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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.