The past decade has seen increased international recognition of the importance of the services provided by natural ecosystems. It is unclear however whether such international awareness will lead to improved environmental management in many regions. We explore this issue by examining the specific case of fish migration and dams on the Mekong river. We determine that dams on the Mekong mainstem and major tributaries will have a major impact on the basin's fisheries and the people who depend upon them for food and income. We find no evidence that current moves towards dam construction will stop, and consider two scenarios for the future of the fisheries and other ecosystems of the basin. We conclude that major investment is required in innovative technology to reduce the loss of ecosystem services, and alternative livelihood strategies to cope with the losses that do occur.
Golden perch Macquaria ambigua (Percichthyidae) and silver perch Bidyanus bidyanus (Terapontidae) are two potamodromous fish species of the Murray-Darling river system in southeastern Australia. Ageing of these species using thin sections of the sagittal otoliths and validation with known-age fish revealed: they live for over 26 years; male and female silver perch reach maturity at 3 and 5 years respectively; male and female golden perch reach maturity at 2 and 4 years respectively; both species exhibit sexual dimorphism with larger females; and growth varies (L ∞ silver perch 331-397 mm, golden perch 354-502 mm) among interconnected river systems. Longevity and opportunistic growth are characteristics that are well suited to the semi-arid and temperate hydrology of this river system. A flood-recruitment model for these two species, consistent with the 'flood-pulse concept', has previously been assumed to be the main mechanism of recruitment. The model appeared appropriate for this large, low-gradient river system with productive floodplains. However, in the middle reaches of the Murray River we found that golden perch recruitment was strong in non-flood years and poor in flood years, and silver perch recruited in all years. These data do not preclude golden perch recruiting during floods as well, because downstream larval drift may have resulted in strong year-classes being swept downstream of the sampling area during high flows. However, the recruitment models for these species need to be re-evaluated to include within-channel flows. Importantly, these flows can be manipulated by river regulation, unlike large floods, and therefore there is potential to enhance recruitment.
Salmonid fishways have been used in many countries for non-salmonid fishes, including Australia, but generally with poor results. Trapping the entrance and exit of a 1:9 gradient salmonid fishway on the Murray River confirmed very poor passage of native fish, with <1% of the most abundant species ascending. Fifty years of fish passage monitoring showed the numbers of three native species declining by 95-100% and non-native fish becoming dominant. Fishways are now being designed for native fish and being quantitatively assessed, but daily flow management also needs to be addressed. The ecological model for passage of potamodromous fishes has changed from passing adults of a few species to one that incorporates the whole fish community, specifically: immature fish of large-bodied species that dominate numbers migrating upstream; a diverse range of movement strategies; and small-bodied species, crustaceans and low numbers of less-mobile species.
Freshwater fishes are one of the most imperiled groups of vertebrates, and population declines are alarming in terms of biodiversity and to communities that rely on fisheries for their livelihood and nutrition. One activity associated with declines in freshwater fish populations is water resource development, including dams, weirs, and hydropower facilities. Fish passing through irrigation and hydro infrastructures during downstream migration experience a rapid decrease in pressure, which can lead to injuries (barotrauma) that contribute to mortality. There is renewed initiative to expand hydropower and irrigation infrastructure to improve water security and increase low‐carbon energy generation. The impact of barotrauma on fish must be understood and mitigated to ensure that development is sustainable for fisheries. This will involve taking steps to expand the knowledge of barotrauma‐related injury from its current focus, mainly on seaward‐migrating juvenile salmonids of the Pacific Northwest, to incorporate a greater diversity of fish species and life stages from many parts of the world. This article summarizes research that has examined barotrauma during fish passage and articulates a research framework to promote a standardized, global approach. The suggested approach provides clearly defined links to adaptive development of fish friendly technologies, aimed at mitigating the threats faced by global freshwater fisheries from the rapid expansion of water infrastructure.
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