Green sturgeon (Acipenser medirostris) COSEWIC assessment and status report: chapter 7

Distribution

Global range

There is debate over whether the Asian and North American forms of green sturgeon should be considered separate species; therefore the range of the Asian form (Sakhalin sturgeon (A. mikadoi)) is included here. The Sakhalin sturgeon has been extirpated throughout Japan, Korea, and China and is reduced in range in Russia to the Tummin River where it is supported by a hatchery (EPIC 2001). Historically, the species ranged from the Sea of Japan and the southwest coast of Korea, north to the Amur River in Siberia and northeast to the Bering Sea, Alaska (McPhail and Lindsey 1970). Further genetic study regarding the relatedness of Asian and North American forms will clarify the global range.

The North American form of green sturgeon is found on the western coast of North America from Mexico to southeastern Alaska, however, they are rarely found below 30°(S) latitude and their greatest abundance is between the 40th and 60th parallels (EPIC 2001; Moyle 2002; Figure 4). Mecklenburg et al. (2002) give the Alaskan range as along the panhandle and north to the Bering Sea based on a 1964 reference to a record off Unalaska Island, as well as older records from the Bering Sea, and a questionable record of 2 specimens in 1897 from the Copper River (Gulf of Alaska, northwest of Controller Bay).


Figure 4. Range map for green sturgeon (A. medirostris); Asian form range excluded as it is currently restricted to one river and relatedness is in debate.

Figure 4. Range map for green sturgeon.


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 kilometres 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, and the Klamath and Sacramento river systems in California (EPIC 2001, Adams et al. 2002, Moyle et al. 1994). Spawning populations have been extirpated from the San Joaquin, Eel, South Fork Trinity rivers and possibly the Umpqua River (EPIC 2001).

Canadian range

The range of green sturgeon in Canada spans the entire length of the Pacific Coast (Houston 1988, Scott and Crossman 1973). Green sturgeon are rarely captured in freshwater, preferring estuaries and marine environments. Reports of green sturgeon caught in the lower Fraser, Nass, Stikine, Skeena and Taku rivers are extremely rare but have been documented (FISS 2003). The extent of utilization of freshwater is unknown, but is thought to be limited as there is no evidence that spawning has ever occurred in Canadian rivers.

Green sturgeon have reportedly always been rare in freshwater in Canada (McPhail and Carveth 1993). Incidents are generally limited to irregular reports from sport fishers and researchers conducting white sturgeon tagging programs. In 1985 and 1986, a tagging study captured two green sturgeon (not positively identified) in the process of tagging approximately 500 white sturgeon 50 to 90 km upstream from the Fraser River mouth (Houston 1988). Conversely, a tagging study from 1995 to 1999 in a similar area (78 to 154 kilometres from the river mouth (rKm)) failed to report any occurrences of green sturgeon (Adams et al. 2002). The lower Fraser River has had more reports of green sturgeon; however, they continue to be rare. Nearly 13 000 white sturgeon have been tagged from the beginning of 2000 to present and 12 to 15 possible green sturgeon have been reported (T. Nelson, Fraser River Sturgeon Conservation Society, Crescent Beach, British Columbia (BC); pers. comm. 2003).

Green sturgeon have been caught incidentally in large bottom trawler hauls off the west coast of Vancouver Island, the Straight of Georgia, and coastal northern BC and in salmon gillnets at the mouths of rivers along the southern coast of BC (Anonymous 1954; Slack and Stace-Smith 1996; Houston 1988; Echols 1995).

Detailed distribution maps for green sturgeon were not developed by the Department of Fisheries and Oceans (DFO) until 1996, when the department began differentiating green and white sturgeon in their catch statistics. However, a local BC fisherman who was concerned that green sturgeon populations were declining conducted a survey of fishermen regarding historical catches back to 1960 (Slack and Stace-Smith 1996). The groups surveyed included members of the commercial trawl, gillnet, longline and sport fishing industry and the results suggested that freshwater reports of green sturgeon have always been uncommon; however, large marine catches have historically occurred. For example, Hart (1973) reported 75 fish weighing a total of 952 kg in one day off Kyuquot Sound. Responses to the survey, however, were sparse and the earliest report occurred in 1960 and therefore may not fully account for the historical range of green sturgeon. Commercial trawl fishing data collected by DFO from 1996 to 2002 recorded green sturgeon bycatch as far north and west as 54° latitude and 131° longitude respectively, which is consistent with the historical range estimations given by Slack and Stace-Smith (1996). Marine range has been estimated based on the recent DFO catch information to be 12 000 to 30 000 km2 or 2.6 to 6.6% of Canadian Pacific waters (B. Lucas, Research Biologist, Department of Fisheries and Oceans, Nanaimo, BC; pers. comm. 2002). Limited information makes an area of occupancy estimate difficult due to the low number of catches and the short time series of catch records.

The small number of recent freshwater sightings of green sturgeon suggest that the species is still rare in freshwater in Canada. The few recorded historical sightings and the rarity of green sturgeon make it impossible to firmly establish whether any range contraction has occurred. Green sturgeon do not seem to have undergone any large-scale range contraction in marine waters as the recent DFO data is consistent with the survey catch data collected by Slack and Stace-Smith (1996). However, the survey by Slack and Stace-Smith likely did not include all incidents of historical green sturgeon catches and therefore, marine range contraction cannot be ruled out entirely.

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