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Green Sturgeon
Biology |
Life History |
Critical Habitat |
Status |
Threats | Conservation Efforts &
Research | References
Biology
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Sturgeons are most closely related to paddlefishes, reedfishes, and numerous
fossil groups within the infraclass Chondrostei that are primary
cartilaginous fish with some degree of ossification (bony structures). They
are not ancestral to modern bony fishes, but represent a highly specialized
and successful offshoot of ancestral Chondrosteans. Their skeleton is
composed of cartilage, and they have a series of external bony plates,
called scutes along their backs and sides. Sturgeon are often likened to
sharks (Chondrichthyes) because of the many features they share, including
spiracles, heterocercal tails, fin and jaw structure, spiral valve, and
Ampullae of Lorenzini. These unique sensory organs allow them to detect
electrical signals given off by prey in murky waters or mucky substrates.
Sturgeon do not have teeth, but instead use their long, flexible “lips” to
suck up food from the bottom.
Twenty-six species of sturgeon can be found in the temperate waters of the
Northern Hemisphere, two of which reside on the west coast of North America:
the green sturgeon, Acipenser medirostris, and the white sturgeon,
Acipenser transmontanus (Moyle 2002). |
Figure 1. A sturgeon’s protrudible
lips (M. Manuel). |
Green sturgeon were first described in San Francisco Bay by Ayres (1857).
Like most sturgeon they are anadromous, but tend to spend more time in the
ocean than most species. They can be found from Alaska to Mexico, but are
most commonly encountered north of Point Conception. Olive green
coloration, barbell placement, narrow snout, prominent lateral and ventral
green stripes, differences in number and sharpness of scutes, and presence
of an additional scute behind the dorsal and anal fins distinguish them from
the co-occurring white sturgeon.
Figure
2. Differences between green sturgeon (left) and white sturgeon (right).
Life History
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Green sturgeon
reach maturity around 15 years of age and can live to be 70 years
old. Unlike salmon, they may spawn several times during their long
lives, returning to their natal rivers every 3-5 years. By
comparing the DNA and movement patterns of tagged fish, researchers
identified two genetically distinct population segments (DPS) of
green sturgeon (Adams et al. 2002; Israel et al. 2004). Although
these fish may look identical, their genetic makeup is very
different. This distinction allows NMFS and other agencies to manage
populations more effectively, and helps preserve this diversity.
Fish that
spawn in the Klamath River in northern California and the Rogue
River in Oregon belong to the northern DPS (nDPS) and are listed by
NMFS as a Species of Concern. Fish that spawn in the Sacramento
River in California belong to the federally threatened southern DPS
(sDPS. Records also show that green sturgeon historically spawned in
the Eel River (California) and possibly the Umpqua River (Oregon),
but have not been found to utilize these areas today.
During
spawning runs, adult sDPS fish enter San Francisco Bay between
mid-February and early May and migrate rapidly up the Sacramento
River (Heublein et al 2009). Spawning occurs in cool sections of
the upper Sacramento River with deep, turbulent flows and clean,
hard substrate. In fall, these post-spawn adults move back down the
river and re-enter the ocean. After hatching, larvae and juveniles
migrate downstream toward the Sacramento-San Joaquin Delta and
Estuary. After rearing in the Delta and Estuary for several years,
they move out to the ocean. As adults, both nDPS and sDPS green
sturgeon migrate seasonally along the west coast, congregating in
bays and estuaries in Washington, Oregon, and California during the
summer and fall months and off northern Vancouver Island, BC, Canada
during the winter and spring months (Lindley et al. 2008). |
Figure 3. Map of sDPS green
sturgeon range and
habitat use variation by age class (NMFS
Santa Rosa). |
Figure
4. Generalized schematic of green sturgeon life cycle and habitat needs (R.
Beamesderfer).
Critical Habitat
Critical habitat was designated for sDPS green sturgeon on October 9, 2009,
and includes marine, coastal bay, estuarine, and freshwater areas (NMFS
2009). In freshwater, critical habitat includes the mainstem Sacramento
River downstream of Keswick Dam (including the Yolo and Sutter bypasses),
the Feather River below Oroville Dam, the Yuba River below Daguerre Point
Dam, and the Sacramento-San Joaquin Delta. Critical habitat within marine
waters include areas within the 60 fathom (= 110 m) depth isobath from
Monterey Bay to the U.S.-Canada border. Many coastal bays and estuaries are
designated as critical habitat, including: San Francisco Estuary and
Humboldt Bay in California; Coos, Winchester, Yaquina, and Nehalem bays in
Oregon; Willapa Bay and Grays Harbor in Washington; and the lower Columbia
River estuary from the mouth to rkm 74. Primary Constituent Elements (PCEs)
considered in the designation of critical habitat include food resources,
substrate type/size, water flow, water depth, water quality, sediment
quality, and migratory corridor.
Status
On April 7, 2006, NMFS
determined that the sDPS warranted listing as a threatened species with a
recovery priority number of 5 under the Endangered Species Act (71 FR
17757). Like many other species of sturgeon, green sturgeon have
experienced substantial population declines over the past century.
Unfortunately, ecology
and life history have received little study, presumably because of their
generally low abundance, limited spawning distribution, and low commercial
and sport fishing value.
Attempts to evaluate the status of sDPS green sturgeon have
been met with limited success due to the lack of consistent long term data
potential confusion with white sturgeon (Adams et al. 2002; Heppell and
Hofmann 2002).
Based on available
scientific data and ongoing conservation efforts, NMFS concluded in the
final rule that sDPS green sturgeon were likely to become endangered in the
foreseeable future throughout all of its range. The most critical factors
in the formulation of this conclusion were: a) Reduction of potential
spawning habitat, b) severe threats to the single remaining spawning
population, c) the inability to alleviate these threats using current
conservation measures, and d) the continued observance of declining numbers
of juveniles collected in the past two decades.
Threats
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Green sturgeon populations successfully
persisted throughout western North America for
two-hundred million years, but are thought to have experienced a
precipitous decline during the past century.
Harvest of adults likely resulted in direct declines
in abundances, and destruction of spawning and rearing habitats led to
reduced population sizes and resilience. With
regulations prohibiting harvest or take now in effect, the most
significant threats to green sturgeon likely relate to loss and
inaccessibility of available spawning habitat.
Much
of this is driven by competing water resource needs between humans and
fish.
Temporary dams, altered flows, and entrainment in water diversions can
impede or inhibit both upstream spawning migrations of adult green sturgeon
and downstream migrations of juveniles (Heublein et al. 2009). Insufficient
freshwater flow rates in spawning areas, contaminants,
fisheries bycatch, poaching, invasive species, impassable barriers,
and unfavorable water conditions are also likely
to threaten the survival and recovery of this
ancient fish.
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Figure 5. Red Bluff Diversion Dam (RBDD),
a known
barrier to green sturgeon migration (J.
Day).
|
Conservation Efforts &
Research
Figure 6. USFWS conducts egg mat sampling on the
Upper Sacramento River to determine when and
where green sturgeon spawn (B. Poytress).
|
Regulations prohibiting the retention of green sturgeon in both recreation
and commercial fisheries throughout California, Oregon, Washington, and
British Columbia have been implemented. These regulations represent a
significant reduction in the risk of loss of green sturgeon to fishing
activities, and are expected to have a substantial conservation impact.
Recent research efforts have focused on monitoring early life history
stages and estimating adult abundance to better evaluate overall species
status (Israel et al. 2010).
Additionally, increased
understanding of the impacts of contaminant exposure, ocean energy
projects, predation by native and non-native species, foraging and
feeding behavior, and baseline population viability data are sought to
support the construction of a sound conservation plan for the ecosystems
upon which green sturgeon depend.
Numerous studies of the distribution, migration, spawning habitat
utilization, and population genetics of green sturgeon are being currently
underway through both collaborative and independent efforts at
University of California at Davis (UCD),
Bureau of Reclamation (BOR),
U.S. Fish and
Wildlife Service (USFWS), Oregon
Department of Fish and Wildlife (ODFW),
California Department
of Water Resources (CDWR),
California Department of Fish and Game
(CDFG),
Washington
Department of Fish and Wildlife (WDFW),
Oregon Department of Fish and Wildlife,
and
National Marine Fisheries Service (NMFS).
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|
CDWR biologists use cutting edge sonar
equipment to monitor
sturgeon presence in the Sacramento and
Feather Rivers (J. Day). |
References
Adams P.B., C. Grimes,
S.T. Lindley, and M.L. Moser. 2002. Status review for North American green
sturgeon, Acipenser medirostris. NOAA, National Marine Fisheries
Service, Southwest Fisheries Science Center, Santa Cruz, CA. 50 pp.
Heppell S.S., and L.
Hofmann. 2002. Green sturgeon status assessment. Oregon State University
report to National Marine Fisheries Service Southwest Fisheries Science
Center, Santa Cruz, CA. 74 pp.
Heublein J.C., J.T.
Kelly, C.E. Crocker, A.P. Klimley, and S.T. Lindley. 2009. Migration of
green sturgeon Acipenser medirostris, in the Sacramento River.
Environmental Biology of Fishes 84(3): 245-258.
Israel J.A., J.F. Cordes,
M.A. Blumberg, and B. May. 2004. Geographic patterns of genetic
differentiation among collections of green sturgeon. N. Am. J. Fish. Man.
24: 922-931.
Israel J.A. and B. May.
2010. Indirect genetic estimates of breeding population size in the
polyploidy green sturgeon, Acipenser medirostris. Molecular Ecology
19: 1058-1070.
Lindley S.T., M.L. Moser,
D.L. Erickson, M. Belchik, D.W. Welch, E. Rechisky, J.T. Kelly, J. Heublein,
and A.P. Klimley. 2008. Marine migration of North American green sturgeon.
Trans. Am. Fish. Soc. 137: 182-194.
Moyle P.B. 2002. Inland
Fishes of California, 2nd edition. University of California
Press, Berkeley and Los Angeles, California. 502 pp.
Ayres W.O. 1857.
Descriptions of new species of fish from San Francisco Bay. Proc. Calif.
Acad. Sci. 1:1-77.
10/05/2011 |
