I ional Agricultural Trade and Policy Center
POLICY BRIEF SERIES
Institute of Food and Agricultural Sciences
An Economic Model for Evaluating
Zebra Mussel Management Strategies
Donna J. Lee and Damian C. Adams *
PBTC 07-03 July 2007
INTERNATIONAL AGRICULTURAL TRADE AND POLICY CENTER
THE INTERNATIONAL AGRICULTURAL TRADE AND POLICY CENTER
The International Agricultural Trade and Policy Center (IATPC) was established in 1990
in the Institute of Food and Agriculture Sciences (IFAS) at the University of Florida
(UF). The mission of the Center is to conduct a multi-disciplinary research, education and
outreach program with a major focus on issues that influence competitiveness of specialty
crop agriculture in support of consumers, industry, resource owners and policy makers.
The Center facilitates collaborative research, education and outreach programs across
colleges of the university, with other universities and with state, national and
international organizations. The Center's objectives are to:
* Serve as the University-wide focal point for research on international trade,
domestic and foreign legal and policy issues influencing specialty crop agriculture.
* Support initiatives that enable a better understanding of state, U.S. and international
policy issues impacting the competitiveness of specialty crops locally, nationally,
* Serve as a nation-wide resource for research on public policy issues concerning
* Disseminate research results to, and interact with, policymakers; research, business,
industry, and resource groups; and state, federal, and international agencies to
facilitate the policy debate on specialty crop issues.
An Economic Model for Evaluating Zebra Mussel Management Strategies
Zebra mussels present an imminent threat to the water resources in Lake Okeechobee in Florida.
The lake is vitally important to consumptive water uses and recreational anglers and provides a
host of ecosystem services. We employ a probabilistic bio-economic simulation model to
estimate the potential impact of zebra mussels to wetland ecosystem services, consumptive water
users, and recreational anglers under alternative public management scenarios. Without public
management, the expected net economic impact from zebra mussels is -$244.1 million over 20
years. Public investment in prevention and eradication will yield a net expected gain of +$188.7
million, a superior strategy to either prevention or eradication alone.
Key Words: Invasive species, cost transfer, surface water, fishing, wetlands, probability
JEL Classifications: C63, Q25, Q52, Q57, Q58
* This document was prepared while the authors were employed as an Associate Professor and a Lecturer in the
Department of Food and Resource Economics in the Institute of Food and Agricultural Sciences, College of
Agriculture and Life Sciences at the University of Florida. For inquiries, contact DLee@Entrix.com.
Donna J. Lee and Damian C. Adams*
An Economic Model for Evaluating Zebra Mussel Management Strategies
Donna J. Lee and Damian C. Adams
Zebra mussels (Dreissena polymorpha) are a small freshwater species native to
Southeastern Europe. In suitable water, zebra mussels become successful invaders. Mature
females can produce up to one million eggs per year (USCACE, 2003). The zebra mussel most
likely crossed the Atlantic Ocean as larvae on a transatlantic ship (Hebert et al., 1989; Griffiths et
al., 1991; Thorp et al., 2002) and disembarked into the Great Lakes. The mussels multiplied
rapidly and began spreading. Today populations are found in twenty-four states as shown in the
map in Figure 1 (USGS, 2007).
Zebra and Ouaqga Mussel Sightings Distribulton
Figure 1. Zebra mussels in the United States (USGS 2007)
The problem with zebra mussels is that they colonize on any submerged surface including
boat hulls, navigational buoys, bridge abutments, and water intake pipes. Their dense mats will
accelerate the rate of corrosion, sink navigational buoys with their weight, and obstruct water
flow in pipes. United States' expenditure for the upkeep required to maintain boat bottoms,
flow in pipes. United States' expenditure for the upkeep required to maintain boat bottoms,
docks pilings, locks, gates, and pipes is estimated to be $60 million per year (USGAO, 2002).
Because zebra mussels are spreading, damages are expected to rise. Future damages are
estimated to be between $3.1 and $5 billion for the period 2002 to 2011 (USGAO, 2002 and
Zebra mussels compete with native flora and fauna for food and space, alter the
composition of the water column, and transform lake bottoms. They will bio-foul rocks, logs,
submerged plants, and the shells of other mussels. In the U.S. more than half of all native
freshwater mussel species are either threatened or endangered. Recovery efforts are significantly
hindered by the presence of zebra mussels (Ricciardi et. al.,1998; USGAO, 2002)
Will Zebra Mussels Invade Florida?
Zebra mussels were first sighted in Florida in 1998 during an inspection of a bait and
tackle shop (University of Florida, 1998). Fortunately a fast acting official collected and
destroyed the animals before they could spread. No other sightings have occurred since, but in
the last decade zebra mussels have made their way south creeping ever closer to the Florida
border. Populations are thriving in Arkansas, Alabama, Kentucky, Louisiana, Mississippi,
Missouri, Tennessee, and West Virginia (USGS, 2007). According to estimates by Drake and
Bossenbroek (2004) zebra mussels are bound to reappear in Florida. The authors estimate that in
the coming years there is a "high" likelihood that zebra mussels will reach north Florida and a
"moderate" likelihood that zebra mussels will reach south Florida. Suitability of Florida's warm
waters was examined by Hayward and Estevez (1997). They judged the rivers in the Florida
panhandle (north Florida) to be unsuitable for zebra mussel propagation because the water is
acidic and contains few minerals. The St. Johns river in north central Florida and Lake
Okeechobee in south Florida both have low acidity and high mineral content and are judged
suitable for sustaining zebra mussels.
This study examines the potential for Lake Okeechobee to become infested with zebra
mussels, describes a simulation model, proffers a series of management scenarios, presents
results, and offers sensitivity tests on key model parameters. Novel contributions include
quantification of potential future damages from zebra mussels, economic trade-offs between
public management expenditures and public and private gains, and comparisons of management
alternatives prevention and eradication.
Lake Okeechobee is an important commercial shipping route, a valuable source of
freshwater, a major recreational resource, and at 448,000 acres, the second largest lake wholly
contained in the United States, (FDEP, 2001). Five counties around the lake pump water for
irrigation, industry, and household uses. Impacted services from an infestation of zebra mussels
would include water supply, water recreation, and wetland ecosystem services.
The Lake Okeechobee waterway is presently free of zebra mussels and the nearest
populations are 750 miles away. Most likely zebra mussels will make the journey by clinging to
the stems of aquatic weeds entwined in a boat propeller or snagged on a trailer. While the
possibility may seem remote, it is worth noting that zebra mussels can survive several days out
of water. In the Great Lakes region, aquatic weeds covered with live zebra mussels were
observed on 1 out of every 275 boats in parking lots while owners were preparing to launch into
non-infested lakes (Johnson and Carlton, 1996).
Lake Okeechobee is a popular destination for local and out-of-state sport fishers,
recreational boaters, and host to several major fishing tournaments each year. Out-of-state
boaters and returning Florida boaters are likely vectors for transporting zebra mussels to Lake
Zebra mussel model
In a previous work, Leung, et al. (2002) used stochastic dynamic programming to model
the probability of a zebra mussel invasion as a decreasing function of prevention effort. Zebra
mussel growth was captured with a logistic function. Damages were expressed in terms of lost
productivity due to reduced water flow. The optimal solution was to reduce the probability of
arrival by 10% using prevention measures. Finnoff et al. (2005) applied a stochastic dynamic
programming model following Leung et al. (2002) to examine the economics of preventing zebra
mussel damages in a Midwest lake. They questioned the importance of including feedback links
and the conditions under which omission would make a difference. One interesting finding was
that over investment or under investment in control could result depending on how the public
manager believes the private entity will respond to the invasion. To compare management
alternatives for eradicating the oyster drill (Ocinebrellus inornatus) an invasive marine mollusk,
Buhle et al. (2005) employed a Markov approach. The authors specified a 2x2 transition matrix
to capture two of the animals' three life stages and ascertained that control efforts targeting the
adult animals would be more cost effective than control efforts targeting the bright egg masses.
For Lake Okeechobee, we assume there is a real threat of zebra mussel introduction. Once
introduced, the small critters are unlikely to be noticed until dense mats are formed or piles of
razor sharp mussel shells wash up on shore. By the time they are noticed, the economic and
environmental damage will already be significant. To characterize this system we use a stylized
model comprising the following four "states of nature": (1) none, (2) introduced, (3) propagating,
and (4) critical mass. The probability that the Lake will be in any of the four states at time t in the
future, is st. At present, there are no zebra mussels so slto = 1 and it follows that s2to = ssto =
S4t=0 = 0. Some additional description of the variable st appears in Table 1.
Table 1. Description of Zebra Mussel States in Lake Okeechobee
Probability of state i Economic and
i Description of state i
at time t ecosystem damages?
1 sit No zebra mussels no
2 S2t Zebra mussels recently introduced no
3 S3t Zebra mussels propagating no
4 S4t Zebra mussels at critical mass yes
The st state probabilities are brought together to form the elements of vector variable St :
(1) St = 2 where 0 < s, <1 and Cs, =1 .
At present the Lake has no zebra mussels, so at t = 0,
(2) So= ,
To derive future state values St+l we define the transition probability a,, which represents the
probability of changing to state i from state j in a single time period. In matrix form, a,j comprise
the elements of Ao the 4x4 matrix of transition probabilities under a natural progression of zebra
a11 a12 a13 a14
(3) A a21 a22 a23 a24
a31 a32 a33 34
a41 a42 a43 a44
St+l is defined as the product ofAo and St from equations (3) and (1):
(4) St+ =AoSt.
Each element of St+ can be obtained:
(5) s,t+1 = alSlt +a12s2t a3S3t +a4s4t fori =1...4 .
Because the natural progression of zebra mussels may be undesirable, prevention measures
are available to reduce the probability of introduction and propagation. Letting fi measure the
effectiveness of a prevention program, the transition probability matrix Ap with a prevention
program in place is expressed:
all-a21fl a12 a13 a14
(6) A a21 (-l) a22 a23 a24
a31 a32 a33 a34
a41 a42 a43 a44
Propagation can be thwarted with early eradication which is defined as the action required
to destroy all zebra mussels as soon as they are detected. With monitoring as a component of the
prevention program, we assume early eradication takes place in state 3 before the zebra mussels
can cause significant damage or loss. The transition probability matrix Am is represented by:
a11 -2lfl 1 1 1
(7) Am a2(1 l) a22 a 23 a 24
(7) Am ~
a31 0 a33 a34
a41 a42 0 0
Without a prevention program in place, we assume there would be no monitoring and
therefore zebra mussels would be detected with the onset of economic damages, i.e. in state 4.
Late eradication is defined to be the measures taken to destroy all zebra mussels in Lake
Okeechobee after reaching state 4. The transition probability matrix Ar is expressed:
all 1 1 1
(8) A= a21 a22 a23 a24
a31 0 a33 a34
a41 a42 0
Post-eradication, we assume the treated lake would be free of zebra mussels for a period of
at n years during time which the transition probability matrix becomes,
With prevention, the zebra mussel state equation during states pre-introduction (state 1)
and introduction (state 2) is defined:
(10) St A St, .
After zebra mussels are established, prevention measures would no longer be practical,
thus prevention measures would be halted. During propagation (state 3) and critical mass (state
4), the transition matrix is Ao and the state equation reverts back to equation (4).
With prevention and early eradication, the state equation from pre introduction through
introduction, propagation, and early eradication is:
(11) St Am St,
With late eradication, the state equation from pre-introduction through introduction,
propagation, critical mass, and eradication is:
(12) St Ar St_
For the remainder of the planning horizon after early eradication or late eradication the state
(13) St =AnSt-l
Economic comparison of the management choices requires estimates of the expected
benefits and costs. For this problem, the management choice variable X is a (4x4) vector
composed of the elements x, and x,.
x, x, 0 0
0 0 0 0
0 0 x, O0
0 0 0 xT
The decision to invest in prevention is given by xp=l and x,=0. The decision to invest in
prevention and early eradication is given by xp=l and x,=l. The decision to invest in late
eradication is given by xp=0 and x,=l. The four management alternatives are shown in Table 2
Table 2. Four management alternatives
p =0 p =1
x, = 0 Do nothing Invest in prevention
(status quo) (prevention)
Invest in prevention and eradicate
Eradicate when zebra mussels become
x, =1 before zebra mussels become
(prevention and early eradication)
Combining the two management choices yields a vector of four management alternatives:
(1 xp)(1- x,)
x, (1- x.)
The unit costs of implementing the management choices x, and x, are c, and c, which
comprise the (2x1) management cost vector q.
The cost of management Ct at time t is the product of unit cost q, management choice X, and
the zebra mussel state St from equations (16), (14) and (1):
(17) C,= q'XS,.
Economic damage from zebra mussel infestation is d an X dependent variable of increased
maintenance expenditure by consumptive water users in Lake Okeechobee. Ecosystem service
loss with zebra mussel infestation e includes diminished wetland functions, loss in wildlife
habitat, and reduced aquatic food supply in state 4. The benefit from zebra mussel infestation b is
the added value to recreational and sport fishers from the improved water clarity and increased
catch rate due to zebra mussel filter feeding. In this model, cost, damage, and loss are expressed
as negative values and benefit is expressed as a positive value. The objective is to choose a
management strategy X that maximizes Z the present value of total expected cost, damage, loss,
and benefit with the threat of zebra mussel infestation. The objective is to:
(18) max Z= (1+r)-t(q'XS,)+(e'+b')S, +u(X)'d(X)'S) ,
subject to equations (1) through (17). In (18), r is the annual discount rate and Tis the number of
years in the planning horizon.
Empirical model parameters
Recreational and sport boats are the primary vector for transporting zebra mussels from
infested lakes to Lake Okeechobee. We examined data from three national tournaments on Lake
Okeechobee during 2006-2007 (Carson, 2007 and Eads, 2007) and observed that half the 926
anglers were from states with zebra mussel infested waters. The potential for trailered boats to
vector zebra mussels was shown by Bossenbroek et al. (2001). They estimated that trailered
boats in the Great Lakes area could convey enough live zebra mussels to colonize an uninfested
body of water in a nearby state with a probability of between 1.18x10-5 and 4.11x10-5. We used
an intermediate probability of 3.78x10-5 per boat, multiplied by 926 boats per year to obtain an
annual probability of zebra mussel introduction of 3.5% per year (a21 = 0.035)
Upon introduction to Lake Okeechobee, zebra mussels would prosper according to
Hayward and Estevez (1997). The scientists computed habitat suitability indices (HSI) of 0.83
and 0.91 for open water and shallow water containing dense aquatic plants. Given the high HSI
values for Lake Okeechobee and the large expanse of suitable habitat, we assumed introduced
zebra mussels would become established and propagate until critical mass was reached with a
probability of 100% (a2 43 = a44 =1.0). 1
1 These values were chosen to enhance the transparency of the empirical model. Simulations assuming a32 a43= a44
= 0.83 and a32 a43= a44 = 0.91 were also conducted. Results from those runs appear in the appendix in Table 6 and
Time to reach carrying capacity according to Nalepa et al., (1995), Strayer et al. (1996),
Burlakova, Karatayev, and Padilla (2006), Borcherding and Sturm (2002), and Lauer and Spacie
(2004) is two to three years after detection. For our model, we assume zebra mussels will grow
to produce dense mats sufficient to cause damages two years after introduction, thus the time lag
between states 2 and 3, and between states 3 and 4 is one year.
Private economic damage
In the Great Lakes area, O'Neill (1996) and Deng (1996) estimated annual expenditure for
chemical, mechanical and thermal maintenance. For a zebra mussel infestation in Lake
Okeechobee, we assume water users would employ mechanical and thermal means to clear
clogged intake pipes and spend $4.90 per million gallons pumped as reported by Deng (1996).
Mean water withdrawal from Lake Okeechobee is 562,589 million gallons per year (USGS,
2006). Multiplying annual water use by average unit expenditure we arrived at economic
damages of $2.76 million per year to consumptive water users (d2 = 2.76). As most pipes in the
Great Lake region are pre-treated with antifouling paint, we apply this damage estimate to
Anti-fouling paint helps to reduce maintenance expenditures by inhibiting zebra mussel
colonization. In the Columbia River Basin, water users applied anti-fouling paint to interior pipe
surfaces at a cost of $25.56 per square foot (Phillips, 2005). Based on a survey of Lake
Okeechobee water users (Adams, 2007) we calculated the average interior surface area of intake
pipes to be 300.58 square feet. With 504 major water users on the Lake, we calculate an intake
pipe surface area of 151,492 square feet, which would require $3.87 million to treat with
antifouling paint. Assuming the paint treatment lasts 10 years, annualized mitigation damage is
$0.387 million (d3 = 0.387).
We assume anti-fouling paint treatment saves consumptive uses about 22% in maintenance
expenditures. Thus without treatment, Lake Okeechobee consumptive water users would pay
$5.98 per million gallons per year pumped to maintain pipes. Annual damages would be $3.37
million (di = 3.37).
Public ecosystem service loss
Surrounding Lake Okeechobee are 29,000 acres of Audubon Society wetlands and 31,000
acres of unnamed wetlands for a total of 60,000 acres of wetlands. Costanza, et al. (2003)
estimated the value of wetland services to be $1,083 per acre per year. Multiplying $1,083 by
60,000 acres yields a wetland damage estimate of $64.98 million per year (e = 64.98).
Private economic benefit
Between 1983 and 2002, anglers spent an average of 1,575,340 hours on Lake Okeechobee
each year (FFWCC, 2003). The Florida Fish and Wildlife Conservation Commission reported
average spending of $20.65 per hour in 2002 (FFWCC, 2003). Using total expenditures to
estimate the recreational value of freshwater fishing, we multiplied hours fished by value per
hour to obtain a total recreational value of $32.5 million per year. Assuming an increase in water
clarity attributable to zebra mussels would yield a 1% increase in fishing hours. The benefit from
zebra mussels is $0.325 million per year in state 4 (b = 0.325).
A plan to monitor and prevent zebra mussels from entering Lake Okeechobee was
proposed in 2003 (U.S. Army Corps of Engineers, 2005). The plan included inspecting
underwater structures, sampling waterway sediments, and distributing education alert materials
to boaters, lake homeowners, and businesses. The cost of implementing the proposed plan is
$152,800 per year (cp = .1528).
In 2006, an infestation of zebra mussels prompted the Virginia Department of Game and
Inland Fisheries to pour 174,000 gallons of potassium chloride into Millbrook Quarry. At 100
parts per million the concentration was double the amount needed to kill zebra mussels but low
enough to avoid harming humans or fish. The single treatment is expected to protect the quarry
from zebra mussel infestation for 33 years. The cost for chemicals and labor was $365,000
(VDGIF, 2007). A similar treatment for Lake Okeechobee would require 628.6 million gallons
of potassium chloride at a cost of $1.320 billion. This cost annualized over 33 years is $55.03
million (c, = 55.03).
A summary of the parameter values used in the Zebra Mussel Model appears in Table 3.
Table 3. Zebra Mussel Model Parameter Values
Symbol Definition Value
all Probability of zebra mussel not being introduced to Lake Okeechobee 0.965
Probability of zebra mussel being accidentally introduced to Lake 0.035
a32 Probability of zebra mussel moving from state 2 to state 3 1
a43 Probability of zebra mussel moving from state 3 to state 4 1
a44 Probability of zebra mussel remaining state 4 1
all other a,i 0
b Economic benefits from zebra mussel $0.325 mil
Cp Cost of arrival prevention and monitoring (per year) $0.1528 mil
cr Cost of eradication (annualized) $55.03 mil
d, Private economic damages without mitigation expenditures (per year) $3.37 mil
d2 Private economic damages with mitigation expenditures (per year) $2.76 mil
d3 Private mitigation expenditures (annualized) $0.387 mil
e Value of wetland services lost with zebra mussels in state 4 (per year) $64.98 mil
fp Effectiveness of prevention measures 0.75
fr Effectiveness of eradication measures 1.00
r Discount rate 0.02
t Year 0,...,19
T Planning horizon 20 years
Four management scenarios
With Management I (do nothing) public management costs are zero. Private water users
become aware of zebra mussels when they incur damages d, in the first year of state 4. In the
second year they will apply anti-fouling paint thereby incurring damages d2 and d3 in subsequent
years. Public ecosystem loss is e for every year the system is in state 4. Public recreation benefit
is b for every year the system is in state 4.
With Management II (prevention), public management cost is cp when the system is in
state 1 and 2 and zero in states 3 and 4. Private damage is d3 during the first year that the system
is in state 3 and d2 and d3 while in state 4. Public ecosystem loss is e for every year the system is
in state 4. Public recreation benefit is b every year the system is in state 4.
With Management III (late eradication), public management cost is c, after the system
reaches state 4. Private water users become aware of zebra mussels when they incur damages d,
during the first year in state 4. In subsequent years private damages drop to zero because the
zebra mussels are eradicated. Public ecosystem loss is e for one year while the system is in state
4. Public recreation benefit is b for one year while the system is in state 4.
With Management IV (prevention and early eradication), public management cost is cp
while the system is in state 1 and 2, c, when the system is in state 3, and zero otherwise. Private
damage is zero. Public ecosystem loss and public recreation benefit are zero as the system never
reaches state 4.
The empirical zebra mussel model was run on GAMS software (GAMS, 1998). A
presentation and discussion of results follow.
The least cost strategy is Management I in which nothing is done to prevent zebra mussels
from entering Lake Okeechobee and nothing is done to arrest propagation after they arrive. Over
20 years management cost is $0. The present value of expected ecosystem damages in terms of
lost wetland functions is -$219.5 million. Private water users sustain -$25.7 million in expected
damages from increased maintenance expenditures and recreational anglers will gain +$1.1
million in expected fishing benefits. The net present value of"do nothing" is -$244.1 million.
The next least costly strategy is Management II in which prevention measures are
implemented. Since prevention is only 75% effective, if zebra mussels arrive, we assume
prevention measures would be halted and no further action would be taken to manage the
growing mussel population. Over 20 years, the present value of expected public expenditure on
prevention is -$2.5 million. The present value of expected ecosystem damages in terms of lost
wetland functions is -$62.4 million. Private water users will endure -$7.2 million in expected
damages due to increased maintenance and mitigation expenditures. Recreational anglers will
enjoy +$0.3 million in expected fishing benefits. The net present value of managing the threat of
zebra mussel with prevention is -$71.8 million, a gain of +$172.2 million over doing nothing.
The most costly strategy is Management III in which zebra mussels are eradicated from
Lake Okeechobee after they begin causing damage. Over 20 years, the present value of expected
public expenditure on eradication is -$185.9 million. The present value of expected ecosystem
damage in terms lost wetland functions is -$23.8 million. Private water users will absorb
expected damages of -$1.2 million and recreational fishers will gain +$0.12 million in expected
fishing benefits. The net present value of late eradication is -$210.8 million, a gain of +$33.3
million compared to doing nothing.
The strategy with the smallest public ecosystem loss, least private economic damage, and
the highest expected net present value is Management IV in which both prevention and
eradication measures are used to mitigate infestation and resulting damages. Over 20 years, the
present value of expected public expenditure on prevention and early eradication is -$55.4
million. Expected loss in ecosystem functions, damage to private consumptive use, and gain to
recreational anglers is $0. The net present value from prevention and early eradication is -$55.4,
a gain of +$188.7 million compared to doing nothing.
Among the four alternatives, the optimal strategy based on the net present value of
expected costs, damages, losses, and benefits over the 20-year planning horizon as defined in
equation 17 is Management IV Prevention and early eradication. A summary of the simulation
model results appears Table 4.
Table 4. Zebra mussel model simulation results
SII III IV
Do nothing Prevention early
Public management cost
Public ecosystem loss
Private economic damage
Private recreational benefit
T = 20 years, r = .02
Results show large gains to investment in prevention. With an expected outlay of $2.5
million for prevention measures over 20 years, more than $170 million in expected losses and
damages can be avoided. If a prevention program is not in place before zebra mussels are
introduced and begin causing damages, eradication may be warranted. Over 20 years, the
expected expenditure for eradication is -$185.9 million which would serve to reduce impending
damages by -$220 million. If a prevention program is in place and zebra mussels are detected
before the can cause damage, early eradication would serve to supplant -$70 million in expected
damages for an incremental cost of-$52.9 million over prevention alone.
Breakpoint parameter values
To test model robustness, we estimated breakpoint values for key parameters in the model.
Here we define breakpointt value" to be the value at which the relative preference of the four
management strategies changes based on expected NPV.
Simulation results show that if the annual probability of zebra mussel arrival was only
0.0004 (rather than 0.035), prevention would not be warranted. The optimal strategy would be to
wait for zebra mussels to arrive and eradicate them when they are detected.
If the probability that introduced zebra mussels will propagate and grow to critical mass is
0.052 (rather than 1), prevention would not be warranted. Instead, managers should eradicate
zebra mussels when they are detected.
Recreational water users may advocate reduced zebra mussel management. Our
simulations show that if the benefit from zebra mussels was instead $71.6 million per year
(compared to our assumed value of $0.325 million), the advantages of allowing zebra mussels to
enter Lake Okeechobee would out weight the projected damages. In this case, neither prevention
nor eradication would be warranted.
We found that if the annual cost of prevention ballooned to $8.7 million per year (versus
our assumed value of $0.1528 million), prevention would be unwarranted as there would be no
advantage over late eradication. There would however be a slight advantage to being able to
eradicate early versus late. Likewise, if the effectiveness of prevention at reducing the arrival rate
fell to 17% (versus 75%), there would be no gain in prevention over eradication. Finally, if
prevention cost was $9.7 million per year, early eradication would be unwarranted and late
eradication would be preferred.
If on the other hand the annual cost of eradication was $1,729 million per treatment (versus
$1320 million), neither late nor early eradication would be warranted. With high eradication
costs, prevention measures take on more importance as a means of mitigating potential damages.
As a reference, $1,729 million is equivalent to an annualized cost of $72 million per year for 33
years or the same treatment at $1,320 million lasting for only 15 years.
Estimated breakpoint values and the relative rankings of management alternatives appear
in Table 5.
Table 5. Breakpoint parameter values for zebra mussel model
SDefi n Parameter Breakpoint I II IV
Symbol Definition Value Value
All Base model parameters Table 3 Table 3 4 2 3 1
a11 Annual probability of zebra mussel not arriving in Lake Okeechobee 0.965 0.99960 3 3 1 2
a21 Annual probability of zebra mussel arrival in Lake Okeechobee 0.035 0.00040
a32 Annual probability of zebra mussel moving from state 2 to state 3 1 .052 3 3 1 2
a43 Annual probability of zebra mussel remaining in state 3 to state 4 1 .052
a44 Annual probability of zebra mussel remaining in state 4 1 .052
b Benefit (to fishing from zebra mussel) $0.325 mil $71.6 mil 1 1 3 2
C1 Annual cost of prevention and monitoring $0.1528 mil $ 8.7 mil 3 2 2 1
C2 Eradication cost $55.03 mil $72.08 mil 2 1 3 1
Eradication cost (annualized) $1,320 mil $1,729 mil
7f Eradication duration (years) 33 15 2 1 3 1
e Value of wetland services (per year) $64.98 mil $48.09 mil 2 1 3 1
Value of wetland services (per acre) $1,083 $801.4 2 1 3 1
f Effectiveness of prevention measures 75% 1% 3 3 2 1
Rank = 1 implies the highest expected NPV
The zebra mussel is expected to reach Florida in the near future and thus poses a threat to
wetland ecosystem services and consumptive water uses. Several years ago the U.S. Army Corps
of Engineers responded to the threat by outlining an education, monitoring, and prevention
program for Lake Okeechobee. The program however was never funded. While bringing live
zebra mussels in the state of Florida is illegal and punishable by fine, there is no other state or
federal program to prevent zebra mussels from entering Lake Okeechobee. In lieu of prevention,
eradication post arrival is an option, albeit a costly one.
This study examined the potential impact of zebra mussels on ecosystem services,
consumptive water uses, and recreation in Lake Okeechobee. A probabilistic model was
developed to simulate the arrival and spread of zebra mussels and assess the cost effectiveness of
alternate management strategies. Results indicate that both prevention and eradication of zebra
mussels are economically justified for Lake Okeechobee.
These findings are based on the data we used to parameterize the model. While we used
the best data available to the study, some questions undoubtedly remain. To tackle these
questions head on and advance the dialog on this topic, we conducted a series of sensitivity tests
around key model parameters. Specifically we tested the probability that zebra mussels would
arrive in Lake Okeechobee and the likelihood that they would survive and reproduce in this new
environment. We also tested our assumptions on the effectiveness a prevention program that
would cost only $152,800 per year and brought into question the cost of a prevention program
that boasted 75% effectiveness. Because documented eradications of invasive mollusks are few,
we reexamined our assumptions regarding how much this action might cost presuming
eradication was technically feasible and environmentally desirable.
The battery of sensitivity tests were presented in the form of breakpoint values, i.e.
borderline values of the tested parameters that would cause a change in the relative ranking of
the preferred alternatives. Under the baseline model parameters, Management IV was most
preferred, that is offered the highest expected net present value. Next preferred were
Management II, III, and I. Our sensitivity tests showed that the cost effectiveness of prevention is
fairly robust over a wide range of model assumptions. For example, probability of arrival, habit
suitability, and prevention effectiveness would have to be many times smaller or the cost of
prevention would have to be many times larger to rule out prevention as a worthwhile public
investment. In contrast a mere 30% increase in the cost of eradication would cause this
management activity to be ruled out based on cost effectiveness. Likewise, it would only take a
26% reduction in projected wetland losses due to zebra mussels to conclude that eradication may
not be worthwhile.
To evaluate the eradication of zebra mussels from Lake Okeechobee we used case studies
from other locations to infer treatment procedures, chemical dosages, and overall cost. Better
information will be required before managers will embark on a venture of this magnitude.
Fortunately the decision to eradicate can be postponed until zebra mussels have arrived at which
time hopefully more will be known. Because of the likely arrival of zebra mussels, their potential
to induce economic and environmental damage and the uncertainty regarding the technical
feasibility and cost effectiveness of eradication, this study provides empirical evidence for
prevention as a sensible management option that is economically justified. While additional
scientific study could lend better data to improve the precision of our model estimates, the
imminent threat of zebra mussels will remain until a prevention program is in place.
Adams, D.C. "Economics and Law of Invasive Species Management In Florida," Ph.D. Dissertation, University of
Florida, May 2007.
Buhle, E.R., M. Margolis, J.L. Ruesink, Bang For Buck: Cost-Effective Control Of Invasive Species With Different
Life Histories, Ecological Economics, 52(2005):355-366.
Borcherding, J., and W. Sturm. "The Seasonal Succession of Macroinvertebrates, in Particular the Zebra Mussel
(Dreissena polymorpha) in the River Rhine and Two Neighboring Gravel-Pit Lakes Monitored Using Artificial
Substrates." International Review ofHydrobiology 87(2002): 165 181.
Bossenbroek, J.M., C.E. Kraft, and J.C. Nekola. "Prediction of Long-Distance Dispersal Using Gravity Models:
Zebra Mussel Invasion of Inland Lakes." Ecological Applications 11(6)(2001):1778 1788.
Burlakova, L.E., A.Y. Karatayev, and D.K. Padilla. "Changes in the Distribution and Abundance of Dreissena
polymorpha Within Lakes Through Time." Hydrobiologia 571(2006):133 146.
Carson, R. Personal Communication. FLW Outdoors, February 2007.
Costanza, R., R. d'Arge, R deGroot, S. Farber, M. Grasso, B. Hannon, K. Limburg, S. Naeem, RV O'Neill, J.
Paruelo, R.G. Raskin, P. Sutton, & M. Van Den Belt, "The Value Of The World's Ecosystem Services And
Natural Capital," Nature, 387(1997): 283-260.
Deng, Y. "Present and Expected Economic Costs of Zebra Mussel Damages to Water Users with Great Lakes
Intakes." Ph.D. Dissertation, The Ohio State University, August 1996.
Drake, J.M., and J.M. Bossenbroek. "The Potential Distribution of Zebra Mussels in the United States." BioScience
54(111 2 1114): 931 941.
Eads, B. Personal Communication. Fishers of Men Tournament, February 2007.
Finnoff, D., J.F. Shogren, B. Leung, D. Lodge, "The Importance of Bioeconomic Feedback in Invasive Species
Management," Ecological Economics, 52(2005):367-381.
Florida Department of Environmental Protection, Division of Water Resource Management. Basin Status Report
Lake Okeechobee. November 2001.
GAMS Development Corporation, Washington DC, 1998.
Griffiths, R.W., D.W. Schloesser, J.H. Leach, and W.P. Kovalak. "Distribution and Dispersal of the Zebra Mussel
(Dreissena polymorpha) in the Great Lakes Region." Canadian Journal oj i i .. .,. andAquatic Sciences
Hayward, D., and E.D. Estevez. Suitability of Florida Waters to Invasion by the Zebra Mussel (Dreissena
polymorpha). Mote Marine Laboratory Technical Report No. 495, 1997.
Hebert, P.D.N., B.W. Muncaster, and G.L. Mackie. "Ecological and Genetic Studies in Dreissena polymorpha
(Pallas): A New Mollusk in the Great Lakes." Canadian Journal o i ,.. ... andAquatic Sciences
Johnson, L.E., and J.T. Carlton. "Post-Establishment Spread in Large-Scale Invasions: Dispersal Mechanisms of the
Zebra Mussel Dreissena polymorpha." Ecology 77(6)(1996):1686 1690.
Lauer, T.E., and A. Spacie. "Space as a Limiting Resource in Freshwater Systems: Competition Between Zebra
Mussels (Dreissenapolymorpha) and Freshwater Sponges (Porifera)." Hydrobiologia 517tl" '41: 137 145.
Leung, B, D.M. Lodge, D. Finnoff, J.F. Shogren, M.A. Lewis, and G. Lamberti, "An Ounce of Prevention or a
Pound of Cure: Bioeconomic Risk Analysis of Invasive Species," Proceeding of the Royal Society Biological
Studies, 2t.'iL 2 "2) 2407-2413
Macaulay, C. E\ oluollon Takes Its Chance In Darwin's Cullen Bay Marina," 1999,
www.marine.csiro.au/leafletsfolder/cullenbay.html, (Accessed June 23, 2007).
Nalepa, T.F., J.A. Wojcik, D.L. Fanslow, and G.A. Lang. "Initial Colonization of the Zebra Mussel (Dreissena
polymorpha) in Saginaw Bay, Lake Huron: Population Recruitment, Density, and Size structure." Journal of
Great Lakes Research 21(4)(1995):417 434.
O'Neill, C.R., Jr. "Economic Impact of Zebra Mussels Results of the 1995 National Zebra Mussel Information
Clearinghouse Study." Great Lakes Research Review 3(1997):35 42.
Phillips, S., T. Darland, and M. Systsma. Potential Economic Impacts of Zebra Mussels on the Hydropower
Facilities in the Columbia River Basin. Pacific States Marine Fisheries Commission Report, 2005.
Pimentel, D., L. Lach, R. Zuniga, and D. Morrison. "Environmental and Economic Costs of Nonindigenous Species
in the United States." Bioscience 50(1999):53 65.
Riccciardi, A, R.J. Neves, and J. B. Rasmussen, "Impending Extinctions Of North American Freshwater Mussels
(Unionoida) Following The Zebra Mussel (Dreissena Polymorpha) Invasion," J. Animal Ecology, 67(1998):613-
Strayer, D.L., J. Powell, P. Ambrose, L.C. Smith, M.L. Pace, and D.T. Fischer."Arrival, Spread, and Early
Dynamics of a Zebra Mussel (Dreissena polymorpha) Population in the Hudson River Estuary." Canadian
Journal oJ i .' ..,. andAquatic Sciences 53(1996):1143 1149.
Strayer, D.L. "Effects of Alien Species on Freshwater Mollusks in North America." Journal of the North American
P 1,ar. mi. i. ,/ Society 18(1)(1999):74 98.
Thorp, J.H., J.E. Alexander, and G.A. Cobbs. "Coping with Warmer, Large rivers: A Field Experiment on Potential
Range Expansion of Northern Quagga Mussels (Dreissena bugensis)." Freshwater Biology 47(2002):1779 -
University of Florida, "Discovery of invasive zebra mussels prompts warnings from state officials," University of
Florida News," //http.news.ufl.edu/1998/10/07/zebra-2/ (Accessed: June 20 2007).
U.S. General Accounting Office (USGAO). Invasive Species Clearer Focus and Greater Commitment Needed to
Effectively Manage the Problems. Report to Executive Agency Officials, GAO-03-1, October, 2001.
U.S. Army Corps of Engineers (USACE) "Okeechobee Waterway Zebra Mussel Monitoring Plan,"
www.saj.usace.army.mil (Accessed: October 12 2005).
U.S. Army Corps of Engineers (USACE). Environmental Effects of Zebra Mussel Infestations. Technical Note
U.S. Geological Survey (USGS). Zebra Mussels Cause Economic and Ecological Problems in the Great Lakes.
GLSC USGS Fact Sheet 2000-6, created August 15, 2000.
U.S. Geological Survey (USGS). USGS real-time water data for Florida. Internet site:
http://waterdata.usgs.gov/fl/nwis/rt (Accessed: October 10, 2006).
U.S. Geological Survey (USGS). 2007. Non-indigenous Aquatic Species Database. Internet site:
http://nas.er.usgs.gov/taxgroup/mollusks/zebramussel/ (Accessed: June 15, 2007).
Virginia Department of Game and Inland Fisheries (VDGIF). Millbrook quarry zebra mussel eradication. Internet
site: http://www.dgif.state.va.us/zebramussels/ (Accessed February 5, 2007).
Table 6. Zebra mussel model simulation results (using HSI = .83)
I II III IV
Do nothing Prevention early
Public management cost -$0 -$2.5 -$150.9 -$45.3
Public ecosystem loss -$178.2 -$50.6 -$19.5 -$0
Private economic damage -$20.9 -$5.9 -$1.0 -$0
Private recreational benefit +$0.9 +$0.3 +$0.1 +$0
NPV -$198.1 -$58.7 -$171.3 -$45.3
ANPV $0 +$139.4 +$26.8 +$152.8
T = 20 years, r = .02
Table 7. Zebra mussel model simulation results (using HSI = .91)
I II III IV
Do nothing Prevention early
Public management cost -$0 -$2.5 -$167.3 -$50.1
Public ecosystem loss -$197.6 -$56.1 -$21.5 -$0
Private economic damage -$23.2 -$6.5 -$1.1 -$0
Private recreational benefit +$1 +$0.3 +$0.1 +$0
NPV -$219.8 -$64.9 -$189.9 -$50.1
ANPV $0 +$154.9 +$29.9 +$169.7
T = 20 years, r = .02