Green sturgeon  (Acipenser medirostris)

The green sturgeon is classified as Near Threatened (NT) on the IUCN Red List (1) and is listed on Appendix II of CITES (3).

The green sturgeon (Acipenser medirostris) is among the largest and longest living freshwater fish species, weighing up to 150 kilograms and living to around 70 years. This peculiar species has remained almost unchanged for more than 200 million years, and has  prehistoric appearance, with more cartilage than bone and rows of bony plates for protection rather than scales (4). 

This remarkable fish is characterised by a row of 1 to 4 bony plates at the rear of the body, as well as about 33 to 35 rays in the fleshy dorsal fin (5), which is positioned far back on the body, near the tail (6). The lower lip is split in the middle, and four barbels hang from below the flattened, elongate snout (2). The backbone of the green sturgeon curves upward into the tail fin, forming a shark-like tail (7). 

The green sturgeon varies in colour from olive to dark green, with whitish-green underparts. Olive green stripes stretch along the length of the body (5) (6).

The most wide-ranging of all sturgeon species, the green sturgeon occurs in marine waters around the Aleutian Islands and the Gulf of Alaska, as well as bays and estuaries down the west coast of North America, as far south as Ensenada, Mexico (1) (5) (7). 

Spawning is restricted to only three rivers: the Rogue River in Oregon, and the Klamath and Sacramento River systems in California, all of which are in the U.S. (1) (7).

Outside of the breeding season, the green sturgeon is found in oceanic waters, bays and estuaries, utilising both salt and brackish waters. It spawns in deep pools in large, fast-flowing, freshwater rivers (4) (5) (7).

The green sturgeon is an anadromous species, meaning it lives much of its life in marine waters but migrates to freshwater systems to breed. Migration typically begins in late February, and spawning occurs from April to June. Some green sturgeon may travel as far up river as 320 kilometres to find spawning sites (1). 

Compared to other sturgeon species, the green sturgeon releases relatively few eggs, although some larger females may still release as many as 140,000 eggs (7). The eggs of the green sturgeon are larger than those of other sturgeons and may also require higher levels of oxygen in the water to develop. The eggs hatch in 7 to 9 days, and the larvae being to feed 10 days after hatching, completing metamorphosis into young fish at around 45 days. The young fish spend one to four years in freshwater habitats, gradually moving to deeper, salty water as they develop (1). 

A long-lived species, the green sturgeon reaches maturity at a relatively old age. Males typically mature at about 15 to 17 years, while females mature at 20 to 25 years. Adult green sturgeons breed every three to five years. After breeding in freshwater, the green sturgeon moves to sea in late autumn to early winter, when temperatures drop below ten degrees Celsius and water flow increases (1). 

The green sturgeon has a varied diet that includes bottom-dwelling invertebrates such as shrimp, crabs and worms, as well as small fish. Juveniles are more opportunistic in their feeding habits than adults and consume a mixture of fish and invertebrates found around the bottom of rivers (1).

Although the green sturgeon has a relatively large population, probably numbering in the tens of thousands, there is growing concern about the effect of the loss of spawning sites on this species. Not only does the green sturgeon breed in just three rivers, but it is thought that four out of seven spawning sites have been recently lost (1) (6) (8). 

The green sturgeon’s requirement to migrate into freshwater rivers from coastal areas makes it particularly vulnerable to human activities. The blockage of rivers through the construction of dikes, as well as damming, channelling and water extraction all restricts this species access to breeding sites and can destroy existing sites. These activities have also caused a decline in water quality through sedimentation, which leads to the loss of the deep pools in which the green sturgeon prefers to breed. The long life span and late age of maturity of the green sturgeon also makes it vulnerable to the accumulation of toxins in its body tissues (1) (6) (8). 

Today’s population of the green sturgeon can be divided into two distinct breeding populations; one in the northern part of the species’ range, and one in the southern part. Most individuals occur in the northern population, with the largest number of spawning females found in the Klamath River, which supports around 760 to 1,500 breeding females each year. The status of the southern population is somewhat precarious, as poor land use practices has greatly impacted the number of breeding green sturgeons there and restricted them to just the Sacramento River (1) (6) (8). 

Fishing was once a great threat to the survival of the green sturgeon. Fishing pressure reached a high in 1986, when around 10,000 individuals were taken. However, as a result of imposed fishing regulations, this threat has been greatly reduced, and the estimated number of fish caught each year is probably around six percent of that taken in 1986 (8).

As well as being listed on Appendix II of CITES, meaning international trade in this species should be carefully monitored (3), the green sturgeon is protected by various legislation. In Canada, it has a designation of ‘Special Concern’, and in British Colombia it has a status of ‘red’, indicating it is at high risk of extinction there (6). 

In the U.S., the National Marine Fisheries Service completed a status review for the green sturgeon in 2002, which determined that this species is comprised of two distinct populations, but that neither warranted listing as Threatened or Endangered under the Endangered Species Act at the time (9). However, an updated review in 2005 suggested that the southern population should be listed as Threatened (8), which took effect in 2006 (10). 

The green sturgeon has also benefited from fisheries restrictions throughout its range. Fishery managers in California, Oregon, Washington and British Columbia have restricted or closed commercial and sport fisheries everywhere the green sturgeon occurs (1) (7). Changes to the operations of the Red Bluff Diversion, which was previously closed all year, but now opens from mid-September to mid-May, has also allowed the green sturgeon access to spawning areas above the dam (7).

View information on this species at the UNEP World Conservation Monitoring Centre.

Find out more about the green sturgeon:

• NOAA Fisheries Service - Green sturgeon:
  http://www.nmfs.noaa.gov/pr/species/fish/greensturgeon.htm