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THE IMPACT OF NUTRIA (Myocastor coypus) AS AN INVASIVE SPECIES AND
ITS POSSIBLE DISTRIBUTION IN WASHINGTON STATE
By
Ryan Kruse
A Thesis
Submitted in partial fulfillment
of the requirements for the degree
Master of Environmental Studies
The Evergreen State College
July 2012
©2012 by Ryan Kruse. All rights reserved.
This Thesis for the Master of Environmental Studies Degree
by
Ryan Kruse
has been approved for
The Evergreen State College
By
________________________
Gerardo Chin-Leo, Ph.D.
Member of the Faculty
________________________
Date
ABSTRACT
THE IMPACT OF NUTRIA (Myocastor coypus) AS AN INVASIVE SPECIES AND
ITS POSSIBLE DISTRIBUTION IN WASHINGTON STATE
Ryan Kruse
Nutria (Myocastor coypus) is a rodent native to South America that has been introduced
throughout various habitats worldwide, mostly through the fur industry. Nutria are
voracious consumers of emergent vegetation and have been known to convert large
sections of wetland habitat into standing water through subsequent erosion, and
additionally can cause damage to embankments and water control structures due to their
burrowing habits. This study reviews nutria infestation in a few locations, such as
England, where nutria have been successfully eradicated, and Louisiana, where nutria are
a pervasive ecological threat. There is a current lack of information on the range and
severity of nutria infestation in Washington State. This thesis uses data from various
agencies and GIS to produce a range map of nutria sightings, a map of where nutria have
been removed or controlled, and a map showing suitable habitat for nutria infestation.
Currently, nutria are distributed across Western Washington, with some evidence
showing a few populations East of the Cascades. The habitat suitability map is consistent
with all nutria sightings, and shows many areas highly suitable for nutria where nutria
have not yet been reported or surveyed. A lack of coordination between agencies
responsible for wildlife management and inadequate follow up to sighting reports has
resulted in uncertainty of where nutria are present, and whether or not they have been
successfully eradicated in some areas. More research, combined with a public awareness
campaign and centralized reporting methods could result in better estimates of the
abundance and distribution of nutria in Washington State, leading to more effective
control.
TABLE OF CONTENTS
Page Number
Chapter 1
Chapter 2
Chapter 3:
References
Introduction
Page 1
Biology and Behavior
Page 2
Nutria as a Pest
Page 6
Thesis Contribution
Page 9
Nutria Infestation by Region
Page 10
Overview
Page 18
Methods
Page 19
Results and Discussion
Page 22
Conclusions and Recommendations
Page 31
Page 35
iv
List of Figures
Figure 1: Image of an enclosure experiment in which nutria herbivory has been
prevented with fencing. The area surrounding the enclosure demonstrates the degradation
of marsh vegetation as a result of nutria feeding. From http://www.nutria.com/site24.php
..............................................................................................................................Page 8
Figure 2: Nutria sightings within Washington are displayed, with polygons representing
watersheds on the HUD12 level, and points reflected nearest town reporting nutria.
Squares represent historical nutria farms. Sightings predating 1980 are displayed
separately.
…………………………………………………………………………….……..Page 24
Figure 3: Number of nutria removed by the USDA wildlife services are displayed with
graduated symbols by nearest populated area. Nutria were removed via trapping or
shooting.
…………………………………………………………………………….……..Page 27
Figure 4: Suitable habitat for nutria infestation in Washington is displayed, based off of a
distance from water parameter (<1200 yards), and an average minimum daily temperature
parameter (>39 degrees F). Average minimum daily temperatures of over 50 degrees F
are shown in darker red to display optimal habitat.
…………………………………………………………………………………...Page 30
v
Acknowledgments
The completion of this thesis required the support and assistance of certain individuals. I
thank my reader, Dr. Gerardo Chin-Leo, for his guidance, feedback, and
recommendations. I must thank Trevor Sheffels, PhD candidate from Portland State
University, for his work on nutria in the Pacific Northwest, and his communication
during the infancy of my research. I thank the United States Department of Agriculture,
Mathew Cleland, the Invasive Species Network, Washington State Fish and Wildlife, and
anyone else who provided me with information on nutria. I thank my parents, for
supporting me throughout my education. I thank Jason Lim, for keeping me company in
the computer center, and using his library skills to hunt for papers. And I thank the
rodents of unusual size, for providing something worth studying.
vi
Chapter 1:
Introduction
Invasive species are capable of causing a number of ecological disturbances as
well as economic problems, and are considered one of the most imminent threats to
biodiversity (Chornesky & Randall, 2003). The nature of many invasive species is not
well understood, and managing or mitigating for these problems is complicated,
particularly in the face of dynamic ecosystems and a changing climate (Chornesky &
Randall, 2003; Hellman et al., 2008). Invasive plants and animals may compete for space
or nutrients, prey upon native species, or disturb ecological balances in a number of
ways. Often, invasive species reproduce faster and reach larger sizes in these new
habitats than in their native range (Buckley et al., 2003; Clout & Poorter, 2005). Once an
invasive has established a considerable population, it is extremely difficult to remove
(Hoagland & Jim, 2006). For this reason, assessing the status of an invasive species in a
given region is necessary in order to make decisions regarding whether the species can be
controlled or eradicated.
Nutria (Myocastor coypus) or the coypu, is a semi-aquatic rodent native to South
America that has been introduced to wetland habitats worldwide (Jacoby Carter &
Leonard, 2002). Its natural habitat is along rivers and lakes and in marshes and estuaries.
Nutria has historically been valued for its fur, which has led to its spread as an invasive
species through escapes from fur farms as well as intentional introduction. The animal is
considered a pest or nuisance in many areas outside of its native range due to its feeding
and burrowing habits (Jacoby Carter & Leonard, 2002).
1
Because nutria are capable of rapid reproduction and are highly adaptive and
capable of increasing their range into new habitats, their detrimental effects are of
concern to stakeholders at multiple levels. There is ample evidence that nutria are capable
of destroying ecologically valuable wetland and marsh habitat, as well as creating
economic costs through crop loss, damage to embankments and dams, and potential
flooding (Jacoby Carter & Leonard, 2002).
Attempting to control an invasive population such as nutria is typically expensive,
and sometimes futile (Bomford & O’Brien, 1995). It requires an understanding of the
extent and significance of the problem in order to understand if the species necessitates
control, and if so, how control can be accomplished. Failure to understand the status of an
invasive species may allow the species to gain a stronger hold and incur larger damages if
action is not taken (Hoagland & Jin, 2006). On the hand, resources could easily be wasted
in an attempt to control a species in a manner that is ineffective or unnecessary. Some
states, such as Louisiana and Maryland, have already instituted nutria control programs
after suffering great ecological damage to nutria infestation (Jacoby Carter & Leonard,
2002). Other states have not studied the effects or extent of nutria infestation in their
locality.
Biology and Behavior
Myocastor coypus is known in many countries as coypu, and as nutria in the
United States. This caviomorph rodent is native to South America, South of 23 degrees
latitude. Nutria look similar to beavers, but are smaller and have a rat-like tail. On
2
average, adults are relatively large (up to 7kg). Other characteristics include webbed hind
feet, orange to yellow colored incisor teeth, and reddish-brown fur (Woods et al., 1992).
Nutria are adapted to an aquatic lifestyle, but are capable of moving quickly whether on
land or in water. They eat primarily aquatic vegetation, but will also consume terrestrial
plants, with a possible preference for monocotyledons (Guichón et al., 2003). In its home
range in South America, it has been documented to occasionally consume mollusks
(Larrison, 1943).
In South America, nutria populations are limited by hunting and the animal is
considered of economic value for as a furbearer. Its status as a pest or asset in non-native
areas is usually tied to its economic viability as fur bearing species. For example, in areas
such as Eastern Europe, where fur is more valued, nutria are considered a resource,
whereas in Western Europe, the fur market is smaller and does not outweigh the
ecological concerns of nutria presence (Jacoby Carter & Leonard, 2002).
Most areas of nutria invasion are considered to be the result of escape from
captive farming, although in some regions nutria were intentionally released as a game
animal or for vegetation control (Jacoby Carter & Leonard, 2002). In favorable habitats,
nutria populations have been established and have increased in range. The main factor for
success in population establishment appears to be temperature, with nutria unable to
survive harsh winters (Gosling, 1986). Consecutive days of frost have been shown to
cause substantial deaths in both adult and juvenile nutria, and in harsher conditions
female nutria have been known to abort litters (Guichón et al., 2003). However, there is
evidence that nutria have a high level of behavioral flexibility. Their ability as a tropical
species to spread to temperate areas is indicative of this flexibility, and nutria have spread
3
to areas previously considered too cold for survival. Nutria, in a pattern similar to many
invasive animals, have been shown to mature earlier and reach larger body sizes in
invaded areas than in their native range (Guichón et al., 2003). This may be, in part, an
evolutionary response to the temperate conditions experienced in its new habitat.
Nutria are described as preferring stagnant fresh water, but are also known to
inhabit salt water and brackish habitat (Jacoby Carter & Leonard, 2002) . They
preferentially eat aquatic and semi-aquatic vegetation in their habitats, such as reeds and
sedges. However, nutria are capable of consuming a wide variety of vegetation and their
diet in a given habitat is highly variable according to what is easily available, including
crops adjacent to their habitat (Guichón et al., 2003). Often, these are the losses most
easily calculated as a result of nutria infestation.
Nutria have been described as abundant in holding ponds and drainage ditches,
and some researchers have provided evidence that the animals thrive in highly eutrophied
water systems, such as sewage lagoons near cattle ranches (Brown, 1975). This may be
due to a fertilizing effect of polluted water on emergent vegetation, which provides a
dense food source for the rodent. For example, in Florida, enriched waters are often
choked with the exotic water hyacinth, which nutria utilize heavily (Brown, 1975).
However, nutria are thought capable of infesting any wetland habitat, including those that
are relatively undisturbed (Usher et al., 1986).
Nutria have a variable home range. Mark-recapture studies in Louisiana have
suggested that typically nutria remain within one general area throughout their lives,
rarely traveling over 1,200 yards from where they are released. Daily movements of
4
nutria are usually within 200 yards. Some nutria, however, moved large distances: 18
miles within 167 days, and 15 miles within 67 days (Chabreck, 1962). This means nutria
are perfectly capable of infesting new areas within a fairly short amount of time, although
the majority of the animals may never leave the habitat of their birth. The degree to
which nutria spread into new habitats may be dependent on the quality and abundance of
the resources in that habitat.
In their native South America, nutria have been described as nocturnal, with an
occasional shift towards diurnal behavior when temperatures are colder (Woods et al.,
1992). However, research on nutria in temperate areas has described nutria as being most
active during dawn and dusk hours (crepuscular). Some tracking of nutria behavior in
Louisiana has shown nutria to be most active at night, with activity decreasing through
dawn (Chabreck, 1962). It is possible that the crepuscular behavior is a misconception, as
unless monitoring includes night hours, nocturnal animals will appear to be most active at
dawn and dusk. Behavior may vary from region to region depending on climate.
Nutria typically live around 3 years, but reproduce rapidly. Females can have two
litters per year, ranging from 5-13 animals per litter. The average number per litter seems
to be highly variable between locations (Brown, 1975). It is this rapid reproduction that
allowed nutria to grow from an estimated 20 escaped individuals to a population of
twenty million nutria within 20 years in Louisiana. Given the high fecundity of nutria,
the chances that even a small number of animals distributing to new habits will lead to
infestation is high.
5
Nutria as a Pest
Nutria pose a number of risks as a non-native species. They are considered
voracious consumers of emergent vegetation, eating up to 25% of their body weight per
day (Shaffer et al., 1992). They also have a tendency to consume the base or stalk of
plants, often uprooting the plant and allowing the unconsumed portions to wash away
(Hailman, 1961). This means a population of nutria is capable of converting wetland or
marsh habitat into open water within a relatively short amount of time. Areas in which
nutria have significantly depleted vegetation are called "eat outs". In addition to
herbivory, the swimming channels created by nutria to move through marshland have a
fragmenting effect and can accelerate the rates of loss (Shaffer et al., 1992). In some
areas these feeding habits have led to the loss of entire wetlands. Louisiana has attributed
a loss of over 600,000 acres of coastal wetlands to nutria invasion (Louisiana Fish and
Wildlife). England, before beginning a control program, attributed major losses of reed
swamp to the feeding habits of nutria (Boorman & Fuller, 1981). Maryland has lost over
7,000 acres of salt marsh in the Blackwater National Wildlife Refuge area, citing nutria
pressure along with sea level rise. This habitat is considered vital for nesting waterfowl
such as the Maryland state-listed black rail.
Areas in which wetlands are being restored and replanted are of particular
concern. For example, wetland restoration in Italy has been slowed as nutria expanded
their range into the newly created habitat and depleted the vegetation (Bertolino et al.,
2005). The disturbance caused by nutria may favor the establishment of invasive plants
through competitive exclusion, as native plants are unable to establish themselves under
the pressure of hervibory.
6
Nutria invasion has the potential to threat rare plants or deplete locally abundant
species. In England, the rare water soldier (Stratiotes aloides) was nearly included on the
IUNC Red List of Endangered Species due to nutria herbivory. Other locally abundant
species in England were nearly or completely eliminated from certain localities (Usher et
al., 1986).
Nutria are additionally considered an agricultural pest which can affect crops
adjacent to their habitats. Crop feeding is more prevalent in winter months, where nutria
have been known to feed up to 1.5 km away from their habitats (Usher et al., 1986). A
variety of crops have been recorded as affected by nutria, although alfalfa, sugarcane, and
rice have been particularly affected. Additionally, they have been reported as destroying
fruit and nut trees, and conifers. In some areas, such as Italy, damage by nutria to crops
have been calculated in the millions of dollars (Bertolino & Viterbi, 2009).
Nutria burrow into banks and water control devices in order to make their dens.
These burrows are usually submerged at the opening and are not easily spotted. These
burrows can undermine the stability of these banks, leading to increased erosion and
flooding. Burrows are of particular concern for farmland areas and other regions in which
water may be at an elevated plain, such as throughout the Netherlands.
Nutria present a further risk as an invasive species as carriers of parasites and
dangerous bacteria, including leptospirosis, or tularemia, which can be spread to humans
through biting, or contact with the animal or its feces (Waitkins et al., 1985). Bacteria and
diseases carried by nutria may be spread to livestock or pets.
7
The true costs of nutria infestation are often underestimated, as ecological
damage, especially to wildlife areas are difficult to price. Nutria may act in tandem with
other factors in wetland loss, such as development, deforestation, and water level rise,
accelerating wetland loss. Control is considered cost effective if the amount spent
controlling the invasive is less than damages that would be suffered without control.
However, future costs are difficult to predict, and externalities are likely to be left out of
calculations. A cost-benefit analysis may fail to consider the cost of controlling nutria
should densities increase.
Figure 1:
Figure 1: Area photograph of an enclosure experiment in which nutria herbivory has been
prevented with fencing. The area surrounding the enclosure demonstrates the degradation
of marsh vegetation as a result of nutria feeding. From http://www.nutria.com/site24.php
8
Thesis Contribution
Nutria have been present in Washington State since the late 1930s, but little is
known about their current range, their population numbers, or the severity of their impact
as an ecological or economic threat in this region. This thesis contributes by adding to
knowledge of this species in the Washington State, including population and range, level
of threat, and current degree of management. Furthermore, this thesis draws conclusions
on how policy can be improved regarding nutria control in Washington State.
I will begin with a brief overview of this species and its history as an invasive
species. I will review nutria infestation in key areas in the US, such as Louisiana and
Maryland, and in Europe, and discuss how these regions been affected by nutria, and how
they have addressed or failed to address this species. These regions will be presented as
case studies in order to inform the outlook of nutria infestation in Washington and the
potential for mitigation.
Using sighting data from a number of agencies, I will provide a range map to
show the known extent of nutria infestation in Washington, as well as discuss knowledge
gaps and communication barriers between agencies involved with this species. Using
information about the preferred habitat of nutria, I will present a habitat suitability map to
discuss areas in which nutria may be likely to spread. Last I will make conclusions and
recommendations in order to address nutria in Washington State.
9
Chapter 2:
Nutria Infestation by Region
England:
Nutria were first brought to England in 1929 for fur farming, and subsequent
escape lead to feral populations. One researcher discussed a few escapes of the animal in
1935, stating "it does not seem as if the coypu would readily establish itself in the first
place, and its extermination, if necessary, should not prove difficult" (Warwick, 1935).
Nutria were observed to increase their range from the 1940s into the 1960s, and
populations were likely around 200,000 animals (Stokstad, 1999).
In the late 1950s, nutria herbivory became a noticeable problem, with damage to
crops and the banks of waterways posing economic concern, and the loss of desirable
plant species becoming and ecological threat. Loss of crops included the sugar beet, with
nutria capable of destroying several acres of the crop by biting off the top of the tap root
and subsequently killing the plant. Other affected crops included kale, brussel sprouts,
potatoes, and mangolds (Norris, 1967).
While trapping began as early as the 1940s, coordinated attempts to decrease the
number of nutria began in the 1960s. Severe winters caused large declines in the
populations, but researchers found the populations thrived and returned in milder winters.
Using weather models, researchers were able to predict the costs of nutria control with
focus on years with mild winters.
10
In 1981, a ten-year attempt to eradicate nutria began as a cost of 4 million dollars.
An Organization called "Coypu Control" was formed, consisting of 24 trappers. Their
methods consisted of cage trapping followed by euthanasia. In 1989 the species was
considered extinct in the area (Jacoby Carter & Leonard, 2002). England remains the
best documented example of a successful effort to eradicate nutria (Jacoby Carter &
Leonard, 2002).
Italy
Nutria may have been introduced to Italy as early as 1928, and is now considered
widespread in the Northern and Central regions (Bertolino et al., 2005). Damage caused
by nutria, particular hampering wetland restoration efforts led to control programs in
some regions. A cost benefit analysis of nutria control in Italy from 1995 to 2000
estimated a cost of € 11,631,721 in damage caused by nutria through damage to crops
and riverbanks. Over 220,000 animals were removed at a cost of € 2,614,408. However,
these efforts failed to control populations or curb the increases in nutria damage. The
reason for ineffectiveness of control efforts may have been timing, with managers only
attempting to control populations at high densities and after damage had already taken
place (Bertolino & Viterbi, 2010).
Attempts to eradicate nutria from Italy have provided evidence that nutria are
resistant to population control. An intensive trapping campaign from 1994 to 1996 in a
wetland area of Northern Italy resulted in the removal of 8,600 animals, yet failed to
reduce the number of nutria present (Cochi & Riga, 2008). Nutria may have
counterbalanced severe winters and the trapping campaign with a higher birth rate and
11
lower fetus reabsorption rate, suggesting that nutria are highly capable of resisting
population control (Cochi & Riga, 2008).
Trends in the United States:
Ranches farming nutria were established in the United States through the 1930s in
Louisiana, Michigan, New Mexico, Ohio, Oregon, Utah, and Washington (Ashbrook,
1948; Jacoby Carter & Leonard, 2002). The fur industry crashed during World War II
due to a decline in the price of fur pelts, and many defunct farms simply released the
animal (Jacoby Carter & Leonard, 2002). Nutria were also intentionally introduced across
the Southeast, where state and federal agencies promoted the animal for weed control.
Nutria are currently thought to have stable or increasing populations in at least 15 states,
although the number could be as high as 20 (Jacoby Carter & Leonard, 2002). Some
states have reported small feral populations that were successfully eradicated; however,
these populations may have already been limited by unfavorable habitat conditions. Other
states have reported small feral populations that may have died off without eradication
efforts (Jacoby Carter & Leonard, 2002). Nutria have shown the ability to spread from
state to state, with Maryland populations likely the source of populations in Delaware and
populations in Louisiana using gulf waterways to establish populations in the Florida
panhandle (Brown, 1975).
Louisiana:
12
Nutria were first introduced in the New Orleans area in the 1930s, but this small
population was thought to have been eradicated. Fur farms beginning in 1938 are thought
to be the source of the current nutria population. There are records of nutria escape from
farms following hurricane damage to pens and fences in 1940; however, there is some
documentation of intentional release prior to that event, possibly to supplement the fur
industry. Two years later, populations were observed 78 km away via water from the
release site (Jacoby Carter & Leonard, 2002). From these initial founders, nutria reached
an estimated population of 20,000,000 by 1960 (Woods et al., 1992). Nutria were
reported frequently as destroying levees, wetlands, and agriculture (Jojola et al., 2009).
Fur trapping contributed to control populations of feral nutria through the 1960s and 70s.
The decrease in the price of fur in the 1980s lead to decreased trapping. Nutria
populations again began to rise, as did reports of damage (Jojola et al., 2009). An
estimated 80,000 acres of wetland have been damaged by nutria in Louisiana, which is
likely an underestimate, as only the most severe damage is detectible via aerial survey
(Louisiana Department of Fish Wildlife).
Nutria in Louisiana are likely the dominant force in destroying wetland habitat
and preventing the reestablishment of vegetation in marshland and swamp forests
(Woods et al., 1992). Plants such as the bald cypress, and Spartina sp. have been
reportedly destroyed as fast as they are planted, disrupting efforts to conserve habitat.
Large and rapid loss of coastal wetlands led the Louisiana government to institute a
bounty program to thin nutria populations: the Coastwide Nutria Control Program in
2002. Since the instatement of the program, the number of impacted wetland sights and
total impacted areas has dropped to as low as 6,900 acres. The program initially offered 4
13
dollars for a nutria tail, raising to 6 dollars to registered participants (Jojola et al., 2009).
Before the inception of the program, nutria harvest in Louisiana was around 25,000 pelts
a year. A few seasons later, numbers were as high as 375,000 nutria harvested per year,
with a stated goal of 400,000 nutria harvested annually (Sheffels & Sytsma, 2007). Other
methods of encouraging nutria harvest used by the Louisiana government include
advertising nutria as a delicacy (Sheffels & Sytsma, 2007).
Maryland:
Nutria were introduced to the eastern shore of Maryland in the early 1940s, and
have since spread across the coastal marshes, including the Backwater National Wildlife
Refuge (Dixon, 2012). Much of the refuge consists of floating vegetation on top of fluid
mud; nutria herbivory damages the root mats that hold the marsh together, cause it to
break up and wash away with tidal action. Initial populations within the refuge were as
low as 150 animals, which has increased to around 50,000 nutria today (Stokstad, 1999).
The animals are estimated to cause 2.8 million dollars of damage, mostly through the loss
of hunting, fishing, and hiking opportunities. Over the past 40 years, the refuge has lost
7,000 acres of salt marsh to nutria herbivory. The fragile marsh systems provide valuable
nesting habitat for water fowl, as well as habitat for varieties of fish and crustaceans.
A two year program, hiring 15 trappers, has been reported to have eliminated the
rodents from the refuge at a cost of 2 million dollars (Fahrenthold, 2004). Control efforts
were modeled after the successful eradication program in England and included efforts to
research populations, reestablish wetlands, and educate the public. Although the
14
Blackwater Refuge may be free of nutria, the animal continues to have a presence across
Maryland.
Oregon:
Nutria farms were established in Oregon in the 1930s and 40s, including in the
Portland and Tillamook areas. There may have been as many as 600 farms raising nutria
at this time (Sheffels & Sytsma, 2007). Feral nutria have spread across Oregon, and
trapping records indicate that feral nutria have been present on either side of the Cascade
mountain range (Winter & Lewis, 2001). Research has suggested that it is the burrowing
habits of nutria, such as damage to water control devices that are the primary concern of
nutria infestation in the Pacific Northwest, as opposed to the ecological destruction in
Louisiana and Maryland (Sheffels & Sytsma, 2007).
The Center for Lakes and Reservoirs at Portland State University was assigned in
2001 to develop an Oregon Aquatic Species Management Plan, which would include
outreach, prevention, detection, research, and mitigation protocol. Accurate
communication and documentation were established as important goals in order to
address nutria efficiently. The CLR conducted a 'Nutria Management in the Pacific
Northwest' workshop and produced a range map focusing on nutria density, mostly in
Oregon, but including some regions of Washington.
Researchers in Oregon conducted an enclosure controlled herbivory study to
analyze the effects of nutria feeding habits in the area. While herbivory was observed on
15
some of the study sites, the research remained inconclusive due to insufficient data
(Sheffels & Sytsma, 2007).
Washington State:
Feral nutria were first described in Washington State as early as 1941 from pelts
collected near Woodinville, Washington. Although information on the locations of nutria
farms is limited, records exist for farms in Seattle, Bothel, Maple Valley, Bellingham,
and Bremerton (Larrison, 1943). It is likely that animals escaped or were eventually
released from some or all of these locations. In the 1940s, nutria sightings in various
areas within Washington were documented in the primary literature, including Lake
Washington, tributaries of the Snohomish and Skykomish Rivers, and the headwaters of
Snoqualmie in the Cascade Range (Ashbrook, 1948). There is evidence that nutria are
spread across the Pacific Coast from Oregon into British Columbia (Jacoby Carter &
Leonard, 2002).
Observations made by researchers Phu T. Van and Filip Tkaczyk at the University
of Washington provide some data on local vegetation consumed by nutria. Nutria were
observed consuming a number of native plants, such as willow (Salix) species, cattail
(Thypha), native rush (Juncus)¸ as well as non-native species such as Himalayan
blackberry (Rubus discolor), Yellow Iris (Iris pseudocorus), Queen Anne's lace Daucus
carota), and canary reed grass (Phalaris arundinacea) (Van & Tkaczyk, 2007).
Reports of nutria damage include herbivory within habitat restoration attempts,
such as a project in the Vancouver area, which may have lost 400,000 dollars to nutria
16
(Sheffels & Sytsma, 2007). Newly established or restored habitat may be the most
sensitive to nutria herbivory. However it is likely that erosion and damage to
embankments and water control devices that may incur the largest and most immediate
costs due to nutria infestation. Nutria populations in the Pacific Northwest, particularly
Washington, may be concentrated in canals and other human modified bodies of water, as
opposed to the coastal marshes and wetland habitats infested by nutria in Southern
Washington (Sheffels & Sytsma, 2007).
Residents in Washington, such as along the Sammamish River, have reported
nutria feeding out of vegetable gardens, with a fondness for cabbage (Larrison, 1943).
The degree to which nutria will consume crops may hinge upon distance from the water
and the availability of aquatic plants near the waterway (Guichón et al., 2003). If a buffer
of vegetation is allowed to grow between the water and an agricultural field or garden it
may provide optimal foraging and prevent nutria from seeking terrestrial plans, even if
they are highly nutritious.
Nutria Control in Washington:
Currently the Washington Department of Fish and Wildlife has focused on other
invasive species such as the zebra mollusk, and has taken the strategy of encouraging the
pubic to take a lead on nutria control on a local basis. Some landowners and associations
have made efforts to control nutria, such as Skagit County, and residents in Portage Bay,
such as the Seattle Floating Homes Association. Skagit County has taken particular
interest in eradicating nutria, as damage to embankments is a frequent occurrence in some
17
areas. A variety of agricultural and wildlife groups managed to raise money for nutria
control and employ a trapper in the area.
Costs of nutria control are the responsibility of the land owner. WDFW
recommends a few extermination options, such as the USDA Wildlife Services, which
eliminates nutria using trapping and shooting at a cost of around 43 dollars per hour.
There are no established methods of tracking the range or population levels of nutria in
Washington. WDFW has published a report, "Living with Nutria" which gives advice on
dealing with the animals from building protective devices for gardens or repelling the
animals by blocking their burrows. The website does not give a method for reporting
nutria presence.
Overview:
Nutria have been proven as damaging, costly pests in a variety of regions. Areas
that have calculated the costs of nutria estimate high damages when populations are
dense. In areas where nutria is considered a pest, damages are reported from wetland loss,
crop damages, and destruction of levees and irrigation devices. Costs have been
calculated from crop loss and repairs, although the ecological damage caused by nutria is
difficult to price, except through lost recreational opportunities and damages to tourism.
Hunting and trapping pressures are capable of suppressing a population to rates that are
considered tolerable, but the appeal of nutria as a furbearer or for meat is limited by low
demand. The success of eradication efforts depends on the density of nutria.
18
Nutria populations in Washington are likely underreported due to the low level of
public education, and a lack of centralized phone number or place for citizens to report
sightings. Nutria has become a concern for some localities, although others have made no
effort to control nutria. Nutria on private property may not be recognized, or ever
reported WDFW, or the National Aquatic Invasives Database.
Eradication has been proven possible, but requires attention at a large scale,
sufficient funds, and communication between agencies. Small and isolated populations
are easiest to address, but are likely to become re-infested. Success is most likely if
control efforts are implemented in a timely manner, before populations become
unmanageable. Infestation is difficult to predict, with some areas experiencing rapid
increases in population, and some areas retaining a stable but consistent nutria presence.
Costs are difficult to document, and the lack of research in the Pacific Northwest has kept
the impacts of nutria infestation an unknown in this area.
Methods:
Current Range/Presence Maps
Nutria sightings and reports were collected across multiple agencies, such as the
Washington State Invasive Species Council, U.S.D.A., U.S.G.S., W.D.F.W., and Portland
State University Center for Lakes and Wetlands. These sightings come from various
efforts to document the presence of nutria, including citizen reporting via hotline and
email, historical documentation, wildlife biologist reports, and documentation of
eradication efforts. Because there is no coordinated effort to document nutria populations,
19
or set protocol for surveying population levels, nutria density was not accessed. Sightings
collected from the USG Nonindigenous Aquatic Species database were available on a
12th level watershed level (HUDC12). Other reports, such as those collected by WDFW
were limited to nearest populated area. These levels were plotted separately to show
confirmed infected watersheds as polygons and reports from populated areas as points.
The separate plotting has the additional benefit of displaying discrepancies between
agencies on known nutria infestation. Locations of documented nutria farms are also
displayed on the range map; these historical farms were discussed in Larrison (1943), and
are likely the source of much of the current nutria infestation in Washington. Sightings
prior to 1980 with no more recent reports of infestation were documented separately
Nutria Control:
Wildlife Services of the U.S.D.A provided records of nutria shot or trapped by
their agency in each Washington State city/town. Information beyond this, such as
addresses, was not available because this information would be protected by a federal
agency. Locations were plotted in G.I.S, showing total number of nutria removed per
area. This map provides the most information on density by giving a minimum number of
nutria present before eradication in a given city or town. It also provides some
information on nutria awareness, as control is requested by public and private entities.
20
Habitat Suitability:
Using overlays of GIS data from the Washington State Department of Ecology
and habitat parameters from past researchers, maps were produced in ARC GIS
highlighting suitable habitat for nutria in Washington. These parameters were based upon
minimum temperature as discussed by Gosling et al. (1983), and access to bodies of
water. A buffer was created around all major streams, lakes, and water bodies, using a
shapefile from the USGS Geospatial Database. This buffer was 1,200 yards, the average
maximum daily range of nutria from Nofio-Clements (2009). The clip tool in ArcGIS
was applied to these buffer zones to select areas with an average daily minimum
temperature of 39 degrees or more. This was based upon data from Gosling (1983, 1986),
which found that nutria activity began to noticeably slow when temperatures fell under 4
degrees Celsius (39.2 degrees Fahrenheit). Average daily minimum was chosen as an
ideal temperature parameter in order to show areas with prolonged exposure to cold
temperatures. Watersheds at the 12th level the included any of these buffer zones were
selected using a select by attributes tool in ArcGIS. Graduated colors feature areas with
the least severe minimum temperatures.
This map provides an estimation of where nutria populations could be supported
within Washington State. This can be compared to known infested areas, which will
provide information on the accuracy of the habitat suitability model (are nutria in areas
that the model does not deem suitable?), as well as make predictions about where nutria
are likely to be reported in the future (are there suitable areas not yet reporting nutria?).
21
Analysis of Mitigation/Awareness
Nutria are reported to different agencies in Washington State and the PNW.
Eradication options are available through USDA Wildlife Services, but are conducted by
individual request and cost. The current status of nutria regulation, control, public
education, and tracking are discussed and compared with other U.S. states to contrast
policy and discuss efficiency of various programs. The effectiveness of eradication
efforts, such as in Maryland, Louisiana, and the U.K. are discussed in order to provide
information on the suitability of various policy for Washington.
It is possible to make conclusions based on available data about the level of
awareness and level of attention for nutria infestation in Washington State. Differences in
documentation demonstrate at what level agencies are coordinated and in communication
about where nutria are present and what impacts they are having on Washington State.
Chapter 3:
Results and Discussion:
Range Map
Nutria populations are evident throughout Western Washington, with a few
scattered populations reported in the Eastern half, such as in the Yakima Valley and
Spokane River (Figure 2).
Infestations are common along the mouth of the Columbia River, and the eastern
shore of the Puget Sound. Infested watersheds range in size from large lakes and rivers
22
to smaller creeks and wetlands. Most reports come from populated areas, such as along
the I-5 corridor and around larger towns and cities. This is not necessarily because nutria
populations are denser in these regions, but is likely due to higher visibility and proximity
to the public. Populations in rural areas may be under reported, as there is less human
traffic to observe nutria.
The towns reporting nutria represents the most recent of data, from 2007 to the
present, with the per watershed data based off data prior to 2007. The more recent reports
show nutria in new areas. This could represent either an expansion in extent of nutria
infestation, or an increase in public reporting.
Historical nutria farms explain much of potential current distribution of nutria.
Nutria farms in King and Skagit County could have led to the introduction and
subsequent spread of the animals in areas such as Lake Washington. Nutria farms in
Oregon (not pictured on the map) could have been the source of nutria infestations along
the Columbia River, and possibly as far North as Olympia.
23
Figure 2:
Figure 2: Nutria sightings within Washington are displayed, with polygons representing
watersheds on the HUD12 level, and points reflected nearest town reporting nutria.
Squares represent historical nutria farms. Sightings predating 1980 are displayed
separately.
24
Nutria Removal Map:
Extermination data on nutria provides minimal data on population density. Over
the past few years, over 300 nutria have been removed from Seattle, over 100 from
Vancouver, and hundreds more from towns along the Columbia River (Figure 2).
Unlike the extermination data from the USDA, the sighting reports used to
generate the range map often do not involve confirmation by a qualified person. Because
this information comes from a variety of sources, mostly public sightings, animals are
susceptible to misidentification as nutria, mainly muskrat and beaver. Without further
investigation by a wildlife biologist or qualified person, sightings can only be considered
probable. However, sightings are relatively consistent with where nutria have been
removed. The range of muskrats in Washington is much greater and includes areas which
have never generated nutria sightings, even if by mistake.
Because nutria are removed by request rather than requirement, these numbers
may indicate that large populations exist in regions of Washington that are not attempting
to control nutria. Nutria have been confirmed and spotted in many areas, but the densities
at which they exist remain unknown. While there is evidence that nutria thrive in
eutrophied systems, they are perfectly capable of inhabiting and degrading pristine
habitat. There is evidence this has occurred in Washington, with nutria reported in creeks
and wetlands in the foothills of the Cascades, in rural Lewis County, and near hiking
trails along the Snohomish River.
25
The greatest economic threats, however, may come from irrigated agricultural
areas, such as Skagit County. This is one area which has taken initiative on nutria control.
The Skagit Nutria Advisory Committee provides an example of cooperation between
stakeholders, such as the Nature Conservatory, Skagit Land Trust, and the Western
Washington Agricultural Association. The website for the SNAC states that no nutria
have been confirmed since 2007 and has deemed the trapping efforts as a likely success.
Indeed, no nutria have been removed from Skagit County since 2007, however, data from
the WDFW Invasive Species Council includes sightings from 2010 in Skagit County.
These are in separate areas than where nutria have been removed in prior years.
Inconsistencies between agencies highlight a lack of communication or proper
documentation regarding nutria infestation. While regional Fish and Wildlife offices
claim no nutria are present in Eastern Washington (pers comm., Howard L. Ferguson),
reports to the Washington State Fish and Wildlife office include sightings in the Yakima
area. It is possible as one source claimed, that the populations there may have been
thinned or eliminated by a long period of freezing days in the late 1970s (Sheffels and
Systma, 2007). Recent sightings of nutria in the Yakima area have been reported to
WDFW, but it is possible these sightings were misidentified, such as from muskrats.
26
Figure 3:
Figure 3: Number of nutria removed by the USDA wildlife services are displayed with
graduated symbols by nearest populated area. Nutria were removed via trapping or
shooting.
27
Suitable Habitat:
The suitable habitat for nutria based upon parameters for minimum temperature
and distance from water explains most of the distribution of nutria sightings (Figure 2).
Some inconstancies can be explained by levels of scale regarding bodies of water, with
nutria being sighted in creeks and wetlands that are too small to be included in the
National Hydrology Dataset. Additionally, there are a few areas in which nutria have
been spotted that are outside the expected range based upon minimum temperature. This
could mean a number of things: that nutria are capable of inhabiting these areas, that the
nutria had extended its range in warmer months, or that the animal sighted was not a
nutria.
The nutria spotted in the Five Lakes area of the Okanogan seems to be an outlier,
isolated from other infestations and in a region colder than expected. This sighting dates
back to the 1940s, and was documented in the primary literature as coming from a Native
American trapper, who trapped the species instead of the targeted muskrat (Larrison,
1943). There are further reports coming from this region, including a sighting by the wife
of the Forest Supervisor of the Colville Reservation, spotting a dog carrying a nutria
carcass. The Five Lakes sighting provides an interesting anecdote. It is impossible to
confirm that this sighting was nutria rather than muskrat, although a trapper familiar with
the species may indeed have been qualified to distinguish between these animals. The
nutria may have been an isolated outbreak, such as from a farming operation, which
would have in theory not survived harsh winters. It is also possible that populations
persist in the area and are not being reported to any agency.
28
The suitable habitat map is constrained by a few factors, mainly, that nutria may
be capable of pushing into colder regions providing they can avoid sustained periods of
freezing temperatures. These colder regions may also provide foraging opportunities in
warmer months before the nutria retreat to core habitat for breeding or wintering. Another
factor is that nutria are capable of finding habitat in drainage ditches, irrigation canals,
and small ponds, such as on private property. This may open up the available habitat for
nutria to areas at considerable distance from major streams, lakes or wetlands.
Although nutria are capable of living in a variety of habitats, including brackish
water, this map does not distinguish between bodies of water based upon salinity. This
distinction was not made in order to include coastal wetlands and areas such as the San
Juan Islands. However, future suitability models may further refine nutria habitat to
eliminate open salt water and other areas, which would not provide vegetated areas.
Further refinement of this model could include using confirmed nutria sightings as
epicenters from which nutria might spread into new areas, or putting focus on areas such
as dairy farms which may provide eutrophied drainage ditches and other suitable nutria
habitat.
A model created by Bertonlino and Ingegno (2009) analyzed current nutria habitat
in Italy in order to project habitat suitability to other areas. This model found that nutria
preferred certain types of farmland over others, and avoided woodland or urban areas. As
the habitat in Italy is vastly different from Washington State, it is difficult to apply such
conclusions. A similar model for the Pacific Northwest would further refine this
suitability model, but would require a large dataset describing the habitat where nutria are
known to exist. Such a dataset does not yet exist.
29
Figure 4:
Suitable Habitat
Temp_F
37 - 39
40 +
Figure 4: Suitable habitat for nutria infestation in Washington is displayed, based off of a
distance from water parameter (<1200 yards), and an average minimum daily temperature
parameter (>39 degrees F). Average minimum daily temperatures of over 50 degrees F
are shown in darker red to display optimal habitat.
30
Conclusions and Recommendations:
Considering that nutria have been present in Washington State for over 60 years,
there is relatively little information on their distribution, density, range, and ecological
impacts. Washington offers a somewhat unique opportunity to research nutria, as its
population dynamics and ecological interactions may differ from other regions with more
pervasive nutria infestations, such as Louisiana. There are indications that nutria in this
region may have significant costs, particularly in areas of wetland reestablishment. These
could have direct impacts on state funding, such as in the success of wetland mitigation
sites conducted by the Washington State Department of Transportation. Damage to
embankments and water control structures could inflict costs on private land owners, or
even cause damage on larger scales. This kind of damage to private residence has been
seen in the Union Bay area of Seattle.
Nutria populations likely remain low in Washington State, as opposed to
Louisiana and Maryland where populations reach the millions. Studies on the success of
invasive species eradication find that the goal of eradication is most feasible when
populations are detected an addressed at an early stage (Rahel et al., 2008). Furthermore,
climate change has been associated with increased range and severity of invasive species
(Rahel et al., 2008). The Washington State Department of Fish and Wildlife recognizes
nutria as a problem and encourages control efforts, but does not directly manage or fund
nutria control. This is due to limited department resources and a focus on high priority
invasive species, such as the zebra and quagga mussel, (pers comm., Alan Pleus).
Unfortunately, much of the public is still unfamiliar with nutria and unlikely to report
their presence or hire a wildlife manager.
31
The lack of a centralized reporting method, such as a single hotline for reporting
nutria, and a shortage of communication between agencies regarding where nutria have
been reported, has led to uncertainty in where nutria remain, where they have been
eradicated, and where they have yet to spread. A single hotline for reporting sightings,
with documented follow up, would create more accurate information on the range of
nutria while reducing reports based upon muskrats or beaver. Although information on
nutria density would aid control or eradication efforts, no method for estimating the
population in Washington currently exists. While a statewide survey could be expensive
and overly ambitious, some local sampling may be a reasonable method of understanding
population densities in Washington. Only a few studies have been conducted on nutria
herbivory preferences and impacts in the Pacific Northwest. Further enclosure studies in
infested areas are needed in order to determine how nutria herbivory affects biomass and
productivity in Washington wetlands. It is possible that the damage caused by nutria to
these ecosystems is significant and quantifiable.
Due to low population levels, a bounty system in Washington is not likely
feasible, or would require a high reward amount to compensate participants. Trapping is a
more safe method than shooting, particularly since the majority of nutria sightings come
from around populated areas. The eradication efforts conducted in Maryland and England
required teams of wildlife control experts working over multiple years. Skagit County
claims similar success by hiring a control expert over a few seasons, although this was in
a few small, isolated areas. Controlling populations in areas such as Union Bay near
Seattle could require more intense efforts. One of the most important factors for the
outlook of nutria control in Washington is detection. This could be accomplished by
32
providing nutria identification guides to individuals familiar with the wetland ecosystems
in Washington, such as local biologists, birdwatchers, wildlife enthusiasts, and duck
hunters.
The process of detecting nutria, particularly in sensitive areas, could be focused to
areas with a higher likelihood of nutria presence. Habitat suitability models, such as the
one produced for this paper can be compared to sighting reports. For example, nutria
have been found in the Columbian River area, as well as Grays Harbor near Aberdeen. It
is likely the Grays Harbor population originated from nutria crossing over Willapa Bay,
which lies between these two areas. Therefore, Willapa Bay would be an area with a high
likelihood of nutria infestation, and also has high ecological value as an estuary. Sensitive
wildlife areas may be the most prudent regions to search for nutria infestation to
minimize ecological consequences. The habitat suitability model suggest that’s nutria
have a wide array of available habitats. Further research could attempt to understand any
possible constraints that have prevented nutria from expanding their range to suitable but
previously uninhabited areas. Such restraints could include quality of vegetation, or
barriers to migration. This analysis would require that these areas are confirmed as being
free of nutria infestation.
While nutria eradication has shown a degree of success in some areas of
Washington, i.e., Skagit County, it is uncertain if intense nutria removal would produce
results if applied to areas in southern Washington, where nutria are likely more
widespread and at higher numbers. Some efforts in Italy have proven futile, but these
populations may have been much higher than any known population in Washington State.
Further difficulty comes with preventing the reestablishment of nutria after population
33
thinning or eradication, such as nutria along the Columbia River or in Oregon
repopulating areas in Southern Washington. A combined effort with Oregon may be a
necessity, and is plausible given the preexistence of nutria control efforts in Oregon.
There is a great deal of unknown regarding nutria in Washington, which reflects a
greater lack of understanding in how invasive species behave in general. Researching
nutria in Washington has a great benefit of adding to scientific knowledge of how
invasive species spread and impact ecosystems, and furthermore how they can be
managed, controlled, or eradicated. The current status of nutria in Washington, although
not entirely clear, may be optimal for eradication efforts, before this species pushes into
new and highly sensitive areas, or populations explode. At minimum, a public awareness
campaign, centralized reporting hotline, and thorough documentation are necessary next
steps for informed policy making.
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