Green sturgeon (Acipenser medirostris) COSEWIC assessment and status report: chapter 8
Habitat
Habitat requirements
Green sturgeon inhabit a range of environments throughout their life cycle, including freshwater streams, rivers, estuarine habitat, and marine waters (Figure 5). The specific habitat requirements of green sturgeon are poorly understood but they are thought to resemble those of white sturgeon (Moyle et al. 1992). There are no known spawning populations of green sturgeon in Canada and they have been rarely caught in freshwater environments. Instead, incidental catches in the commercial trawl and salmon gillnetting fishery suggest that marine and estuarine environments are the main habitats utilized by green sturgeon in Canada. However, not much effort has been made to understand the extent and nature of its presence in Canadian waters and it is possible that Canadian (spawning) populations are too small to be detected, as is their presence in freshwater. In order to provide a holistic assessment of the habitat requirements of green sturgeon throughout its life cycle, its freshwater habitat requirements in the US are included here.
Green sturgeon spawning is thought to primarily occur in deep pools in areas of large cobbles, but has also been observed in areas of clean sand or bedrock in turbulent river mainstreams (Moyle et al. 1992). Adults in the Sacramento River spawn in temperatures ranging from 8°C to 14°C (EPIC 2001). Optimal temperature for larval growth is estimated to be 15°C (Cech et al. 2000) and growth is substantially reduced outside the 11°C to 19°C range. Temperatures above 20°C are lethal to green sturgeon embryos, and temperatures above 24°C cause a significant reduction in larval growth (Cech et al. 2000). Minimum and maximum pH tolerance and dissolved oxygen requirements of eggs and larvae for green sturgeon are unknown. Green sturgeon eggs are adhesive and are broadcast over substrate where they settle and stick on the bottom in breaks in the substrate (Deng 2000; Cech et al. 2000). Their eggs do not form a thick jelly coat as do white sturgeon eggs and subsequently exhibit poor adhesion to substrate (Deng 2000; Cech et al. 2000). The comparatively lower adhesiveness of green versus white sturgeon eggs suggests that a higher turbidity load may be more detrimental to green sturgeon spawning in comparison to white sturgeon, since the increased turbidity may interfere with adhesion to the substrate (Moyle et al. 1994; Moyle 2002). Furthermore, the larger eggs and higher growth rates of developing green sturgeon in comparison to white sturgeon suggests 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 (USFWS 1995b).
Figure 5. Conceptual Model of green sturgeon life cycle with limiting factors
Beamesderfer and Webb 2002 used with permission.
During the 1 to 4 years of freshwater residence, juveniles gradually move to deeper and more saline areas as they grow (Beamesderfer and Webb 2002). As juveniles grow, they exhibit greater tolerance to salinity and achieve seawater tolerance at 7 months or sooner (Allen and Cech 2003). Juveniles are frequently captured in off- channel, low flow habitats and in estuaries (Nakamoto et al. 1995).
After migrating from freshwater, green sturgeon are captured both offshore and in estuaries. The Department of Fisheries and Oceans (DFO) trawl fishery catch statistics between 1996 and 2002 recorded green sturgeon at a maximum depth of 610 m (mean=82 m). After spending over 15 years in the marine environment and estuaries, adults return to freshwater and can travel significant distances up rivers to spawn. Green sturgeon have been reported to spawn up to 160 km upstream in the Klamath and Rogue rivers, and over 300 km upstream in the Sacramento River (Beamesderder and Webb 2002). Tagged adults from the Rogue River spent upwards of 6 months in freshwater; contrary to previous information indicating that adult green sturgeon spend limited amounts of time in freshwater (Erickson et al. 2002). Furthermore, green sturgeon preferred specific sites that were deep (>5m), low gradient reaches, or off-channel coves. Tagged green sturgeon emigrated to sea after spawning from late autumn to early winter when temperatures dropped below 10°C and flows increased above 100 m3s-1 (Erickson et al. 2002).
Trends
Green sturgeon are generally encountered in marine and estuarine environments in Canada, thus habitat quality and restriction in these areas is of most concern. Estuaries in British Columbia (BC) are used for log storage and can be the eventual sinks to freshwater pollutants. An assessment of the quality of the lower Fraser River ecosystem was done in 1993 to 1994 by Healey et al. (1994) to determine whether species assemblages had substantially shifted since a similar survey was done by Northcote et al.’s (1976) in 1973 to 1974. A 2.5 fold increase in species density was found in the 1993 to 1994 survey and no dramatic changes in species assemblages were determined to have occurred over the 21 years between the studies. The BC Ministry of Water, Land and Air Protection assessed commercial and public usage of 51 BC estuaries and found that usage had increased from 2.1 to 4.7% from 1986 to 2001 of the total area assessed. 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) suggest 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).
Freshwater environments are still available to green sturgeon in Canada with no damming occurring in the lower portions of rivers where they have been historically observed. There is the possibility that some areas have been lost due to pollution from agriculture and industry. A die-off of white sturgeon in the FraserRiveroccurred in 1993 to 1994 and thirty-four dead “huge sturgeon”, mostly females, were found (MELP 1997). Pollution, thermal stress and the consumption of decomposing sockeye were suggested causes of the die-off. Although no green sturgeon were reported, the possibility that some green sturgeon could have been affected exists as all affected sturgeon may not have been found.
Protection/ownership
Green sturgeon are subject to protection from harmful alteration, disruption or destruction of fish habitat by Section 35 of the federal Fisheries Act.
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