Green sturgeon (Acipenser medirostris)
Green sturgeon (Acipenser medirostris)
Sturgeons are among the largest and most ancient of cartilaginous fishes. They are placed, along with paddlefishes and numerous fossil groups, in the infraclass Chondrostei, which also contains the ancestors of all other bony fishes. The sturgeons themselves are not ancestral to modern bony fishes but are a highly specialized and successful offshoot of ancestral chondrosteans, retaining such ancestral features as a heterocercal tail, fin structure, jaw structure, and spiracle. They have replaced a bony skeleton with one of cartilage and possess a few large bony plates, known as scutes, instead of scales. Sturgeons are highly adapted for preying on bottom dwelling animals, which they detect with a row of extremely sensitive barbells on the underside of their snouts. They protrude their extraordinarily long and flexible "lips" to suck up food. Sturgeons are confined to waters of the Northern Hemisphere. Of 25 extant species, only two live in California, the Green sturgeon and the white sturgeon (A. transmontanus).
The Green sturgeon is similar in appearance to the White sturgeon, except the barbels are closer to the mouth than the tip of the long, narrow snout. The dorsal row of scutes numbers 8-11, lateral rows, 23-30, and bottom rows, 7-10; there is one large scute behind the dorsal fin as well as behind the anal fin (both lacking in White sturgeon). The scutes also tend to be sharper and more pointed than in White sturgeon. The dorsal fin has 33-36 rays, the anal fin, 22-28. The body color is olive green with a stripe on each side; the scutes are paler than the body.
Green sturgeon can reach 7 feet (210 cm) in length and weigh up to 350 pounds (159 kg).
On April 7, 2006, the National Marine Fisheries Service (NMFS) issued a final rule listing the Southern distinct population segment (DPS) of North American Green sturgeon (Acipenser medirostris) as a threatened species under the United States Endangered Species Act. Included in the listing is the Green sturgeon population spawning in the Sacramento River and living in the Sacramento River, the Sacramento-San Joaquin Delta, and the San Francisco Bay Estuary. This threatened determination was based on the reduction of potential spawning habitat, the severe threats to the single remaining spawning population, the inability to alleviate these threats with the conservation measures in place, and the decrease in observed numbers of juvenile Southern DPS Green sturgeon collected in the past two decades compared to those collected historically.
Critical habitat under the United States Endangered Species Act has not been designated for the Southern DPS of Green sturgeon.
The northern DPS of the Green sturgeon (which spawn in the Rogue River, Klamath River, and Umpqua River) is a U.S. National Marine Fisheries Service Species of Concern. Species of Concern are those species about which the U.S. Government's National Oceanic and Atmospheric Administration, National Marine Fisheries Service, has some concerns regarding status and threats, but for which insufficient information is available to indicate a need to list the species under the U.S. Endangered Species Act.
Life history and habitat requirements
Sturgeons have adopted a temporal strategy to controlling risks. Sturgeons live a long time, delay maturation to large sizes, and spawn multiple times over their lifespan. The sturgeon's long life span and repeat spawning in multiple years allows them to outlast periodic droughts and environmental catastrophes. The high fecundity that comes with large size allows them to produce large numbers of offspring when suitable spawning conditions occur and to make up for years of poor conditions. Adult Green sturgeon do not spawn every year and only a fraction of the population enters freshwater where they might be at risk of a catastrophic event in any year. The widespread ocean distribution of Green sturgeon ensures that most of the population at any given time is dispersed among areas where they are not vulnerable to catastrophic losses.
The ecology and life history of Green sturgeon have received little study, evidently because of the generally low abundance, limited spawning distribution, and low commercial and sport fishing value of the species. Green sturgeon is the most marine species of sturgeon, coming into rivers mainly to spawn Green sturgeons do not spawn every year, and it is believed that the majority of adult Green sturgeons are in the ocean at any given time.
The United States Fish and Wildlife Service (USFWS) 1995 reported that for the Klamath River, Green sturgeon life history could be divided into three phases:
- freshwater juveniles: less than 3 years old
- coastal migrants: 3-13 years old for females and 3-9 years for males
- adults: more than 13 years old for females and more than 9 years old for males
Northern DPS 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. Spawning takes place in deep, fast water. Preferred spawning substrate is likely large cobble, but it can range from clean sand to bedrock. Eggs are broadcast and externally fertilized in relatively fast water and probably in depths greater than 3 m. Female Green sturgeon produce 59,000-242,000 eggs, about 4.34 millimeters (mm) in diameter (Van Eenennaam et al. 2001 and 2006).
Temperatures of 23-26°C affected cleavage and gastrulation of Green sturgeon embryos and all died before hatch (Van Eenennaam et al. 2005). Temperatures of 17.5-22°C were suboptimal as an increasing number of Green sturgeon embryos developed abnormally and hatching success decreased at 20.5-22°C, although the tolerance to these temperatures varied between progenies (Van Eenennaam et al. 2005). The lower temperature limit was not evident from the Van Eenennaam et al. 2005 study, although hatching rate decreased at 11°C and hatched green sturgeon embryos were shorter, compared to 14°C. The mean total length of hatched green sturgeon embryos decreased with increasing temperature, although their wet and dry weight remained relatively constant (Van Eenennaam et al. 2005). Van Eenennaam et al. 2005 concluded that temperatures 17-18°C may be the upper limit of the thermal optima for green sturgeon embryos. Growth studies on younger juvenile green sturgeon determined that cyclical 19-24?C water temperature was optimal (Allen et al. 2006).
Green sturgeon fertilization and hatching rates are 41.2% and 28.0%, compared with 95.4% and 82.1% for the white sturgeon (Deng et al. 2002). However, the survival of green sturgeon larvae is very high (93.3%) (Deng et al. 2002). Female green sturgeon invest a greater amount of their reproductive resources into maternal yolk for nourishment of the embryo, which results in larger larvae (Van Eenennaam et al. 2001). Five day old Green sturgeon larvae have almost twice the weight of white sturgeon larvae (65 versus 34 milligrams (mg)) (Van Eenennaam et al. 2001). This greater reserve of maternal yolk and larger larvae could provide an advantage in larval feeding and survival (Van Eenennaam et al. 2001). Compared with other acipenserids, Green sturgeon larvae appear more robust and easier to rear (Van Eenennaam et al. 2001). Juveniles continue to grow rapidly, reaching 300 mm in 1 year and over 600 mm within 2-3 years for the Klamath River (USFWS 1995). Juveniles spend from 1-4 years in fresh and estuarine waters and disperse into salt water at lengths of 300-750 mm (USFWS 1995).
A conceptual model of early behavior and migration of Green sturgeon early life intervals based on the Kynard et al. 2005 study follows: Females deposit eggs at sites with large rocks and moderate or eddy water flow that keeps the large, dense, poorly adhesive eggs from drifting, so eggs sink deep within the rocks. CH2M Hill (2002) assumed that hatchling Green sturgeon embryos drift downstream like hatchling white sturgeon embryos. This was incorrect. Hatchling Green sturgeon embryos seek nearby cover, and remain under rocks, unlike white sturgeon which drift downstream as embryos (i.e. newly hatched Green sturgeon do not exhibit pelagic behavior like newly hatched white sturgeon) (Deng et al. 2002). After about 9 days fish develop into larvae and initiate exogenous foraging up- and downstream on the bottom (they do not swim up into the water column, unlike white sturgeon). After a day or so, larvae initiate a downstream dispersion migration that lasts about 12 days (peak, 5 days). At the age of ten days, when exogenous foraging begins, Green sturgeons are 19 to 29 mm in length (mean 24 mm) (Deng et al. 2002). At the age of 15 to 21 days, Green sturgeon are 30 mm or greater in length (Deng et al. 2002). At the age of 45 days, metamorphosis is complete and Green sturgeon are 70 to 80 mm in length (Deng et al. 2002). All migration and foraging during the migration period is nocturnal, unlike white sturgeon. During the first 10 months of life, Green sturgeon are the most nocturnal of any North American sturgeon yet studied, and this was the case for all life intervals during any activity (migration, foraging, or wintering). Post-migrant larvae are benthic, foraging up- and downstream diurnally with a nocturnal activity peak. Foraging larvae select open habitat, not structure habitat, but continue to use cover in the day. When larvae develop into juveniles, there is no change in response to bright habitat, and no preference or avoidance of bright habitat. In the fall, juveniles migrate downstream mostly at night to wintering sites, ceasing migration at 7-8°C. During winter, juveniles select low light habitat, likely deep pools with some rock structure. Wintering juveniles forage actively at night between dusk and dawn and are inactive during the day, seeking the darkest available habitat.
For the Klamath River Green sturgeon, an average length of 1.0 m is attained in 10 years, 1.5 m by age 15, and 2.0 m by 25 years of age (USFWS 1993). The largest reported Green sturgeon weighed about 159 kg and was 2.1 m in length (USFWS 1993). The largest Green sturgeon have been aged at 42 years, but this is probably an underestimate, and maximum ages of 60-70 years or more are likely.
Little is known about Green sturgeon feeding at sea, but it is clear they behave quite differently than white sturgeon (CDFG 2005a). Green sturgeons are probably found in all open Oregon estuaries, with a lot of movement in and out of estuaries and up and down the coast (ODFW 2005a). Adults feed in estuaries during the summer (ODFW 2005a). Stomachs of Green sturgeons caught in Suisun Bay contained Corophium sp. (amphipod), Crago franciscorum (bay shrimp), Neomysis awatchensis (Opossum shrimp) and annelid worms (Ganssle 1966). Stomachs of Green sturgeon caught in San Pablo Bay contained Crago franciscorum (bay shrimp), Macoma sp. (clam), Photis californica (amphipod), Corophium sp. (amphipod), Synidotea laticauda (isopod), and unidentified crab and fish (Ganssle 1966). Stomachs of Green sturgeons caught in Delta contained Corophium sp. (amphipod), Neomysis awatchensis (Opossum shrimp) (Radtke 1966). Radtke 1966 also reported that while the Asiatic clam (Corbicula fluminea) was abundant throughout the Delta, Suisun Bay and San Pablo Bay, it was not utilized as a food source by Green sturgeons.
Threats to the Green sturgeon include being taken as bycatch in salmon gillnet and other fisheries, water development projects that affect migration or decrease habitat quality, and other land use stressors that affect habitat quality. Exotic species negatively affect the southern DPS. Commercial fisheries have been prohibited in the Columbia River and Willapa Bay since 2001.
Current and historical distribution
Prehistoric fish distributions have been mapped by Gobalet et al. 2004 based on bones at Native American archaeological sites. Data were reported on dozens of sites throughout California and summarized by county. Sturgeon remains were observed in 12 counties, all in the Central Valley. Observations were concentrated at San Francisco Bay and Sacramento-San Joaquin and delta sites (Contra Costa, Alameda, San Francisco, Marin, Napa, San Mateo and Santa Cruz counties). Historical 18th-century accounts report the aboriginal gillnetting and use of tule balsa watercraft for the capture of sturgeon, and fishing weirs were also likely employed on bay tidal flats (Gobalet et al. 2004). Most sturgeons were unidentified species but Green sturgeons were specifically identified from Contra Costa and Marin County sites. Sturgeon remains (unidentified species) were also identified from lower Sacramento River counties (Sacramento, Yolo, Colusa, Glenn, and Butte counties). No sturgeon remains were found in samples from the upper Sacramento River although other fish species including salmonids were reported in those areas.
Green sturgeons which spawn in the Rogue River, Klamath River, and Umpqua River are the Northern DPS Green sturgeon, while the Green sturgeons which spawn in the Sacramento River system are Southern DPS Green sturgeon (NMFS 2005). Both the Northern DPS Green sturgeon and Southern DPS Green sturgeon occur in large numbers in the Columbia River estuary, Willapa Bay, and Grays Harbor, Washington (NMFS 2005).
A number of presumed spawning populations (Eel River and South Fork Trinity River) have been lost in the past 25-30 years. Moyle 1976 reported Green sturgeon spawning in the Mad River, but does not mention the Mad River in 2002. Scott and Crossman 1973 reported potential spawning in the Fraser River in Canada, but Moyle 2002 reported that there was no evidence of Green sturgeon spawning in Canada or Alaska. Green and white sturgeon occasionally enter the Feather River system, but intensive sampling in recent years found no evidence of spawning and there is no data that spawning occurs now or occurred in the historical time frame (Beamesderfer et al. 2004). No current use by sturgeon of Sacramento River tributaries, other than the Feather River system, has been reported (Beamesderfer et al. 2004, Moyle 2002). No evidence was found to indicate that Green sturgeons were historically present, are currently present, or were historically present and have been extirpated from the San Joaquin River upstream from the Delta (Beamesderfer et al. 2004). There is no evidence of Green sturgeon spawning in the Columbia River or other rivers in Washington (Moyle 2002, ODFW 2005a and 2005b). In contrast to those studies, samples from Green sturgeon collected in the Columbia River suggest the existence of one or more spawning populations in addition to the Sacramento system, Klamath, and Rogue populations, suggesting not all spawning populations have been identified (Israel et al. 2004).
The Green sturgeon is the most widely distributed member of the sturgeon family Acipenseridae, and is also the most marine-oriented of the sturgeon species. Green sturgeon are known to range in nearshore marine waters from Mexico to the Bering Sea, with a general tendency to head North after their out-migration from freshwater (NMFS 2005). They are commonly observed in bays and estuaries along the western coast of North America, with particularly large concentrations entering the Columbia River estuary, Willapa Bay, and Grays Harbor during the late summer (Moyle 2002, NMFS 2005). While there is some bias associated with recovery of tagged fish through commercial fishing, the pattern of a northern migration is supported by the large concentration of Green sturgeon in the Columbia River estuary, Willapa Bay, and Grays Harbor, which peaks in August (NMFS 2005).
Individual Southern DPS Green sturgeon tagged by the California Department of Fish and Game (CDFG) in the San Francisco Estuary have been recaptured off Santa Cruz, California; in Winchester Bay on the southern Oregon coast; at the mouth of the Columbia River; and in Grays Harbor, Washington (USFWS 1993 and Moyle 2002). Most tags for Southern DPS Green sturgeon tagged in the San Francisco Estuary have been returned from outside that estuary. Green sturgeons remain present in all documented historic habitats and ranges in Oregon (ODFW 2005b).
White and Green sturgeon juveniles, subadults, and adults are widely distributed in the Sacramento-San Joaquin Delta and estuary areas including San Pablo (Beamesderfer et al. 2004). White sturgeon historically ranged into upper portions of the Sacramento system including the Pit River and a substantial number were trapped in and above Lake Shasta when Shasta Dam was closed in 1944 and successfully reproduced until the early 1960s (Beamesderfer et al. 2004). Landlocked white sturgeon populations have been widely observed in the Columbia and Fraser systems but no landlocked Green sturgeon populations have ever been documented in any river system (Beamesderfer et al. 2004), indicating that Green sturgeon likely did not historically spawn in the upper reaches of rivers prior to the construction of large dams as NMFS 2005 has assumed.
Common Names: Barbel Sturgeon, Green Sturgeon