Acipenser transmontanus

Gene sequencing data of Birstein and DeSalle (1998) indicate that there are least three main clades within Acipenser: A. sturio-A. oxyrinchus, A. schrenckii-A. transmontanus, and all Ponto-Caspian species plus A. dabrysnus and A. brevirostrum.

Krieger et al. (2000) examined phylogenetic relationships of North American sturgeons based on mtDNA sequences and found that (1) nucleotide sequences for all four examined genes for the three Scaphirhynchus species were identical; (2) the two Acipenser oxyrinchus subspecies were very similar in sequence; (3) A. transmontanus and A. medirostris were sister taxa, as were A. fulvescens and A. brevirostrum (in contrast to Birstein and DeSalle 1998).

This species has a large range along the eastern Pacific Ocean and uses the ocean environment during migration and other parts of its life cycle. This species is managed for harvest and conservation purposes. Several threats, all of which are on-going, have reduced populations in some areas while populations are stable in others. Dams and other passage barriers continue to limit the riverine and estuarine habitat distribution of this species along the North American west coast.

This species inhabits fresh water riverine and estuarian waters from the Gulf of Alaska south to about Ensenada, Mexico and spawns in estuaries and other suitable habitat associated with major coastal rivers. This species may occur in landlocked populations spawning in river bottoms and freshwater estuaries or utilize the open western Pacific Ocean for accessing coastal estuaries. Spawning has been documented in the Fraser, Kootenay, and Upper Columbia Rivers (British Columbia, Canada), the Lower Columbia River (Washington and Oregon, USA) and Willamette River, (Oregon; pers. com., L. Heironimus 2024), the Kootenai River Basin (Idaho and Montana, USA), and the Sacramento, Feather, and occasionally San Joaquin Rivers (California, USA) (Moyle et al 2015). Current spawning is thought to be limited to the Fraser, Columbia, and Sacramento-San Joaquin River Systems (COSEWIC 2013, Dubois et al 2011). Using the Hammerson and Master (2009) method for determining Range Extent for freshwater and anadromous fish, this species likely occupies over 1,000,000 square kilometers in over 100 HUC 8 watersheds. Natureserve recognizes and maintains records for 6 populations of this species as defined by COSEWIC and the USFWS for conservation purposes. These populations are used for management and do not necessarily reflect genetic units. They are: Kootenai River in Idaho and Montana, USA (Pop 1; USFWS 1994), and the Upper Columbia River (Pop 2; COSEWIC 2013), the Nechako River (Pop 3; COSEWIC 2013), the Lower Fraser River (Pop 4; COSEWIC 2013), the Upper Fraser River (Pop 5; COSEWIC 2013), and Middle Fraser River (Pop 6; COSEWIC 2013), Canada. The other portions of the species' range (e.g., in California and Oregon) are not currently designated by US or Canadian authorities.

There are likely between 15 – 30 populations of this species including between 12 and 25 in Canada (Lower Fraser, Mid Fraser, Lower Fraser, Nechako, Upper Columbia, and Kootenay), one to a few in Washington and Oregon, USA, (JCRMS 2015), one in Idaho and Montana, USA, (Lower Kootenai River) and possibly three in California, USA, (Upper Sacramento River, Feather River, and San Joaquin River). Most of these populations are not discrete; where not landlocked, individuals migrate in the open ocean to various rivers and estuaries along the Pacific North American coast. However, the described populations are associated with current spawning regions and for purposes of this assessment are considered the occurrences.

Passage barriers and dams are likely the single most significant threat to this species. They prevent fish and other aquatic species from accessing upstream habitat that was often available historically thus reducing overall habitat availability for the species. They also fragment populations reducing allelic diversity throughout their range, alter the natural hydrograph impacting the timing and duration of spawning migrations and aggregations, and block the movement of native fish that bring marine-derived nutrients into upriver environments limiting prey availability and the overall growth potential for individuals located upstream of the dams. Local water diversions – many of which are undocumented and unregulated – reduce stream flows over and above constraints imposed by dams creating passage barriers that fragments remaining habitat. Operation of new and existing agricultural lands adjacent to rivers and streams occupied by this species are also a threat. Conversion to agriculture appears to be increasing in some areas where viticulture, marijuana, and other agricultural production effect flow regimes by reducing overall flows. Agricultural operations introduce pesticides and fertilizers into streams that can create toxic conditions for this species which are particularly vulnerable to pollutants (Moyle et al 2015). In California, USA, this species spends much of its life cycle in coastal estuaries associated with the San Francisco Bay, Suisun Bay, Suisun Marsh and San Pablo Bay. Estuaries associated with other major coastal rivers including the Columbia and Fraser River, Canada, and likely others along the west coast of North America are used by this species. Estuary degradation has reduced available habitat for this and other species along the North American Pacific coast. Recent predation by Steller sea lions (Eumetopias jubatus), California sea lions (Zalophus californianus), and harbor seals (Phoca vitulina) has been implicated as a decline factor within the Columbia River system (JCRMS 2024a) and may be a factor in other locations. Similarly, mortality from commercial salmon fishery by-catch has been identified as a potential threat in some parts of this species range and may be a threat in others (COSEWIC 2013). The effects of Climate change on freshwater fish vary regionally but include changes in water quality (especially water temperature), changes in timing and duration of flows, and extended drought and variable water availability. In a comprehensive review of climate vulnerability of at-risk freshwater fish, Moyle et al (2013) assessed 164 taxa against exposure and sensitivity criteria within four generally accepted and relevant climate scenarios. This species was classified as “vulnerable” and likely to become endangered in California, USA. Climate change will likely impact this species in a similar way in other parts of its range as well (Columbia Basin Trust 2017, WDFW 2024).