Green sturgeon (Acipenser medirostris) COSEWIC assessment and status report: chapter 3

Logo of COSEWIC
COSEWIC
Executive summary

Green sturgeon
Acipenser medirostris

Species information

The green sturgeon (A. medirostris) is easily distinguished from other families of fish by a combination of features including four barbels in front of a subterminal mouth, five rows of bony scutes, a heterocercal tail, an elongate snout, a single fleshy dorsal fin located near the caudal peduncle, and a largely cartilaginous endoskeleton. Green sturgeon are generally dark olive green with a white belly. Due to range overlap and similar appearance, green sturgeon can be easily confused with the white sturgeon (A. transmontanus). The green sturgeon spawns in freshwater but spends the majority of its lifecycle in the marine environment and is reported to reach a maximum length and weight of 2.3 m and 159 kg.

There are conflicting genetic studies regarding the relatedness of the North American and Asian forms of green sturgeon. There is more evidence suggesting that they should be considered separate species; however, further study is required to address this issue. Discrete northern and southern populations have been identified for the green sturgeon in North America by the National Marine Fisheries Service (NMFS), with the latitudinal boundary being found at Eel River, California (CA). The northern population is likely the source population of individuals found in Canada; however, as green sturgeon undertake large northern migrations and the genetic population structure of individuals in Canada is unknown, this cannot be confirmed at this time.


Distribution

Green sturgeon are found along the Pacific Coast of North America extending from the northern Mexico border up to southern Alaska. There are no known spawning populations located in Canada and green sturgeon are principally found in marine waters. Although rare, reports of freshwater captures in the lower Fraser, Nass, Stikine, Skeena, and Taku rivers have been documented. Known spawning populations are restricted to three rivers found in the US: the Rogue and Klamath rivers in Oregon, and the Sacramento River system in California. Green sturgeon are frequently caught up and down the coast and are thought to be present in limited numbers in most estuaries. 


Habitat

Green sturgeon have diverse habitat needs ranging from freshwater streams, rivers, estuarine habitat as well as marine waters depending upon their life stage. The specific habitat requirements for green sturgeon are poorly understood but are thought to resemble those of white sturgeon. Green sturgeon spawning is thought to occur in deep pools in areas of large cobbles, but can range from clean sand to bedrock in turbulent river mainstreams. The larger eggs and higher growth rates of developing green sturgeon in comparison to white sturgeon suggest that a higher oxygen demand may be required for proper embryonic development. Therefore, green sturgeon may subsequently require colder, cleaner water for spawning relative to white sturgeon. Marine residents utilize benthic habitat to feed on various invertebrates and fish species.


Biology

Green sturgeon are anadromous, slow growing and mature slowly. The first 1 to 4 years are spent in freshwater and juveniles gradually adapt to waters of higher salinity as they grow older. After leaving freshwater, green sturgeon migrate and forage for benthic invertebrates and fish in estuaries and marine areas. Adults return to natal spawning sites and can spend up to six months in freshwater and migrate up to 300 km upriver. Maximum size is reported to be 230 cm and 159 kg. Males mature slightly earlier than females at 15 years, whereas females reach maturity in the range of 17 to 25 years (Adams et al. 2002). Generation time is between 27 and 32 years; however precision and accuracy of aging techniques is still considered poor. Green sturgeon have the largest egg size of any sturgeon species and consequently have a lower relative fecundity than other anadromous species of sturgeon. Fecundity ranges from 51 000 to 224 000, eggs which are less adhesive and have a much thinner chorionic membrane than white sturgeon suggesting that better water quality is needed for successful green sturgeon reproduction in comparison to white sturgeon.


Population sizes and trends

A paucity of information is available to estimate population sizes and trends in both Canada and the US. In Canada, green and white sturgeon were not differentiated in catch statistics until 1996 with the inception of the fisheries observer program. Limited anecdotal survey data are available prior to this date and when compared to the Department of Fisheries and Oceans (DFO) observer catch information, the possibility of a substantial decline in green sturgeon abundance exists. However, because the DFO data are of such a short time series, and the accuracy and precision of an anecdotal survey are questionable, this claim cannot be made with certainty.


Limiting factors and threats

Sturgeon exhibit a combination of morphological, life history and habitat requirements that make them highly susceptible to negative impacts from human activities. Green sturgeon are rarely captured in freshwater in Canadawith the majority of sightings being in estuaries and marine waters. Therefore, negative anthropogenic impacts are mostly limited to those affecting prey species and fishery impacts. As no spawning of green sturgeon has been observed in Canada and freshwater utilization is thought to be limited, freshwater impacts may be more generally limited to spawning and rearing habitats in the US.


Special significance of the species

Green sturgeon are among the largest and longest living species found in freshwater, living up to 70 years, reaching 2.3 m in length, and weighing up to 159 kg. Literally surviving contemporaries of the dinosaurs, green sturgeon are one of the world's most ancient species, having remained virtually unchanged since they appeared in the fossil record more than 200 million years ago. The reported disagreeable taste and rarity of green sturgeon has limited its utilization in Canada. The majority of fish are captured as bycatch and are frequently discarded. However, there is a tribal fishery in the US located on the Klamath River, Oregon (OR).


Existing protection or other status designations

Green sturgeon was given the status rank of rare by the Committee on the Status of Endangered Wildlife in Canada (COSEWIC) in 1987 and has been reclassified to the status rank of Special Concern due to a change in the ranking definitions. In BC, green sturgeon has a status of red indicating it is a candidate for extirpation, endangerment, or threatened status. It is illegal to retain green sturgeon while fishing in both marine and freshwater in Canada.

In the US, green sturgeon has Federal Species of Concern status. In Oregon, Washington, and Alaska it has no special protection. In California, it is classified as a species of special concern but has no protection under the California Endangered Species Act. There are fishing regulations such as size, bag and slot size limits in these states.

The International Union for Conservation of Nature (IUCN) assessed the green sturgeon in 1996 and classified it as Vulnerable (A1ac). The Sturgeon Specialist Groupconsidered them to be facing a high risk of extinction in the wild in the medium-term future, based on direct observation and a decline in area of occupancy resulting in a reduction of population size of at least 20% over the last three generations. The Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) listed green sturgeon under Appendix II in June 1997 and they remain listed.

Page details

Date modified: