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Summary
The uptake and release of ballast water and associated sediments by ships is
one of the predominant means by which new aquatic invasive species are intro-
duced around the world. Many of these nonindigenous species have caused ex-
tensive environmental, economic, and human health impacts in receiving sys-
tems. The prospect for future invasions has inspired world-wide efforts to re-
duce, if not eliminate, the transport and release of living organisms in ships’
ballast water. At the present time, a diverse range of governmental organiza-
tions and private interests throughout the world are advancing policy (regula-
tions) and approaches (treatment methods) to reduce ballast-mediated invasions.
Over the past ten years, ballast water exchange to reduce the densities of
coastal organisms transferred among global regions has been the most common
practice to mitigate ballast-mediated transfers of species. In an effort to go
beyond the protectiveness afforded by ballast water exchange, the U.S. Envi-
ronmental Protection Agency (EPA) and the U.S. Coast Guard (USCG) are de-
veloping standards limiting the density of organisms in ballast water discharged
to U.S. waters. These agencies requested the National Research Council’s
(NRC) Water Science and Technology Board (WSTB) to undertake a study to
provide technical advice to help inform the derivation of numeric limits for liv-
ing organisms in ballast water for their regulatory programs.
INTRODUCTION
Each year there are more than 90,000 visits, on average, of commercial
ships greater than 300 metric tons to U.S. coastal waters including the Great
Lakes, discharging about 196 million metric tons of ballast water. There is sig-
nificant variation among U.S. ports in terms of the number and frequency of
vessel arrivals, ballast volume discharged, and source region of the ballast. The
organisms collected from unexchanged ballast water and sediments span orders
of magnitude in size, ranging from fishes (30 cm) down to microorganisms (~20
nm). Over 15 animal phyla have been detected in ballast (especially common
are mollusks, crustaceans, worms, hydromedusae, and flatworms), as well as
algae, seagrasses, viruses, and bacteria. The concentration of organisms present
within a ship’s ballast water exhibits tremendous temporal and spatial variation
1
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2 Propagule Pressure and Invasion Risk in Ballast Water
because of differences in organism abundance among sources and seasons, par-
ticular voyage conditions, characteristics of the ships themselves, and the nature
of ballast water management practices, among other factors. Numbers as high
as 300 million cysts of toxic dinoflagellates have been detected in a single tank.
Thorough ballast water exchange serves to reduce concentrations of live coastal
organisms by, on average, 88 to 99 percent. The efficacy of exchange is highest
when source ports are freshwater locations and lowest when it involves source
ports of high salinity. A final consideration is that much ballast water in the
future will be treated to a level beyond what can be accomplished by ballast
water exchange, and it is this water that will be the primary target of ballast dis-
charge standards.
Studies on the invasion history of North American waters have involved
mining occurrence records from the literature and diverse research programs,
rather than an organized field-based monitoring program designed explicitly to
detect invasions. Thus, today’s knowledge about invasions represents an unde-
restimate of the total number of nonindigenous species that have colonized.
Among the best studied freshwater systems in the world are the Laurentian
Great Lakes, where more than 180 invaders have been detected and described.
At least 55 percent of the nonindigenous species that established populations in
the Great Lakes from 1959 onward are attributed to ballast water release. For
coastal marine ecosystems, California and western North America have received
the most in-depth analyses of aquatic invasions, with over 250 nonindigenous
species of invertebrates, algae, and microorganisms having become established
in tidal (marine and estuarine) waters of California alone. Of these, only about
10 percent are attributed solely to ballast water as a vector, but greater than 50
percent include ballast water as a possible vector among others. Indeed, ballast
water is only one of many potential vectors that now transport marine, estuarine,
and freshwater species between continents and oceans; others include vessel
biofouling, aquaculture, live bait industries, aquarium and pet industries, the live
seafood industry, and the availability of thousands of species on the Internet for
unregulated purchase and distribution to the public at large. Despite the chal-
lenges that polyvectism implies, the enduring principle of vector management is
that limiting (reducing) species transfers decreases invasions. It is for this rea-
son that ballast water (and other vector) management and supervision are critical
in protecting and preserving the beneficial uses and the indigenous populations
of fish, shellfish, and other wildlife in the nation’s waters.
The goal of this NRC report is to inform the regulation of ballast water by
helping EPA and the USCG better understand the relationship between the con-
centration of living organisms in ballast water discharges and the probability of
nonindigenous organisms successfully establishing populations in U.S. waters.
The NRC was not asked to propose specific ballast water discharge limits, as
that is a risk management decision, nor was it asked to evaluate matters related
to the technical and engineering aspects of testing, installing, and using ballast
water treatment systems. The NRC committee’s statement of task, given below,
was to evaluate the risk–release relationship in the context of differing environ-
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Summary 3
mental and ecological conditions, including estuarine and freshwater systems as
well as the waters of the three-mile territorial sea. With respect to development
of allowable concentrations of living organisms in discharged ballast water (in-
oculum density), the committee was asked to:
1. Evaluate the state of the science of various approaches that assess the
risk of establishment of aquatic nonindigenous species given certain concentra-
tions of living organisms in ballast water discharges.
What are the advantages and disadvantages of the available ap-
proaches?
Identify and discuss the merits and practical utility of other addi-
tional approaches of which the National Academy of Sciences is aware.
How can the various approaches be combined or synthesized to
form a model or otherwise more powerful approach?
What are the data gaps or other shortcomings of the various ap-
proaches and how can they be addressed within the near and long term?
Can a “natural invasion rate” (invasion rates based on historic in-
vasion rates), or other “natural” baselines, be reliably established, and if so,
how? What utility might such baselines have in informing EPA’s deriva-
tion of allowable numeric limits for living organisms in ballast water dis-
charges? Can such baselines be established on a national basis, or would
this need to be done on a regional or ecosystem basis?
2. Recommend how these approaches can be used by regulatory agencies to
best inform risk management decisions on the allowable concentrations of living
organisms in discharged ballast water in order to safeguard against the estab-
lishment of new aquatic nonindigenous species and to protect and preserve ex-
isting indigenous populations of fish, shellfish, and wildlife and other beneficial
uses of the nation’s waters.
3. Evaluate the risk of successful establishment of new aquatic nonindigen-
ous species associated with a variety of ballast water discharge limits that have
been used or suggested by the international community and/or domestic regula-
tory agencies.
With regard to Task 3, as discussed briefly below and in greater detail through-
out the report, the available methods for determining a numeric discharge stan-
dard for ballast water are limited by a profound lack of data and information to
develop and validate models of the risk–release relationship. Therefore, it was
not possible with any certainty to determine the risk of nonindigenous species
establishment under existing discharge limits. To address Tasks 1 and 2, this
report outlines a process of model development and data collection critical to
informing future numeric ballast water discharge standards.
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4 Propagule Pressure and Invasion Risk in Ballast Water
POLICY CONTEXT FOR REGULATING ORGANISMS
IN BALLAST DISCHARGE
Chapter 2 discusses the regulatory context surrounding ballast water man-
agement, including state, federal, and international guidelines and regulations
that are the foundation for the current ballast discharge standards. The two main
federal programs for regulating ballast water in the United States are the EPA’s
Vessel General Permit under the Clean Water Act and the U.S. Coast Guard’s
authority under the National Invasive Species Act (NISA). Given the prominent
role of two independent agencies, as well as 20 years of federal and state legisla-
tive activity, a complex network of regulatory arrangements has been created
around ballast water discharge. Internationally, the regulatory arrangement is
simpler, as exemplified by the Inter-national Maritime Organization’s (IMO)
International Convention for the Control and Management of Ship’s Ballast
Water and Sediments. The Convention identifies two key standards; D-1 is a
ballast water exchange standard, and the D-2 performance standard sets maxi-
mum permissible limits on live organisms in ballast effluent based on the size or
taxonomic category of organisms.
The statutes that guide the EPA and USCG regulatory programs ap-
pear to provide the essential considerations and scope to successfully im-
plement scientifically based numeric standards, given sources of variation
(described in Chapter 3). For example, the CWA is capable of addressing
place-specific issues, such as characteristics of the receiving system, through the
State certification process. Meanwhile, the NISA program is comprehensive
with respect to the ship-related modes of introduction. Both statutes allow the
implementing agency to be sensitive to critical risk factors such as voyage pat-
terns and frequencies through variable enforcement intensity. Specifically, ri-
gorous enforcement of reporting and implementation actions by industry, incor-
porating data gathering on living organisms density and diversity in discharge,
will greatly facilitate better understanding of the relationship between propagule
pressure1 (in terms of inoculum abundance and density) and the probability of
invasion.
SOURCES OF VARIATION INFLUENCING
THE PROBABILITY OF INVASION AND ESTABLISHMENT
An assumption in the development of a numeric standard for live organisms
per unit volume ballast water discharged is that there is a direct and quantifiable
1
Propagule pressure is a general term expressing the quantity, quality, and frequency with which
propagules are introduced to a given location. Propagules are any living biological material (such as
particles, cells, spores, eggs, larvae, and mature organisms) transported from one location to
another.
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Summary 5
relationship between the density of individuals released in a ballast discharge
and the probability of their eventual establishment. While a relationship be-
tween inoculum density and establishment probability may exist, many other
factors also affect establishment success in aquatic systems, as discussed in
Chapter 3. These include the identity (taxonomic composition), sources, and
history of the propagules; their frequency of delivery; and their quality. Further
influencing the outcome of propagule release is a host of factors that include
both species traits and the recipient region’s environmental traits. Given the
significant differences between source regions; the diversity, abundance, and
density of entrained organisms; and the compatibility of source and recipient
regions, the prospect of developing a ballast water standard that can be applied
to all ships and yield a desired result is daunting.
It is abundantly clear that significantly reducing propagule pressure
will reduce the probability of invasions, when controlling for all other va-
riables. There is both strong theoretical and empirical support for this, across a
diverse range of habitats, geographic regions, and types of organisms. However,
the precise nature of the response can vary enormously over species, time, and
environments. In short, while inoculum density is a key component, it is but one
of scores of variables that can and do influence invasion outcome. Thus, any
method that attempts to predict invasion outcomes based upon only one
factor in the multi-dimensional world of the invasion process is likely to
suffer from a high level of uncertainty.
RELATIONSHIP BETWEEN ESTABLISHMENT RISK
AND PROPAGULE PRESSURE
Chapter 4 presents the theory underlying the relationship between ballast
water organism concentration and the risk (probability) of nonindigenous spe-
cies establishment (the risk–release relationship), which is focused on the role of
propagule pressure. It evaluates the mathematical models that have been devel-
oped to express this relationship, discussing their data needs as well as other
strengths and weaknesses. The models range from descriptive models that simp-
ly represent the shape of the relationship to mechanistic models that define the
processes generating the relationship. The models are also distinguished by
whether they consider a single species vs. multiple species. Three of the me-
thods discussed are the reaction-diffusion approach, the population viability
analysis, and the per capita invasion probability approach—previously reviewed
by EPA for their prospects in helping to set a numeric ballast discharge standard.
In principle, a well-supported model of the relationship between invasion risk
and organism release could be used to inform a ballast water discharge standard.
For a given discharge standard, the corresponding invasion risk could be pre-
dicted, or, for a given target invasion risk, the corresponding target release level
could be obtained.
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6 Propagule Pressure and Invasion Risk in Ballast Water
Ballast water discharge standards should be based on models and be
explicitly expressed in an adaptive framework to allow the models to be
updated in the future with new information. Before being applied, it is es-
sential that candidate models be tested and compared, and their compounded
uncertainty be explicitly analyzed. Only a handful of quantitative analyses of
invasion risk–release relationships thus far have tested multiple models and
quantified uncertainty.
In the short term, mechanistic single-species models are recommended
to examine risk-release relationships for best case (for invasion)-scenario
species. Candidate best-case-scenario species should be those with life histories
that would favor establishment with the smallest inoculum density, including
fast-growth, parthenogenetic or other asexual reproductive abilities, and lecitho-
trophic larvae. The greatest challenge in this approach will be converting the
results of small-scale studies to an operational discharge standard.
Developing a robust statistical model of the risk-release relationship is
recommended. It is anticipated that this approach will be more fruitful at a
local scale than a nationwide scale. Within a region, this relationship should be
estimated across multiple time intervals, rather than from a single point. The
effect of temporal bin sizes on the shape of the relationship should be examined.
Currently, the greatest challenge in this approach is the insufficient scope and
scale of the available data. Since long-term historical data on ballast-organism
density are limited, the committee recommends an extremely careful analysis
and validation of any proxy variables. There is no evidence that any proxy vari-
able used thus far is a reliable stand-in for organism density.
Models of any kind are only as informative as their input data. In the
case of ballast water, both invasion risk and organism density discharged from
ballast are characterized by considerable and largely unquantified uncertainty.
At the multi-species scale in particular, the existing data (historical invasion
records vs. recent ballast surveys) are substantially mismatched in time, and
patchy in time, space, and taxonomy; current statistical relationships with these
or proxy variables are of dubious value.
OTHER APPROACHES TO SETTING
A BALLAST WATER DISCHARGE STANDARD
In the absence of data and models necessary to support a science-based
quantitative approach to setting ballast water discharge standards, expert opinion
has been a common alternative. Chapter 5 discusses the strengths and weak-
nesses of non-quantitative, expert opinion-based methods for setting ballast wa-
ter discharge standards, including two previously reviewed by EPA (the zero-
detectable discharge standard and the natural invasion rate approach).
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Summary 7
Regarding standards for living organisms in ballast water discharge, expert
opinion processes have taken a number of forms and produced a wide range of
outcomes. While each expert opinion process discussed has conceptual merit,
each is compromised by assumptions, data limitations, or operational difficul-
ties. Despite these drawbacks, expert opinion has resulted in various standards
that could provide a manageable baseline for developing scientific models and
serve to reduce propagule supply. For example, as ships attempt to meet the
IMO D-2 standard, high density discharges and much of the variation in densi-
ties of live organisms in ballast discharge will be modulated. This standard pro-
vides a starting point for the regulatory process and can facilitate progression to
a scientific model.
THE PATH FORWARD
Approaches to setting ballast discharge standards have relied primarily on
expert opinion to evaluate the risk–release relationship. The associated history
and process have resulted in an array of different international, national, and
state discharge guidelines and regulations that seek to reduce propagule supply
below that of untreated ballast water. These differences result from both uncer-
tainty about the risk–release relationship and from the diverse approaches of
different decision makers and stakeholders. Of the more scientifically based
approaches suggested to date and reviewed in this report, descriptive statistical
modeling with proxy variables (such as the per capita invasion probability me-
thod) is currently available to empirically examine the risk–release relationship
because there are data available for ballast volume (derived indirectly from ves-
sel arrival data) and historical invasion rates across estuaries. However, it must
be cautioned that these are extraordinarily coarse-level data because (a) vessel
arrival data often do not directly translate into a measure of ballast water actual-
ly discharged, (b) when actual ballast volume data are available, these do not
translate well into known propagule supply and, (c) there is no significant rela-
tionship between ballast volume and invasions. Thus, while statistical model-
ing has been applied to current datasets, the data are not sufficient in
present form to characterize a biologically meaningful relationship, much
less estimate the associated uncertainty, to be able to identify with confi-
dence the invasion probabilities associated with particular discharge stan-
dards.
Several actions are needed to advance a robust understanding of the risk–
release relationship in order to inform future decisions about ballast water dis-
charge standards. As a logical first step, a benchmark discharge standard
should be established that clearly reduces concentrations of coastal organ-
isms below current levels resulting from ballast water exchange (such as the
IMO D-2 standard). This will serve to reduce the likelihood of invasion in
coastal ecosystems beyond that of the present time.
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8 Propagule Pressure and Invasion Risk in Ballast Water
Following the setting of an initial benchmark, a risk–release model or
models should be selected as the foundation for the data gathering and
analysis effort. One or several of the models described in Chapter 4 should be
pursued in the months and years ahead. What model or models is ultimately
chosen will reflect the available resources, in terms of time, data, and personnel.
At the present time, none of the available models has been validated, due mainly
to a lack of key data. Using multiple models with the same data could be valua-
ble to test for concordance. This would also allow one to assess the range of
outcomes that would result from proposed ballast water discharge standards.
Furthermore, there is considerable worth in transitioning from simple, statistical
single-species models to more complicated, multi-species mechanistic models as
more data become available.
Finally, a two-track approach should be pursued to obtain both expe-
rimental and field-based (descriptive) data. Important early steps should be
taken to develop sampling protocols, standardize methods and analytical
processes, and create the framework necessary to produce high-quality data spe-
cifically needed to populate risk-release models. Experiments can be used to
evaluate this relationship and should deliver results over the next three to five
years. Field-based descriptive data should also be collected and analyzed to
parameterize the same types of models, providing real-world validation of expe-
rimental data. Results from such field efforts would be expected to materialize
in about ten years.
Recommendations for Experiments
Experiments should be used to estimate the effect of propagule pressure on
establishment success, using statistical and probabilistic models. The experi-
ments should (a) be conducted in large-scale mesocosms designed specifically to
simulate field conditions, (b) include a diverse range of taxa, encompassing dif-
ferent life-histories and species from known source regions of potential inva-
sions, and (c) include different types of environments (e.g., freshwater, estua-
rine, and marine water) where ballast discharge may occur.
Initial experimental efforts should focus on single-species risk–release rela-
tionships. Ideally, these would include taxa and conditions that are selected as
“best-case” scenarios, seeking to maximize invasion success and provide a con-
servative estimate of invasion probability. Thus, rather than experiments that
examine complex and interactive effects of many different environmental and
biological variables, a premium is placed on relatively simple initial experiments
that provide a significant amount of data across “model” taxa and conditions in a
short amount of time. This approach should be applied to multiple species, and
serious consideration should be given to selecting the appropriate organisms and
conditions.
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Summary 9
The experiments should be advanced aggressively, in a directed fashion, to
yield results in a three- to five-year time horizon. While this represents a signif-
icant investment in effort and resources, it is the more cost- and time-efficient
path to obtaining critical data needed to parameterize risk–release models com-
pared to field-based measures. Experiments could potentially identify a solid
interim basis for discharge standards, noting the inherent challenges in working
with a limited number of species and the assumptions that these would be repre-
sentative of a broad array of potential invasions. Importantly, these data may
also have direct application to other vectors, in addition to ballast water, as they
test basic questions about establishment that are relevant to propagule pressure
arising from all vectors.
Recommendations for Descriptive Studies
In addition to experiments, descriptive field-based measures are recom-
mended to ground-truth the models, providing a critical validation step to con-
firm that (a) risk–release relationships are consistent with experimental results
and (b) observed invasion rates are consistent with these predictions. Imple-
menting such an effort at one location is not sufficient. This should occur at
selected sentinel estuaries (e.g., San Francisco Bay, Chesapeake Bay, and Tam-
pa Bay), chosen to include different coasts, ship traffic patterns, source regions,
and environmental conditions. For each sentinel estuary, measures of propagule
supply (in ships’ ballast) and invasion rate would be made repeatedly over a
minimum of a ten-year time horizon to provide a data set for independent analy-
sis and validation of experimental results.
The specific design of data collection needs to be defined explicitly, consi-
dering the model(s) being used and making sure that the output will represent
the risk–release relationship and directly translate to a discharge standard.
While it may be reasonable to explore potential proxy variables as one compo-
nent, it is critical to not focus extensively on proxies or other variables that may
not represent the risk–release relationship. Also critical is an a priori estimate
of the uncertainty explicit at all scales, as well as sampling effort (number and
frequency of measures), in order to properly design measures and interpret and
compare predictions. The same data could be used for statistical and probabilis-
tic models, moving toward increasing resolution (e.g., hierarchical probability
models described in Chapter 4) if and as appropriate data are available. A com-
prehensive model would require sampling many vessels (stratified by vessel
type, season, and source) and quantifying the concentration of each species
present in discharged ballast (as well as volume per discharge event). Field sur-
veys to detect invasions of these species would also need to be conducted coin-
cident with ballast measures.
One possible strategy would be to focus on a subset of target species dis-
charged in ballast water to multiple estuaries. This would considerably reduce
the effort required for analysis of ballast water, compared to characterizing the
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10 Propagule Pressure and Invasion Risk in Ballast Water
entire community. It may also serve to reduce the sampling effort, and increase
the probability of detection, of the target species in field surveys. Intuitively, it
would make sense to focus particular attention not only on species that can be
identified and counted in ballast samples, but also on species that are unlikely to
be polyvectic (such as copepods and mysids), providing the clearest signal for
analysis of risk–release relationships associated with ballast water. With this
strategy, selection of taxa is critical and should take into consideration biological
and environmental requirements (especially whether suitable conditions exist in
the specific estuary). A challenge is how generally representative any such spe-
cies would be. Nonetheless, this would result in single-species models (for mul-
tiple species) in parallel to the experimental approach outlined previously.
While collection of field-based descriptive data required for a meaningful
analysis of the risk–release relationship is somewhat daunting in scope, recent
developments make this more feasible than in the past. First, pending interna-
tional and national regulations require commercial vessels to install sampling
ports that provide representative and standardized samples of ballast discharge.
This will provide an important platform for ready access and standardized, com-
parable samples across vessels and locations. Second, the implementation of
ballast water treatment systems will reduce the concentrations, and possibly the
diversity, of organisms in ballast water. This may serve to simplify sampling,
having less biological material to process for quantitative analysis. Third, the
use of molecular genetic tools has dramatically expanded the capacity (and re-
duced the time, effort, and cost) to detect species, based on DNA. It is perhaps
useful to point out that field-based measures outlined above would also serve a
broader range of applications, such as providing critical feedback for adaptive
management, identifying performance of discharge standards to reduce inva-
sions, and tracking invasions from other vectors concurrently.
***
To date, there has been no concerted effort to collect and integrate the data
necessary to provide a robust analysis of the risk–release relationship needed to
evaluate invasion probability associated with particular ballast water discharge
standards. Existing experimental and field data are of very limited scope. There
is currently no program in place to implement either ship-based ballast sampling
or field surveys to detect new invasions across sites. On-going research pro-
vides confidence that this approach is feasible, but it is scattered across sites and
usually short-term in nature. Several models exist which can quantify the risk–
release relationship, given sufficient data that are now lacking. This report out-
lines the paths, using multiple methods over different time frames, that could
address these data gaps, and thus provide a robust foundation for framing scien-
tifically supportable discharge standards for ballast water.
