Green sturgeon (Acipenser medirostris) COSEWIC assessment and status report: chapter 9

Biology

General

Green sturgeon are anadromous, long-lived, slow growing and reach sexual maturity at an advanced age (Houston 1988). In comparison to other sturgeon species, green sturgeon invest a greater amount of their reproductive resources into individual eggs, thus having the largest egg size of any sturgeon species and a resultant lower fecundity in comparison to other anadromous sturgeon (Van Eenennaam et al. 2001). A distinct characteristic of green sturgeon is that their fertilized eggs have a thin chorion layer and show poor adhesion, which is not observed in either white or Atlantic sturgeon (Van Eenennaam et al. 2001).

Reproduction

There are no known green sturgeon spawning populations in Canada; however, they are currently known to spawn in the Rogue and Klamath rivers in Oregon and the Sacramento River system in California (Moyle et al. 1994).

Green sturgeon eggs and larvae are comparatively larger than those of other sturgeon species. For example, Cech et al. (2000) reported an egg diameter of 4.34, 3.40 and 2.62 mm for green, white, and Atlantic sturgeon (A. oxyrinchus) respectively 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. 2001; Cech et al. 2000)

Similar to white sturgeon, artificially spawned green sturgeon captured from the Klamath River larvae hatch after 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). Nakamoto et al. (1995) reported that fish from the Klamath River reach 30 cm in length within the first year and 60 cm after 2 to 3 years. In contrast, green sturgeon reared under hatchery conditions can grow to 40 cm in the first six months after hatching (Deng 2000). Growth rates under normal conditions of both sexes is approximately 7 cm year-1 until sexual maturation when growth rates reduce in response to the energetic demands of reproduction (USFWS 1982; Nakamoto et al. 1995).

The maximum size of green sturgeon is reported to be 2.3 m and 159 kg (Scott and Crossman 1973). Similarly aged males are generally smaller than females with males from the northern distinct population segments (DPS) achieving a maximum size of 168 cm (Adams et al. 2002). Sexually mature males and females from southern DPS range in size from 139 to 199 cm and 157 to 223 cm, respectively (Adams et al. 2002). Maximum age has been estimated to be 42 years (Nakamoto et al. 1995); however, there is some debate that this may be an underestimate with maximum age approaching 70 years (Moyle 2002). Estimating longevity precisely for green sturgeon is difficult due to reduced growth rates of older fish and poor formation of spawning checks caused by long marine migrations and adverse environmental conditions (Nakamoto et al. 1995). Variability in age estimation may also be a product of a seasonal pattern of alternating winter feeding and summer fasting which has been observed in other sturgeon species (Sulak and Randall 2002) but has not been confirmed for green sturgeon.

Green sturgeon are oviparious broadcast spawners. Similar to other sturgeon species, males reach sexual maturity before females (Nakamoto et al. 1995). Males reach sexual maturity around 15 years and females slightly later at 17 years (Adams et al. 2002). Spawning peaks in mid-April to mid-June but is spread from March to July (Moyle et al. 1992) and occurs every three to five years (Adams et al. 2002) with males generally spawning more frequently. Spawning occurs in the mainstem of large rivers in relatively fast water flows (Emmet et al. 1991). Long sperm motility (100% motility for 5 minutes) has been observed in artificial spawning studies (Van Eenennaam et al. 2001) and may be an adaptive fertilization strategy when spawning in fast moving water. The growth rate of larval green sturgeon is much greater than white sturgeon with Wang et al. (1987) reporting five-day-old green sturgeon larvae to be almost twice the weight of white sturgeon larvae of the same age (65 vs 34 mg; (cited in Cech et al. 2000). The faster growth rate is likely due to the larger egg size of green sturgeon (Cech et al. 2000).

Fecundity has been reported to range from 51 000 to 224 000 eggs per female (mean of 127 500; n=26) for fish caught in the Klamath River (USFWS 1982) with quantity being largely dependent upon the age and subsequent size of the fish. Using these figures, a fecundity estimate of approximately 2800 eggs per kilogram bodyweight for green sturgeon can be calculated. This is approximately half that of white sturgeon, which has a fecundity estimate of 5648 eggs per kilogram bodyweight (Moyle 2002).

Detailed age structure analysis of green sturgeon is not available for fish captured in Canadian waters. Preliminary age structure data of the 1999 to 2000 Klamath spawning run indicated an age range of spawners to be 17 to 33 with most being 25- to 31-years-old (Van Eenennaam and Doroshov 2001). However, this short time series may not be representative for the species.

Generation time, the average age of parents in the current cohort, reflects the turnover rate of breeding individuals in the population (Committee on the Status of Endangered Wildlife in Canada [COSEWIC]). For green sturgeon, females mature at a later age than males (seventeen vs. fifteen years; Adams et al. 2002), and generation time is calculated as the mean age of female parents. Generation time is estimated as the age at which 50% of females are mature + 1/M, where M is the instantaneous rate of natural mortality. Neither age of maturity nor spawning frequency, however, has been firmly established (Williamson 2003). Furthermore, natural mortality of juveniles and adults is unknown but is estimated to be below 10% (Beamesderfer and Webb 2002). Therefore, a minimum generation time is approximated to be 17 + 1/10% = 27 years. However, age of reaching sexual maturity for females has also been estimated to be between 20 to 25 years (Beamesderfer and Webb 2002). Therefore, taking the average age of maturity of 22.5 years and maintaining the estimated 10% mortality value, generation time would be 22.5 + 1/10% = 32.5 years. Taking these values as a range, generation time is estimated to be between 27 to 33 years. As the precision and accuracy of aging green sturgeon is questionable, this estimate should be used with caution.

Survival

Green sturgeon have few known predators other than humans and some marine mammals (Fitch and Lavenburg 1971; Emmet et al. 1991). Larval and young green sturgeon are likely preyed upon by other species present in spawning areas. Adults have relatively few direct threats from natural predators, which is due to a life history design based on achieving large size and possibly using freshwater as refugia from predators.

Year class failure is not uncommon for sturgeon populations (Sulak and Randall 2002). A study examining age class structure of gulf sturgeon (A. oxyrinchus desotoi) reported that population age class structure can be highly dynamic and unstable with strong and weak recruitment and persistent recruitment failures in populations under low exploitation (Sulak and Randall 2002). Furthermore, other long-term data series of sturgeon species indicate periodic, widely spaced successful year classes with periodic year class failures (Sulak and Randall 2002). Therefore, green sturgeon are likely able to handle occasional year class failures and still maintain overall population stability

Physiology

Of all sturgeon species, the green sturgeon is the most widely distributed and spends the most time in marine waters (EPIC 2001). The development of anadromy in some sturgeon species enables them to exploit otherwise unavailable rich benthic invertebrate resources in estuarine and marine habitats (Sulak and Randall 2002).

A common trait among anadromous sturgeon is the tendency for adults and sub adults to fast in freshwater and feed only in marine and estuarine environments (Sulak and Randall 2002). Although green sturgeon fasting is not a confirmed behaviour, adult green sturgeon stomachs are frequently found to be empty during their presence in freshwater during some estuarine summer concentrations (USFWS 1982; Beamesderfer and Webb 2002).

The preferred temperatures and upper and lower lethal ranges for adult green sturgeon are unknown. A radio tagging study by Erickson et al. (2002) suggested that green sturgeons emigrate to sea once temperature dropped below 10°C. This temperature was also associated with a flow rate greater than 100 m3s-1, however, which may have been the actual trigger for migration out of the system.

Optimal embryonic growth rates are reached at 15°C and upper lethal temperature threshold for developing embryos was determined to be 20°C. Growth of larval green sturgeon is significantly reduced at 24°C (Cech et al. 2000).

Movements/dispersal

Green sturgeon in Canada are thought to originate from spawning populations in the US as there is no record of green sturgeon ever spawning in Canada. However, the frequent misidentification and rarity may result in underestimation of habitat use in Canada. Green sturgeon are the most widely distributed of sturgeon species with their range extending from Mexico to Alaska (Adams et al. 2002).

Green sturgeon spend their first 1 to 4 years in freshwater gradually exposing themselves to estuarine environments as they get older (Beamesderfer and Webb 2002). During their marine sub-adult phase, green sturgeon are frequently observed in estuarine concentrations at major river systems where spawning is not known to occur. Concentrations have been observed in the Columbia River estuary during the late summer and early fall (Adams et al. 2002). Green sturgeon migrate along the coast of Oregon and are thought to be present in most open estuaries (Williamson 2003).  Reasons for these concentrations are unclear as spawning does not occur in the Columbia River and there is no evidence of feeding during these concentrations (Adams et al. 2002). An explanation of this behaviour is suggested in the evolutionary life history of sturgeon species. As an anadromous lifestyle is a secondary adaptation of sturgeon, their physiology is most closely adapted to, and least stressed, in lower salinity waters (Sulak and Randall 2002). Furthermore, marine mammals are predators of adult green sturgeon and presence close to freshwater may offer access to areas of predator refugia during summer marine mammal migrations.

When green sturgeon enter their marine migratory phase they either occupy estuarine holding areas or undergo a northern migration (Adams et al. 2002). Limited tagging studies exist; however, tagged individuals from the Columbia River have been recaptured off the west coast of Vancouver Island (Adams et al. 2002).

As mentioned previously, two distinct population segments were identified by National Marine Fisheries Service (NMFS) with the Eel River being the north/south boundary between the Oregon and California populations. Unique alleles not present in the other spawning populations, however, were found at low frequencies in the sampled Columbia River population. Israel et al. (2002) suggested that the presence of these alleles might indicate the existence of a currently unknown spawning population(s) or may be an artifact of low sample sizes and sampling methodology. Further study is needed to clarify this issue.

Nutrition and interspecific interactions

Juveniles are opportunistic benthic feeders with a diet consisting of various invertebrates and fish (EPIC 2001; Moyle 2002). Stomach content analysis indicates that 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. 

Behaviour/adaptability

Sturgeons are generally not known as a schooling fish; however, a high level of group cohesion is observed in green sturgeon during migration and summer concentrations in seasonal holding areas. Summer estuarine concentrations and irregular large catches by fishing vessels in marine waters (one haul in Canadian waters in January 2000 was over 1000 kg and one catch in 1960 off the west coast of Vancouver Island was reportedly 9000 kg (Slack and Stace-Smith 1996) suggest that green sturgeon have a level of group cohesion above most other sturgeon species. Schooling behaviour may allow individuals to acquire learned behaviour and/or to chemically imprint upon habitat sites (Sulak and Randall 2002). Therefore, a minimum population size including older individuals may be necessary to maintain a behavioral and habitat “knowledge base” within the species.

Larvae of green sturgeon are nocturnal and do not have a direct swim-up or post-hatching stage which is uncharacteristic to other sturgeon species (Van Eenennamm et al. 2001).

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