Invasive and Non-native Vertebrates in the Lower Coos Watershed


  • Monitoring of striped bass and American shad in the Coos estuary have largely been suspended since populations of both non-native fishes has sharply declined since their height in the 1940s and 1950’s.

  • Invasive nutria cause substantial economic and ecological harm to coastal communities. All indications suggest populations are strong in the project area.

Figure 1. ODFW’s American shad and striped bass data collection sites. Figure shows fewer sites than were actually sampled since not all ODFW sites included lat/long information.

Figure 1. ODFW’s American shad and striped bass data collection sites. Figure shows fewer sites than were actually sampled since not all ODFW sites included lat/long information.

striped bass

striped bass



Due to the shift in focus, it should be noted that seining
efforts were not identical in all years (e.g.,
sites were missed at different years).
According to the ODFW database (2013),
American shad were found throughout the
estuary, most often and in the highest numbers
at Station 63 on the South Fork Coos
River (Figure 1).

They were most recently captured in 2008 at the same station. The age
class most frequently captured was juveniles,
often in exceedingly high numbers (500 –
1,000 in one seine was not uncommon, with a
high of 2,500 juvenile shad caught in a single
seine). Some (generally two or less per seine)
intermediates and adults were also caught.
A related ODFW study (2009) monitored
American shad and striped bass over-wintering
populations in Catching and Isthmus
Sloughs from 1979 to 1998. Researchers set
overnight gillnets in late spring (April-June) at
four sites in Catching and five sites in Isthmus
Sloughs. Small numbers (<10) of shad were found each year.

What’s happening?

This data summary describes available information
regarding the three most economically
and ecologically important non-native vertebrate
species found in the project area: two
fish species, American shad (Alosa sapidissima)
and striped bass (Morone saxatilis), and
one large rodent, nutria (Myocastor coypus).

American shad
Alosa sapidissima
The Coos basin once supported major yearly
runs of American shad each spring; large
enough to support a commercial fishery
in southwestern Oregon (Rothman 1968).
Shad populations have now dwindled to the
point that the run does not occur every year.
Starting in the late 1950’s, a large bacterial
outbreak began infecting American shad
populations during the spawning season in
southern Oregon coastal streams (including
the Coos watershed), causing large adult shad
population losses (Rothman 1968). Rothman
suggests this was the beginning of the sharp
decline in American shad populations along
the southern Oregon coast. A very small
recreational fishery (several anglers/year)
still exists for shad in the Coos watershed (G.
Vonderohe, pers. com. 2015).

By contrast, in the Columbia River, American
shad still represent the largest anadromous
spawning run, outnumbering all native
salmon (wild and hatchery combined), with
spawning adults estimated at 4-8 million in
recent years (Hasselman et. al. 2012b).

For nearly 30 years (1965-present), the Oregon
Department of Fish and Wildlife (ODFW)
conducted long-term fish monitoring, initially
targeting both juvenile American shad
and striped bass. Emphasis on those species
ended in 2006 as their populations declined;
monitoring efforts were re-focused to Chinook
salmon populations. However, incidental
catch data on the two non-native fishes
are still recorded when caught.


Non-native species – also called “alien”
species, this is a species that has been
introduced (either intentionally or accidentally)
to a location outside its native range.

Invasive species – a non-native species
that aggressively outcompetes native species
or causes significant economic loss.

Source: Clinton 1999

Other American shad population information:

Carlton (1989) listed shad as “abundant”
in the South Slough National Estuarine
Research Reserve (SSNERR)(Figure
1). Although no targeted studies have
reassessed the SSNERR population since
that report, it’s likely the populations in
South Slough have declined like the rest
of the Coos estuary. This is supported by
preliminary findings from a fish assemblage
study of the South Slough estuary.
Between July 2015 and June 2016 only 15 American shad have been caught.

An older study by Anderson (1985) found
that American shad larvae outnumbered
striped bass larvae 10 to one, and that
American shad juveniles outnumbered
those of striped bass 335 to one in 1983,
and 585 to one the following year in the
Coos River (Figure 1).

American shad were intentionally introduced
from their native US east coast habitats to the
Pacific coast habitats in 1871 when 10,000
fry were released into the Sacramento River,
CA (Hasselman 2012b). Subsequent introductions
to the Sacramento River occurred over
the next 10 years, totaling 574,000 fry (Hasselman

Five years after their initial introduction to
the Sacramento River, shad were being found
in the Columbia River, (Hasselman 2012b).
Numbers soon increased when nearly 1 million
shad fry were transplanted to the Columbia
River basin in the mid 1880’s (Hasselman
2012b). By the late 1800’s, shad were found
throughout Oregon estuaries.

Striped bass
Morone saxatilis
In 1914, some 35 years after striped bass
were intentionally released in the San Francisco
estuary as a commercial fish species
imported from the US east coast, the first
striped bass were caught in the Coos estuary.
By the mid-1920’s striped bass populations
were robust enough to support a commercially
fishery here (Parks 1978, Waldman et
al. 1998). By 1945, adult striped bass populations
peaked at an estimated 69,000 individuals
in southwestern Oregon (Parks 1978,
Waldman et al. 1998). Since then, striped bass populations have
declined drastically; fewer than 1,000 adults
were counted during monitoring in the 1990’s
(Waldman et al. 1998).

According to Moyle (2002), San Francisco
continues to maintain the highest striped
bass breeding population on the west coast.
In southwestern Oregon, striped bass populations
are the greatest in the Umpqua and
Coquille river estuaries (based on angler
effort and catch data)(G. Vonderohe, pers.
com. 2015).

Although they are considered an anadromous
fish on the west coast, striped bass spend
much of their life in the estuary. Striped bass
are also opportunistic and voracious predators,
feeding on juvenile salmon and other
small native fish (e.g., anchovies) and invertebrates
(e.g., bay shrimp)(Moyle 2002). Striped bass are still a highly regarded sport
fishing species in Oregon despite the commercial
fishery having closed years ago.

During ODFW’s long-term fish monitoring
program (2013)(described above under American
shad), striped bass (both hatchery and
wild) were found throughout the estuary,
most often and in the highest numbers at station
63 on the South Fork Coos River (Figure
1). Striped bass were most recently captured
in 2000 at the same station. After 2000, no
striped bass were caught, despite the same
stations being sampled specifically for striped
bass and shad until 2006. The age class most
frequently captured was juveniles (sometimes
by the hundreds), followed by intermediates
and then adults.

Winter gillnetting by ODFW (also described
above) found that overwintering striped bass
in Catching and Isthmus Sloughs fluctuated by
year and month captured. Striped bass generally
represented a higher catch rate compared
to American shad.

Anderson (1985) examined striped bass populations in the Coos River and estimated 912
wild striped bass in the Coos River system in
1983 and 1,003 in 1984. Anderson also found
that American shad larvae outnumbered
striped bass larvae 10 to one and American
shad juveniles outnumbered striped bass 335
to one in 1983 and 585 to one in 1984. The
majority of striped bass juveniles (hatchery
and wild) were found in the South Fork Coos
River – the majority of those at site #6 (Figure
1). No wild juveniles were found on the Millicoma

Curiously, the Coos population of striped
bass has unprecedented high levels of hermaphroditism
(one individual has both male
and female reproductive organs), a condition
that is exceedingly rare in striped bass
populations in their native range. (Waldman
et al. 1998). Waldman and colleagues (1998)
speculate that this anomaly is likely due to
a genetic bottleneck from the few founders
that strayed to the Coos estuary from San
Francisco Bay. This, along with subsequent
declines in the Coos population caused rare
genes (which expressed themselves through
hermaphroditism) carried by the remaining
individuals to pass on their genes to offspring,
which subsequently caused the gene to
spread throughout the population.

However, Elgethun and colleagues (2000) postulate
that elevated levels of contaminants
such as tributyltin accumulate in striped bass
(a higher order predator), causing reproductive
anomalies. Elgethun’s team measured
butyltin exposure of fish in the Coos estuary
and found that striped bass caught in Catching
Slough had elevated concentrations of
tributyltin (110 μg/kg) and total butyltins
(130 μg/kg)(Figure 1). Striped bass from Isthmus Slough were also measured and had
lower concentrations the same sampling year
(1992) at 40 μg/kg and 50 μg/kg respectively.

Like salmon, American shad are anadromous.
They spend a year in the estuary as juveniles
and adults spend 3-6 years in the ocean before
returning to their natal stream to spawn
(Pearcy and Fisher 2011). In the Coos estuary,
adults return to spawn in May and June, and
juveniles use the rivers and estuary as nursery
habitat (August-November) before returning
to the ocean (Monaco and Emmett 1990).

As a zooplankton-consuming species with
a similar diet to Chinook salmon, American
shad compete with native species for food
and space (Hasselman 2012a). Large numbers
of shad on the Columbia River have created
migratory delays for native fish due to shad’s
dense accumulation at the base of hydroelectric
dam fish ladders (Hasselman 2012a).

In addition, shad are parasitic hosts that can
inadvertently increase population densities
and ranges of parasites that also infect native
fish species (Hasselman 2012a). For example, Hasselman (2012a) describes how a parasitic
nematode was historically restricted to
marine waters where its native herring host
lived, until shad caused an ecological expansion
of the parasite into freshwater systems.
On the other hand, American shad, can also
alleviate avian and mammalian predation on
native fish, including salmon, by virtue of the
attractiveness of their population densities to
predators (Hasselman 2012a).

Figure 2. Estimated relative nutria densities in Oregon watersheds. Modified from: Sheffels and Sytsma 2007.

Figure 2. Estimated relative nutria densities
in Oregon watersheds. Modified
from: Sheffels and Sytsma 2007.

Myocastor coypus
There is very little information currently available
with which to estimate nutria status and
trends in the project area. Resource managers
understand from anecdotal observation
and the studies described below that nutria
continue to be a growing problem on Oregon’s
south coast. One contributing factor to
nutria’s spread especially in western Oregon
is their preference for regions with mild winters
such as the Oregon coast. According to
LeBlanc (1994), summer densities in Oregon
can range as high as 56 animals per acre.
The US Geological Survey’s (USGS) Nonindigenous
Aquatic Species program, consider
nutria to be established in the following
project area subsystems at least since 2007:
Coos River, Isthmus Slough, Haynes Inlet,
South Slough, Upper Bay, and in the Lower
Bay along the North Spit (USGS 2015).

While there’s currently no standard protocol
used to assess nutria distribution or abundance
in Oregon, Sheffels and Sytsma (2007)
created a distribution map based on ODFW
wildlife biologists’ best estimates for nutria
density at the 6th field HUC watershed scale
(Figure 2). According to Sheffels and Sytsma,
nutria densities in the project area subwatersheds
are considered to be medium (11-100 individuals/subsystem) in North Slough,
Haynes Inlet, Coos River, Catching Slough,
Isthmus Slough, and South Slough subsystems
and high (>100 individuals/subsystem)
in Pony Slough, Upper Bay, and Lower Bay

Witmer and Lewis (2001) describe nutria as
a large semi-aquatic rodent, first introduced
in Lincoln and Tillamook Counties, in the
late 1930’s for fur farming. Due to nutria
fur farming’s low economic returns, it was a
short-lived industry with lasting consequences.
Many failed nutria farmers released their
nutria stock into the wild where they soon became
naturalized. With the ability of female
nutria to produce 2-3 litters per year and
4-5 offspring per litter, nutria have become exceedingly abundant in the wild, especially
in central and western Oregon (Sheffels and
Sytsma 2007, Wentz 1971, citations within
Sheffels 2013).

Nutria cause numerous ecological and socio-
economic problems. They cause soil erosion,
reduced water quality, damage to native
flora, structural damage to channel banks and
levees, and are carriers of diseases and parasites
that can pass to humans, livestock and
pets (e.g., rabies)(Witmer and Lewis 2001).
Nutria can denude vast areas of vegetation
through foraging and creating grooming
platforms, trails, and dens (citations within
Witmer and Lewis 2001). Meyer (2006)
found that nutria selectively feed on forbs
(non-grass herbaceous vegetation) in coastal
Oregon wetland habitats and have denuded
large areas of both natural and restored tidal
wetlands in South Slough (Cornu, pers. com.
2015). Meyer also documented considerable bank
erosion in areas with nutria populations compared
to areas without nutria. The associated
excess turbidity in adjacent waters, affecting
fish species. Perhaps most importantly to coastal economies,
nutria significantly destabilize and
ultimately destroy waterway structures (e.g.,
dikes and levees) when they burrow into
banks (Sheffels and Sytsma 2007). These
sometimes extensive burrows can extend
up to 18’ in length and 2’ in diameter, and
include complex interconnecting passages
(Link 2004).


Anderson, D. A. 1985. A Study of the Early
Life History of the Striped Bass, Morone saxatilis
in Coos River Estuary, Oregon. Thesis
(M.S.)–University of Oregon, 1985.

Carlton, J. T. 1989. “Man’s Role in Changing
the Face of the Ocean: Biological Invasions
and Implications for Conservation of Near‐
Shore Environments.” Conservation Biology.
3(3): 265-273.

Clinton, W. B. 1999. Invasive Species. Federal
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Elgethun K, C Neumann, and P Blake. 2000.
“Butyltins in Shellfish, Finfish, Water and
Sediment from the Coos Bay Estuary (Oregon,
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Hasselman, D. J., R. A. Hinrichsen, B. A.
Shields, and C. C. Ebbesmeyer. 2012a.
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Invasive Species or Benign Introduction?.”
Fisheries. 37(3): 115-122.

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Wildlife Damage Management, viewed 1 May
2015: http://icwdm.org/handbook/rodents/

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Northwest. University of Washington Press,
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Moyle, P. B. 2002. Inland Fishes of California.
Berkeley: University of California Press.

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(ODFW). 2009. [Coos Bay spring gillnetting].
Unpublished raw data.

Oregon Department of Fish and Wildlife
(ODFW). 2009. [Coos spring gillnetting]. Unpublished
raw data.

Oregon Department of Fish and Wildlife
(ODFW). 2013. [Coos fish monitoring seine
program]. Unpublished raw data.

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sapidissima) along the Pacific coast of North
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coypus) Populations in the Pacific Northwest
and Development of Associated Control and
Management Strategies, with an Emphasis on
Metropolitan Habitats. Thesis (Ph.D.) —Portland
State University.

Sheffels, T. and M. Sytsma. 2007. Report on
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