The Mekong River presently supports the world’s largest inland fishery at 2.6M tonnes of annual harvest but nearly 200 dams are completed, under construction or planned along the river. John W Ferguson reviews fish passage activities designed to maintain salmon in the US’ Columbia River, and provides guidance for developing environmentally sustainable hydropower in the Mekong.
Large river systems provide a multitude of services, including biological production from river ecosystems, affordable and reliable transportation of commerce, and municipal and industrial water supplies, to name just a few. Two services that are especially important are food supply from fisheries and hydropower production. However, there exists a conflict between hydropower and fisheries production because water resource development has significant effects on the structure and function of river ecosystems (Ward and Stanford 1979; Winston et al. 1991; Reyes-Gavilán et al. 1996; WCD 2001; FAO 2005). Indeed, considerable attention has been focused recently on the effect of development over the past century on riverine environments, as well as the attendant attributes it has delivered to society (e.g., NRC 1996, McCully 2001; Scudder 2005).
The Mekong River is one of the world’s greatest rivers, extending 4909km from the Tibetan Plateau in China to its mouth in southern Vietnam. Its physical diversity, tropical location and high productivity has fostered the evolution of a diverse fish community that is second only to that of the Amazon River in terms of species richness (Froese and Pauly 2010).
An estimated 1100 species inhabit the Mekong River ecosystem when coastal and marine species are considered (Hortle 2009a), and 135 of these conduct extensive migrations within freshwater to complete their life cycle and reproduce (Baran 2006). For example, the Mekong giant catfish (Pangasianodon gigas) grows to over 3m in length and 300kg in weight, and undergoes extended migrations to spawning grounds in northern Thailand and Lao Peoples Democratic Republic (PDR).
The Mekong’s productivity supports the world’s largest inland fishery with an annual harvest of 2.6M tonnes (Hortle 2007) and a combined value from natural harvest and aquaculture of US$3.6-6.5B (Hortle 2009b). These values do not consider food security and employment benefits the fishery provides to millions of people with limited livelihood alternatives, nor do they recognise that Mekong fish are the main source of animal protein, vitamins, and calcium for 60M people in the Lower Mekong Basin (LMB) below China (Baran et al. 2007; Hortle 2007).
The Mekong basin is located in an extraordinarily dynamic hydrological region, and rapid development is transforming the economy, the landscape and the river system itself (MRC 2005). This development drives a desperate need for electricity, and nearly 200 dams are completed, under construction or planned in the Mekong and its tributaries (Baran and Myschowoda 2009a; MRC 2010). In China, four mainstem dams have been constructed, one is under construction, and three more are planned (MRC 2010). In the LMB, 11 mainstem dams are proposed that will range in height from 6-40m and would generate nearly 14,000MW.
There is no doubt that hydropower resources in the Mekong will be developed and their impacts to Mekong fish and fisheries could be substantial. Dams in large river systems are obstacles to fishes that require movement to complete their life cycle (Larinier 2001; Winter and Van Densen 2001; Zigler et al 2004). These obstacles can result in decreasing population trends (NRC 1996; Parrish et al 1998; Jackson and Marmulla 2001), and reductions in species diversity and catch per unit effort of fishes for both short and long distance migratory species (Fernandes et al 2009).
In 2008, the mekong-river-commission (MRC) convened a workshop to discuss potential challenges associated with developing fish passage programmes to mitigate hydropower development in the LMB. The workshop was attended by fisheries scientists and fish passage engineers from Europe, North and South America, Asia, Southeast Asia, and Australia, and its findings were summarised by Dugan (2008).
The workshop identified the challenge of developing hydropower in a balanced, sustainable manner. To summarise, development of the river in a manner that maintains its biodiversity and balances both food and hydropower production is considerably hindered by a lack of basic information on fish population abundance, migration timing, behaviour and life history patterns, and a lack of tested and effective fish passage system designs.
In rivers where fisheries play a prominent role in national economies and rural livelihoods, considering the ‘lessons learned’ from other large river systems can help to identify and inform development approaches.
Therefore I have synthesised the important lessons learned from fish passage research in the Columbia River in the US from my own perspective, having worked on these issues for the past 30 years. This synthesis relies heavily on Ferguson et al (2011) and the reader is directed to that paper for a more detailed discussion of many of the points raised herein.
The Columbia River
The Columbia River is 2000km long and drains an area of 567,000km2 from seven western US states and one province of Canada. It is the fourth largest river in the US based on volume. The fish community of relevance here is comprised of seven species of Pacific salmon and trout, including five that are harvested mainly in the ocean (sockeye (Oncorhynchus nerka), pink (O. gorbuscha), chum (O. keta), coho (O. kisutch), and Chinook salmon (O. tshawytscha)), and two species harvested mainly in freshwater recreational fisheries (steelhead (O. mykiss) and cutthroat trout (O. clarkii)). Other key species include Pacific lamprey (Lampetra tridentata), green sturgeon (Acipenser medirostris), and eulachon (Thaleichthys pacificus), and American shad (Alosa sapidissima) that inhabit the river but are not native. These species are defined by their anadromy, which means the adults become sexually mature in marine habitats and migrate into freshwater to spawn, and juveniles leave freshwater habitats after various durations of natal rearing and migrate to the ocean to grow to adulthood.
Historically, an estimated 7.5-10 million adult salmon returned to the river annually (Chapman 1986; NRC 1996). Extensive harvest by Euro-Americans in the mainstem Columbia River began after the first salmon cannery was built in 1866. Harvest declined starting in the early 1900s from overfishing, tributary habitat degradation, blocked access to spawning habitats from dams, and mortality during passage at mainstem dams (NRC 1996; Lichatowich 1999), but has increased somewhat recently due to dam passage improvements, habitat restoration, improved ocean productivity, and production from hatcheries.
Rock Island dam, installed in 1930, was the first mainstem dam to be constructed on the Columbia River and adult salmon ladders were an integral component of its design. Other key dams include Bonneville and Grand Coulee, which were completed in 1938 and 1941, respectively. Today, ten private and federal mainstem dams span the Columbia River below Grand Coulee, four additional federal dams span the lower Snake River, and more than 130 large private and federal dams in the basin are used for hydropower production, flood control, commercial transportation and irrigation (NRC 1996).
There are 13 dams across the Snake and Columbia River that adult and juvenile salmon must pass to reach spawning habitats. The dams are approximately 30m high and have rated capacities ranging from 693-2480MW. Each was constructed with adult fish collection and ladder systems which have performed well overall based on research conducted since the 1960s (Ferguson et al. 2005).
In contrast, the most recently constructed dam system for diverting downstream juvenile migrants through non-turbine routes performed poorly (Williams and Matthews 1995; Ferguson et al. 2007). Impacts to juvenile salmon during passage through multiple dams became especially apparent during the extreme low flow years of 1973 and 1977 when survival through the hydropower system was estimated at less than 3% (Williams and Matthews 1995).
This low survival heightened the awareness within the Columbia region that salmon stock levels could not be maintained unless the poor survival of juvenile salmon passing dams was addressed. A series of major actions were implemented starting in 1977, including:
• Collecting juvenile fish at upper dams and transporting them in trucks or barges to release sites below Bonneville dam (initiated in 1980).
• Passage of the Pacific Northwest Electric Power Planning and Conservation Act in 1980 which authorised the states of Idaho, Montana, Oregon and Washington to form the Northwest Power and Conservation Council (NPCC) to protect, mitigate and enhance fish and wildlife of the Columbia River Basin affected by the construction and operation of hydroelectric dams, while assuring the region has an adequate, efficient, economical and reliable electric power supply.
• Identifying a volume of water stored in flood control reservoirs for use in augmenting river flow during salmon migrations to aid their migration timing (1982).
• Spilling water at dams to pass fish through non-turbine routes (the amount varies but ranges up to 60% of project flow; 1982).
• Installing specialised systems that guide juvenile fish away from turbines and around dams (1975).
• Maintaining gas super-saturation levels below 115% in dam forebays and 120% in tailraces (1996).
• Developing surface-oriented passage routes for juvenile salmon at dams (1990s).
• Developing new turbine designs to pass juvenile salmon more safely (1990s).
These actions improved the survival of juvenile salmon to where survival through eight dams is now similar to historical survival rates through four dams (Williams et al. 2001), and survival during a severe drought in 2001 was an order of magnitude larger than was observed during a similar low-flow event in 1977 (Williams et al. 2005).
Today, management plans guide water use regimes (TMT 2009) and mainstem dams are operated to achieve juvenile salmon survival rates of 96 and 93% during spring and summer, respectively. Furthermore, power production can supersede fish protection only if needed for system stability and public safety, and even then additional mitigation measures may be implemented to offset impacts to salmon (NPCC 2001). Even with these actions, an estimated 30% of the historic Columbia River salmon populations have been extirpated (Gustafson et al. 2007), and many remaining stocks have undergone a significant reduction in abundance since the 1970s. Many of these stocks are listed as threatened or endangered under the federal Endangered Species Act (1973) and receive additional protections to rebuild their status.
There has been an upward trend in the number of adult salmon counted at Bonneville Dam in recent years. While positive, this trend includes an increased reliance on production from 178 hatchery programmes associated with 351 salmon and steelhead populations in the basin to sustain fisheries and maintain stock levels (HSRG 2009).
Use of hatcheries to supplement natural production initially was based on the simple assumption that salmon abundance was limited by mortality in freshwater, and would increase in direct proportion to the number of eggs that survived a controlled environment (NRC 1996).
However, we now know that this was a simplistic and erroneous view of salmon ecology. For example, hatchery fish have a lower fitness in natural environments than wild fish (Risenbichler and Rubin 1999; Araki et al. 2008), wild-born descendents of captive-bred parents also have reduced reproductive fitness (Araki et al. 2009), and hatchery production can affect wild populations through competition (Levin et al. 2001).
Lessons for the Mekong
These issues associated with water resource development in the Columbia River reveal the following lessons for the Mekong:
1. Overall impacts from hydropower development will be large.
Dams can reduce stock productivity through a variety of effects (NRC 1996), including:
• Blocking or reducing upstream passage of spawning adults (Lundqvist et al. 2008).
• Mortality on adults migrating downstream after spawning (Ferguson et al. 2008).
• Direct mortality to juveniles migrating downstream through dam passage routes (Muir et al. 2001).
• Mortality manifested below a dam (Ferguson et al. 2006) or hydropower system (Williams et al. 2005).
Dams can also affect the survival of juvenile fish migrating through a series of dams by slowing migrations and altering ocean-entry timing (Muir et al. 2006), and modify juvenile rearing patterns after impoundment (Connor et al. 2005). Indeed, initial assessments suggest the effects of dams on Mekong fisheries could be significant (Barlow et al. 2008a,b; Baran et al. 2009b; Dugan et al. 2010).
Dams can also result in changes in productivity patterns when tropical rivers are impounded (Jackson and Marmulla 2001).
Baseline studies and monitoring programmes in the Mekong will be required to assess impacts to fish from river development and disentangle any hydropower impacts from other impacts, such as climate change and increased pressure on fisheries from human population growth.
2. Impacts on migratory fish will differ between mainstem and tributary development.
Effects from mainstem dams are likely to be greater than from tributary dams because of how migratory fish differentially use various habitats available to them in large river systems. This is especially important given the large and taxonomically varied number of species in the Mekong for which effective passage systems at dams have not been developed (Dugan 2008).
A Strategic Environment Assessment that accounts for economic, environmental, and social considerations is being conducted by the MRC to indicate the point along the mainstem where dams should be built. These types of assessments should also consider the need for some tributaries to remain undammed to conserve fish diversity, retain connectivity among key river components, and allow successful spawning and recruitment of key migratory species. However, additional information is needed on how the various species use the Mekong ecosystem to understand their vulnerability to dams, flow regulation and gas saturation.
3. Multiple, adaptive approaches will be needed to mitigate migratory fish resources impacted by mainstem dams.
Mitigating the effects of mainstem dams on Columbia River fish relied on multiple engineering solutions that took up to 40 years to develop, even when the research infrastructure, funding and political support were in place to support the effort. Furthermore, fish passage systems designed for salmon cannot simply be pulled off the shelf and expected to work in a multitude of other situations. Each system must be designed for a specific site and hydraulic conditions, fish species, and life stage present.
For example, Moser et al. (2005) reported passage rates of 50% for Pacific lamprey attempting to migrate past dams using ladders designed for salmon, compared with rates of > 90% for the salmon. Furthermore, simply installing fish passage facilities does not guarantee that fish passage impacts will be reversed. These facilities must be evaluated to ensure they meet performance criteria, are modified after construction if needed, and maintained properly (Porcher and Travade 2002).
The Mekong has nearly 30 times the number of species and 100 times the biomass of rivers in North America (Dugan 2008), which compounds the challenge of designing successful systems. Since design guidelines will differ greatly among species (Oldani and Baigun 2002; Haro et al. 2004), a variety of facilities that operate simultaneously may be needed to address fish passage in river systems with high biodiversity. Facilities will also need to accommodate the scale of fish migrations in the Mekong, where tens of different species are migrating simultaneously at magnitudes often exceeding one million fish per day (Baran et al. 2005).
The Columbia River experience also suggests that once dams are installed they become integral components of fish migration corridors, and power operations will need to be integrated with fish requirements to maintain fisheries.
4. The ability of salmon to negotiate upstream fish passage facilities is comparatively greater than that of tropical species.
Swimming capabilities vary greatly among fish species with differing body forms, size and muscle structure (Wardle 1975; Videler 1993). Within a species, swimming ability also varies according to life stage (juvenile or adult) and with environmental conditions such as water temperature and dissolved oxygen (Videler 1993). Adult salmon are extremely strong swimmers, capable of leaping falls and weirs and swimming at burst speeds of 8m s-1 (Bell 1991).
When encountering migration obstacles, salmon repeatedly search for openings to pass the obstruction, and adult fish ladders at Columbia River dams were designed based on this knowledge and required few modifications. These ladders allowed adults to pass dams and complete their spawning migrations, and thus allowed populations to be maintained while solutions for downstream migrating juvenile fish were developed. Had the adult systems not been installed during dam construction or had they performed poorly, the decline in stocks would have been rapid and many additional populations would have been lost while solutions were developed. For example, Oldani and Baigun (2002) observed that fish passage efficiency past dams was quite low (< 2% for all species) in South America’s Paraná River and inadequate to maintain its populations.
5. Mitigation for dam passage based on artificial production is problematic.
Hatchery-produced fish play a major role in sustaining salmon abundance in the Columbia River and ocean fisheries. Techniques to artificially rear juvenile salmon had been in use for 60 years when the first federal mainstem dam became operational in 1938, and research on these techniques continues today. Thus, it could take decades to develop successful artificial production techniques for the numerous Mekong species. Even if technically feasible, it is doubtful that the long-term funding and political support needed for stocking programmes would be made available in developing countries on the scale necessary to compensate for the potential loss of wild fisheries resources.
6. Regional efforts to balance multiple river services are needed.
The MRC has convened several workshops to build awareness in the region of the difficulty of developing hydropower production in a way that minimises impacts to fisheries. The MRC is analogous to the in the Columbia River basin, and will likely play a similar role of providing regional leadership on ways to develop environmentally sustainable hydropower resources. Regional bodies such as these are essential for fostering debate and discussion among the various proponents of hydropower development, stakeholders, and scientists and engineers working on mitigation strategies.
7. Columbia River versus other river systems.
While the Columbia River experience provides the ‘lessons learned’ discussed herein, it is important for Mekong River planners to recognise that South American experiences may be a better model to use. South American resource managers and scientists are studying ways to maintain fish populations in the face of hydropower development in large, neotropical rivers. Their challenges (Agostinho et al. 2004; Oldani and Baigun 2008) provide a realistic assessment of just how difficult it is to achieve high levels of fish passage success initially, or retrofit dams to address fish passage in river systems characterised by high species abundance and diversity. The lessons learned from South American systems will greatly inform Mekong development discussions, and have already done so to a limited degree (Dugan 2008).
8. Consider the potential effects of climate change.
Scientists in the Columbia River basin are starting to translate (ie downscale) global climate change scenarios into effects at local scales that can be used to assess potential changes in salmon habitats (Crozier et al 2008). These types of studies have been initiated in the Mekong (MRC 2009) and are needed to understand the effects, and potential for synergistic effects, that dams and altered hydrology may have on fisheries.
In a river system in Brazil, Fernandes et al. (2009) demonstrated that both fish species abundance and richness declined as habitat connectivity and flood duration decreased. Climate change may disrupt hydrological characteristics of the Mekong River. These disruptions may alter key fisheries such as the bagnet fishery in the great Tonle Sap Lake in Cambodia which depends on certain flow regimes (Nguyen et al 2008; Campbell et al. 2009). These considerations further emphasise a need for detailed baseline information and the monitoring of fish resources in the Mekong.
Informed management of water resource development in the Mekong River will require that planners have reliable information on fishery sensitivities to the interacting effects of the major factors potentially affecting the biodiversity of such a large and important river ecosystem: climate, land use and river control structures (dams, obstructions, and diversions).
Incorporating the lessons learned from water resource development in the Columbia River and large South American rivers, and looking ahead to potential changes in the hydrologic regimes and landscape, are important considerations when planning water resources development in the Mekong River.
John W Ferguson, Northwest Fisheries Science Centre, 2725 Montlake Blvd, East Seattle, WA 98112, US. Email: (email@example.com)
The author would like to thank Tim Burnhill of the MRC for developing a map of the mainstem dams proposed in the Mekong basin that was used to develop Figure 1, and the MRC for supplying the photos. Thanks also to the US Army Corps of Engineers for providing the adult salmon count data from Bonneville dam used to develop Figure 3. Finally thank you to James Peacock of NOAA’s Northwest Fisheries Science Center for his assistance in developing and formatting Figures 1, 3 and 4 for publication. Points of views or opinions expressed in this document are those of the author and do not reflect an official view or position of the author’s affiliation.