies with age and size, but has been estimated to be in the order of 2,800 eggs per kilogram bodyweight; approximately half that of White Sturgeon (Acipenser transmontanus) at 5,648 eggs per kilogram bodyweight (Moyle 2002).
The eggs do not form a thick jelly coat as do those of other North American sturgeon and exhibit poor adhesion to substrate (Deng 2000, Cech et al. 2000), which would indicate sensitivity to turbidity loading (Moyle et al. 1994, Moyle 2002). Furthermore, the larger eggs and higher growth rates of developing Green Sturgeon in comparison to other North American sturgeons suggests that a higher oxygen demand may be required for proper embryonic development; thus Green Sturgeon may require colder, cleaner water for spawning. (USFWS 1995).
Eggs hatch in 7 to 9 days at 15°C (Van Eenennaam et al. 2001). Cech et al. (2000) reported that temperatures above 20°C are lethal to embryos and temperatures above 24°C significantly reduce five-day larval growth rates. Larvae begin to feed at 10 days post hatch and complete metamorphosis into juveniles at 45 days (Adams et al. 2002). Juvenile Green Sturgeon grow rapidly reaching 60 cm within 2–3 years and they spend 1–4 years in fresh and estuarine waters before dispersal to saltwater (Beamsesderfer and Webb 2002). Green Sturgeon are commonly observed in bays and estuaries up and down the coast and large concentrations enter the Columbia River estuary, and Washington's Grays Harbor and Willapa Bay during late summer (Moyle et al. 1995). Limited tagging studies show that tagged individuals from the Columbia River have been recaptured off the west coast of Vancouver Island (Adams et al. 2002).
During the 1 to 4 years of freshwater residence, juveniles gradually move to deeper and more saline areas as they grow (Beamesderfer and Webb 2002). As juveniles grow, they exhibit greater tolerance to salinity and achieve seawater tolerance at seven months or sooner (Allen and Cech 2003). Adults return to freshwater to spawn, and Green Sturgeon have been reported to spawn up to 160 km upstream in the Klamath and Rogue rivers, and over 300 km upstream in the Sacramento River (Beamesderder and Webb 2002). After spawning, they to sea in late autumn to early winter when temperatures drop below 10°C and flows increased above 100 m3s-1 (Erickson et al. 2002).
Juveniles are opportunistic benthic feeders with a diet consisting of various invertebrates and fish (EPIC 2001, Moyle 2002). Stomach content analysis indicates that adult Green Sturgeon have a marine diet consisting of various benthic invertebrates including shrimp, crabs, worms, amphioids, and isopods (EPIC 2001), but have also been observed feeding on sand lances (Ammodytes hexapterus) and other fish.
Sturgeon exhibit a combination of morphology, life history and habitat requirements that make them highly susceptible to negative impacts from human activities (Boreman 1997). Anthropogenic activities known to impact sturgeon include: exploitation blockage of available freshwater spawning habitat through diking, damming causing inadequate flow regimes, channelization, elimination of backwater areas, dewatering of streams, destruction of thermal refugia, loss of deep pools, inundation of habitat by reservoirs, and exposure to bioaccumulating industrial and municipal pollution, (Boreman 1997, EPIC 2001, Adams et al. 2002).
The long life span and late age of maturity makes sturgeon vulnerable to chronic and acute effects of bioaccumulation. White Sturgeon, for example have been found to have the greatest contaminant concentrations compared to salmonids, suckers, Walleye (Sander vitreus), Pacific Lamprey (Lampetra tridentata), and Eulachon (Thaleichthys pacificus), [USEPA 1999]. Whole body concentrations of hexachlorobenzene (19 ųg/kg), DDT (787 ųg/kg), p,p’DDE (620 ųg/kg), Aroclors (173 ųg/kg), and dioxins were an order of magnitude higher in concentration than all other species tested. Although Green Sturgeon are less exposed to anthropogenic contaminants due to their marine migratory phase, there is the potential for exposure when entering freshwater to spawn and during estuarine concentrations.
Marine and estuarine environments in Canada are of concern as they are heavily impacted by a number of activities including logging, aquaculture, agriculture and urbanization, and can be the eventual sinks to freshwater pollutants. However, a 1994 assessment of the quality of the lower Fraser River ecosystem indicated that no dramatic changes in species assemblages were determined to have occurred since the previous study in 1974 (Healey et al. 1994) despite an increase in usage. Area managed for conservation also increased from 23 to 69% over the same time period with 80% of the increase being accounted for by conservation efforts on the Fraser River estuary (MWLAP 2002). The increase in estuary protection and the results from Healey et al. (1994) suggests that estuarine habitat loss that may effect Green Sturgeon is likely not substantial in Canada.
In the US, where all known spawning populations occur, Green Sturgeon have lost spawning habitat to poor land use practices and habitat alteration through water management projects (EPIC 2001). This has caused a decline in general water quality in some areas through increased sedimentation as well as the loss of deep pools which Green Sturgeon are known to prefer. Furthermore, damming of river systems can block previously available spawning habitat, affect natural flow regimes, potentially reduce areas of thermal refugia, and change sediment transport characteristics of the river which may cascade and impact sturgeon by modifying ecosystem community structure (EPIC 2001). For example, Green Sturgeon were historically observed hundreds of kilometers upstream in the Sacramento and Columbia rivers, but are currently restricted in the Columbia River to the lower 60 km downstream of the Bonneville Dam (Moyle 2002).
Beamesderfer and Webb (2002 suggest that habitat conditions throughout the Green Sturgeon range have stabilized or are improving, but the results of spawning surveys have yet to substantiate that (NMFS 2002). Most of the northern population segment spawns in the Klamath River. Potential threats to this population include concentration of spawning, harvest (especially in mixed-stock estuarine fisheries), and loss of spawning habitat such as the Columbia River, and the Eel and South Fork, and Trinity Rrivers in California (Adams et al. 2002).
The southern population segment (south of the Eel River) is more of a concern because it has fewer spawners (limited to the Sacramento-San Joaquin system), which makes it more susceptible to catastrophic events. These fish also face potentially lethal temperature limits, entrainment by water projects, and may be adversely affected by pesticides and other toxic substances and exotic species (Adams et al. 2002). It is probable that Green Sturgeon spawning habitat has been lost behind dams and water diversions throughout the Central Valley (e.g., Red Bluff Diversion Dam and Glenn-Colusa Irrigation District pumping plant).
Green Sturgeon harvest is all bycatch in Green Sturgeon harvest is all bycatch in two three fisheries. The smaller bycatch occurs in Klamath River Tribal salmon gill net fisheries while the larger portion is in the White Sturgeon commercial and sport fisheries (Adams et al. 2002). Total annual harvest of Green Sturgeon declined substantially to 1,192 fish in 1999–2001 from 6,871 fish in 1985–1989. Most of this earlier harvest came from the Columbia River (51%) and Washington coastal fisheries (28%). In recent years, Columbia and Washington coastal harvest was substantially reduced and in 2001 these two bycatch fisheries and that of the Klamath River tribes were about equal in number. Catch reduction in the Columbia River is a result of increasingly restrictive regulations. For the sturgeon sport fishery, slot limits prohibit retention of fish (white or green) less than 107 cm (42 inches) or more than 152 cm (60 inches). Green Sturgeon retained as bycatch in the commercial fishery must be between 122 and 168 cm (48-66 inches).
Klamath Tribal fisheries (Yurok and Hoopa) accounted for an average 266 adult fish annually from 1986–2001 with no apparent trend. This fishery is monitored but not regulated. California sport catch of Green Sturgeon, primarily in San Pablo Bay, is not monitored, but is thought to be only a few fish each year (Adams et al. 2002). California slot size limits for both sturgeon species is 117–183 cm (46–72 inches).
There is also a bycatch in the domestic trawl fishing industry in B.C., which takes about 171 Green Sturgeon annually (Davis 2004).
There is no information on the extent of illegal exploitation of Green Sturgeon, but poaching activity on White Sturgeon in the lower Fraser River is a concern (Ptolemy and Vennesland 2003).
Fishery managers in California, Oregon, Washington and British Columbia have recognized the sensitivity of Green Sturgeon to fishing and have actively restricted commercial and sport fisheries everywhere Green Sturgeon occur. None of the reviews examined mentioned any legal, regional, national or international trade in Green Sturgeon products. This species is not considered as good to eat as White Sturgeon and markets for their flesh are limited.
Due to life history characteristics that make it particularly sensitive to human activities or natural events Green Sturgeon has a designation of "Special Concern" in Canada (Houston 1988, Davies 2004), and no harvest is allowed in Canadian waters.
In response to a petition to list Green Sturgeon under the U.S. Endangered Species Act, the National Marine Fisheries Service (NMFS) completed a status review for this species (Adams et al. 2002). NMFS determined that the species is comprised of two distinct population segments (DPS) but that neither warrants listing as a threatened or endangered species at this time (NMFS 2003). However, because of remaining uncertainties about their population structure and status, both DPS populations were added to the agency's list of candidate species and will be reevaluated in five years provided sufficient new information becomes available.
It is listed on CITES Appendix II.
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Deng, X. 2000. Artificial reproduction and early life stages of the green sturgeon (Acipenser medirostris). Unpublished PhD thesis. University of California, Davis.
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|Citation:||St. Pierre, R. (US Fish&Wildlife Service) & Campbell, R.R. (COSEWIC Freshwater Fishes SSC) 2006. Acipenser medirostris. The IUCN Red List of Threatened Species. Version 2014.3. <www.iucnredlist.org>. Downloaded on 19 April 2015.|