Sturgeon exhibit a combination of morphology, life history and habitat requirements that make them highly susceptible to negative impacts from human activities (Boreman 1997). Anthropogenic activities known to impact sturgeon include: exploitation blockage of available freshwater spawning habitat through diking, damming causing inadequate flow regimes, channelization, elimination of backwater areas, dewatering of streams, destruction of thermal refugia, loss of deep pools, inundation of habitat by reservoirs, and exposure to bioaccumulating industrial and municipal pollution, (Boreman 1997, EPIC 2001, Adams
et al. 2002).
The long life span and late age of maturity makes sturgeon vulnerable to chronic and acute effects of bioaccumulation. White Sturgeon, for example have been found to have the greatest contaminant concentrations compared to salmonids, suckers, Walleye (Sander vitreus), Pacific Lamprey (Lampetra tridentata), and Eulachon (Thaleichthys pacificus), [USEPA 1999]. Whole body concentrations of hexachlorobenzene (19 ųg/kg), DDT (787 ųg/kg), p,p’DDE (620 ųg/kg), Aroclors (173 ųg/kg), and dioxins were an order of magnitude higher in concentration than all other species tested. Although Green Sturgeon are less exposed to anthropogenic contaminants due to their marine migratory phase, there is the potential for exposure when entering freshwater to spawn and during estuarine concentrations.
Marine and estuarine environments in Canada are of concern as they are heavily impacted by a number of activities including logging, aquaculture, agriculture and urbanization, and can be the eventual sinks to freshwater pollutants. However, a 1994 assessment of the quality of the lower Fraser River ecosystem indicated that no dramatic changes in species assemblages were determined to have occurred since the previous study in 1974 (Healey et al. 1994) despite an increase in usage. Area managed for conservation also increased from 23 to 69% over the same time period with 80% of the increase being accounted for by conservation efforts on the Fraser River estuary (MWLAP 2002). The increase in estuary protection and the results from Healey et al. (1994) suggests that estuarine habitat loss that may effect Green Sturgeon is likely not substantial in Canada.
In the US, where all known spawning populations occur, Green Sturgeon have lost spawning habitat to poor land use practices and habitat alteration through water management projects (EPIC 2001). This has caused a decline in general water quality in some areas through increased sedimentation as well as the loss of deep pools which Green Sturgeon are known to prefer. Furthermore, damming of river systems can block previously available spawning habitat, affect natural flow regimes, potentially reduce areas of thermal refugia, and change sediment transport characteristics of the river which may cascade and impact sturgeon by modifying ecosystem community structure (EPIC 2001). For example, Green Sturgeon were historically observed hundreds of kilometers 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).
Beamesderfer and Webb (2002 suggest that habitat conditions throughout the Green Sturgeon range have stabilized or are improving, but the results of spawning surveys have yet to substantiate that (NMFS 2002). Most of the northern population segment spawns in the Klamath River. Potential threats to this population include concentration of spawning, harvest (especially in mixed-stock estuarine fisheries), and loss of spawning habitat such as the Columbia River, and the Eel and South Fork, and Trinity Rrivers in California (Adams et al. 2002).
The southern population segment (south of the Eel River) is more of a concern because it has fewer spawners (limited to the Sacramento-San Joaquin system), which makes it more susceptible to catastrophic events. These fish also face potentially lethal temperature limits, entrainment by water projects, and may be adversely affected by pesticides and other toxic substances and exotic species (Adams et al. 2002). It is probable that Green Sturgeon spawning habitat has been lost behind dams and water diversions throughout the Central Valley (e.g., Red Bluff Diversion Dam and Glenn-Colusa Irrigation District pumping plant).
Utilisation
Green Sturgeon harvest is all bycatch in Green Sturgeon harvest is all bycatch in two three fisheries. The smaller bycatch occurs in Klamath River Tribal salmon gill net fisheries while the larger portion is in the White Sturgeon commercial and sport fisheries (Adams et al. 2002). Total annual harvest of Green Sturgeon declined substantially to 1,192 fish in 1999–2001 from 6,871 fish in 1985–1989. Most of this earlier harvest came from the Columbia River (51%) and Washington coastal fisheries (28%). In recent years, Columbia and Washington coastal harvest was substantially reduced and in 2001 these two bycatch fisheries and that of the Klamath River tribes were about equal in number. Catch reduction in the Columbia River is a result of increasingly restrictive regulations. For the sturgeon sport fishery, slot limits prohibit retention of fish (white or green) less than 107 cm (42 inches) or more than 152 cm (60 inches). Green Sturgeon retained as bycatch in the commercial fishery must be between 122 and 168 cm (48-66 inches).
Klamath Tribal fisheries (Yurok and Hoopa) accounted for an average 266 adult fish annually from 1986–2001 with no apparent trend. This fishery is monitored but not regulated. California sport catch of Green Sturgeon, primarily in San Pablo Bay, is not monitored, but is thought to be only a few fish each year (Adams et al. 2002). California slot size limits for both sturgeon species is 117–183 cm (46–72 inches).
There is also a bycatch in the domestic trawl fishing industry in B.C., which takes about 171 Green Sturgeon annually (Davis 2004).
There is no information on the extent of illegal exploitation of Green Sturgeon, but poaching activity on White Sturgeon in the lower Fraser River is a concern (Ptolemy and Vennesland 2003).
