The Council requested the Independent Scientific Advisory Board (ISAB) to provide a review of the biological and economic impacts of native and non-native predators, the effectiveness of predator management control efforts currently implemented, and the potential impacts on the Columbia River Basin (Basin) from the introduction and spread of northern pike. The request included six science questions and two initial economic questions. This report addresses the science questions. The Council obtained the assistance of economists, who will address the economic questions in a separate companion report.
Review approach. The ISAB's answers to the Council questions, conclusions, and recommendations are based on a targeted but not exhaustive literature review and a series of scientific briefings by experts working in the Basin and elsewhere. The ISAB considered native and non-native fish predators and native avian and pinniped predators in the Basin. In past reports, the ISAB, Independent Scientific Review Panel (ISRP), and Independent Economic Analysis Board (IEAB) have highlighted important assumptions and observations about the biological and economic impacts of native and non-native predators and the effectiveness of predator control efforts in the Basin. These assumptions and observations were considered in this report. The ISAB offers summary answers to the Council’s six science questions early in this report. These summary answers are followed by detailed explanations for a subset of predator species in the Basin.
Brief answers to the Council’s questions
- What information is needed to develop a common metric of impact across all predator species? To measure the impact of predation, we need to know (1) the total predation by a predator at each point in time or space where predation occurs; (2) the subsequent predation by all other predators at each point in time or space; and (3) the cumulative survival probability over the full life cycle of the fish or to a consistent point of reference (e.g., Bonneville Dam). An ecosystem-wide, multi-predator, multi-prey approach must be taken to fully understand predation impacts.
- What type and level of effort are needed? A system-wide, ecosystem-based approach for assessing and managing fish, avian, and pinniped predators collectively will create a more effective and consistent framework for developing and implementing control actions. Assessing impacts of all potential predators throughout the Basin will require integrated analytical tools, such as life-cycle models for salmon and steelhead, measurement of SARs, and density dependence analysis. This type of analysis would allow managers and policymakers to identify (a) locations where life stages of prey species are most susceptible to different predators and (b) the relative benefits of decreasing salmonid mortality at different life stages, making control efforts more biologically and economically effective.
- Would additional predator management be effective in improving focal species survival? The ISAB cannot simply answer this question as a “yes/no” option without better information on predation in the Basin. Even with more rigorous information on predation, a portfolio approach (including hydrosystem management, habitat restoration, as well as predator control) for reducing mortalities of salmon and steelhead will likely provide the most biologically and cost-effective management alternatives.
- Can we rank predator impacts and then rank which current management activities would be most effective in reducing impacts? Accurately ranking the impacts of the predators on focal species cannot be accomplished across all combinations of predators and their salmonid prey without synthesis of available Basin-wide information and analysis of salmonid survival throughout their life cycles. Research and monitoring have tended to examine subsets of juvenile and adult salmonids, and a subset of predators, resulting in very uneven coverage and a dearth of information on certain combinations of species and life histories. The ISAB compiled tables of the major predators and categorical values of vulnerability of salmonid prey. The tables illustrate some important points, such as effects of seasonal timing of adult returns and differences in body size on differences in vulnerability. Biological effectiveness and cost-effectiveness of management activities vary by predator and prey species, life cycle, and location. However, recent work on avian and pinniped predators is heading in a direction that may lead to a better evaluation of the efficacy of management actions.
- Do we know what level of suppression (exploitation) is needed to reduce the northern pike population in Lake Roosevelt to a level sufficient to reduce risk of emigration? Based on the inexorable invasion by northern pike downstream over the past 65 years, it is likely that even with the best efforts in public education, early detection, and control or eradication, pike will eventually invade the anadromous zone. There is no simple estimable relationship between abundance and the probability of emigration from Lake Roosevelt because, for example, each individual female pike produces tens of thousands of eggs so emigration by even one male and one female pike could produce thousands of juveniles. Moreover, evidence indicates that about as many invasions have been caused by illegal stocking, often to distant locations, as by dispersal of pike themselves. It is essential to develop a monitoring program throughout the anadromous zone and a rapid response program to eradicate new invasions at their earliest stages when it may be possible. After populations become established, control efforts will need to be extensive, river-wide, and must continue indefinitely to be successful in reducing mortality of focal salmonids. Suppression in Lake Roosevelt could reduce risks of downstream establishment by reducing the number and (average) body size of downstream dispersers.
- What are the likely ecological impacts of northern pike should they enter the Basin’s anadromous waters? Pike are highly invasive and through predation are likely to drastically reduce salmonid abundance, especially in low-gradient river segments with wide floodplains. Pike prefer salmonids and are capable of driving preferred prey species to very low levels or extinction, and all sizes and ages of pike (yearling and older) can eat salmon fry, parr, and smolts. If salmonids have no refuges from predation (i.e., habitats that are unsuitable for pike), pike are likely to reduce the salmonid numbers and can cause salmonid populations to collapse. Salmonids that migrate in open water near the surface may be less vulnerable than others that forage or overwinter in habitats occupied by pike.
In this review, the ISAB made a number of conclusions regarding predation management and specific predator species in the Basin:
A coupled ecosystem/socioeconomics approach to predation management and metrics. Predation management requires both biological and socioeconomic information, especially as it pertains to the spread of non-native fish such as northern pike. A coupled ecosystem/socioeconomic approach is required to fully understand both the impacts of predation on focal species and the efficacy of predator management. As the ISAB stated in its Predation Metrics Report (ISAB 2016-1), compensatory mortality is the most important uncertainty to address when developing predation metrics or management plans. Compensatory mortality occurs when predation mortality at one life stage is offset to some degree by decreased mortality at the same or subsequent life stages. The costs and benefits of invasive species prevention, control, and eradication are critical to making sound policy decisions—including the ways in which people’s behavior can promote or frustrate those efforts. Given these high levels of uncertainty and often novel ecological interactions, quantitative evaluation of alternative management or policy responses is sometimes not possible, but a potential approach will be described in the forthcoming economic report.
Northern pikeminnow. The primary native piscivorous fish in the Basin is the northern pikeminnow for which there is a bounty removal program based on predator studies conducted over 30 years ago. The current efficacy of this program is unknown due to potential changes in the relative abundance of pikeminnow and other piscivore predators, and the distribution of prey in their diets since the early studies. These studies need to be updated; the evaluation of the program must do more than count pikeminnow removed; an ecosystem approach is needed.
Northern pike. Invasive northern pike are now distributed throughout Lake Roosevelt and have invaded to within a few miles of Grand Coulee Dam. It is likely that even with the best efforts in public education, early detection, and control or eradication, pike will eventually invade the anadromous zone, either naturally or by human agents. Pike are likely to drastically reduce salmonid abundance, especially in low-gradient river segments with wide floodplains. Pike are highly adaptable and fecund. Nevertheless, reducing the numbers of fish emigrating from Lake Roosevelt is likely to reduce the chances that pike will establish new populations downstream and hence delay the invasion. Illegal introductions by humans are difficult to control, and more could be invested in efforts to measure, understand, and reduce illegal stocking of pike. A species distribution model could be developed to estimate the habitat in the Basin most likely to be invaded. Genetic tools (eDNA) are cost-effective for rapid early detection of the presence of pike, and releasing fish with “Trojan” sex chromosomes (YY males) may provide a means for their control but could require several decades to develop and implement. Early detection and rapid suppression efforts are cost-effective and paramount for eradicating this species or slowing its spread compared to the cost-effectiveness of efforts after the pike are established. Northern pike have been eliminated from individual lakes and reservoirs, and two small watersheds in California and Alaska, and could be removed from small ponds and lakes in the Basin, especially those on the floodplain where they are likely to be introduced illegally.
Other non-native fish. Other non-native fish in the Basin also prey on focal species and include smallmouth bass, which have been in the Basin for over 100 years, and walleye. At this time, neither the abundance of these and other non-native predators nor their impacts on focal species are accurately known. Non-native lake trout are also present in several lakes in the Basin where they are impacting native species such as kokanee, bull trout, and cutthroat trout.
Avian predation. Large numbers of colonial, piscivorous water birds, such as Caspian terns and double-crested cormorants, nest in the Basin and are believed to be one of the greatest sources of mortality—if not the single-greatest source—for emigrating juvenile steelhead and yearling Chinook salmon from the upper Columbia and Snake rivers.Modeling avian predation of juvenile steelhead has found an inverse relationship between the level of predation and smolt-to-adult-returns (SARs). Current management of avian predators focuses on dissuading the birds from nesting in some areas of the Basin. While this strategy has reduced the numbers of birds on specific colonies, the actual change in predation is unknown as the birds may have established new colonies in the Basin and resumed preying on juvenile salmon (or other focal species). A Basin-wide monitoring plan is needed to evaluate total avian predation and evaluate birds control methods.
Marine mammals. The negative impacts of pinniped (sea lion and seal) predation on adult salmonids and other focal fish species (eulachon, sturgeon, lamprey) are better understood than those occurring at juvenile life stages. Recently, there has been substantial progress in estimating Basin-wide impacts of California sea lions on adult Chinook salmon, but the results are equivocal given recent dramatic annual fluctuations in pinniped and salmonid populations and many other important uncertainties. Recent authorization for lethal removal of sea lions may provide relief from this predator at Bonneville Dam and Willamette Falls, but its efficacy must be evaluated as part of a Basin-wide ecosystem examination of predator-prey interactions.
Evaluating effectiveness. The ISAB has evaluated and provides recommendations about the statistical methods agencies use to monitor and estimate predation effects by fish, birds, and marine mammals. The ISAB also evaluated the effectiveness of predation management. Evaluating the effectiveness of predator control programs is a two-step process. First, the magnitude of the predation impact on focal species must be ascertained, and second the effectiveness of control methods must be evaluated. Estimates of total predation and the estimates of the total number of predators gives the marginal gain from reducing a particular predator population by one individual in one particular time and location. However, marginal gains may fail to reflect the benefits of the control program because of compensatory behavior of the prey (e.g., density dependence), and compensatory behavior of this and other predators (e.g., removing one predator species may increase predation by another predator species; predators displaced by hazing, for example, may move elsewhere). Consequently, evaluation of predator control programs must do more than simply count the number of predators removed. The evaluation must monitor responses from other predators to the predator removals and evaluate responses of the salmon over the remainder of its life cycle up to a common point such as Bonneville Dam.
An altered ecosystem. Human alterations have changed the dynamics of both juvenile and adult anadromous salmonids, abundance and distribution of native and non-native predators, vulnerability of salmonids to predation, and complexity of food webs in the Columbia River Basin. Predator management in the Columbia River Basin currently focuses on individual predator species and survival of the portion of their prey that are salmon and steelhead. Most predation analyses to develop management actions in the Basin are fragmented and ignore other factors (e.g., hydrosystem operations, habitat degradation) that influence survival of focal species. A Basin-wide, ecosystem-based approach for assessing and managing fish, avian, and pinniped predators collectively is needed to create a more effective and consistent approach for developing more biologically and economically effective predator control actions.