Rivers provide unrivaled opportunity for clean energy via hydropower, but little is known about the potential impact of dam-building on the food security these rivers provide. In tropical rivers, rainfall drives a periodic flood pulse fueling fish production and delivering nutrition to more than 150 million people worldwide. Hydropower will modulate this flood pulse, thereby threatening food security. We identified variance components of the Mekong River flood pulse that predict yield in one of the largest freshwater fisheries in the world. We used these variance components to design an algorithm for a managed hydrograph to explore future yields. This algorithm mimics attributes of discharge variance that drive fishery yield: prolonged low flows followed by a short flood pulse. Designed flows increased yield by a factor of 3.7 relative to historical hydrology. Managing desired components of discharge variance will lead to greater efficiency in the Lower Mekong Basin food system.

Abstract. Species' responses to seasonal environmental variation can influence trophic interactions and food web structure within an ecosystem. However, our ability to predict how species' interactions will vary spatially and temporally in response to seasonal variation unfortunately remains inadequate within most ecosystems. Fish assemblages in the Tonle Sap Lake (TSL) of Cambodia-a dynamic flood-pulse ecosystem -were studied for five years (2010-2014) using stable isotope and Bayesian statistical approaches to explore both within-and among-species isotopic niche variation associated with seasonal flooding. Roughly 600 individual fish specimens were collected during 19 sampling events within the lake. We found that fishes within the same species tended to have a broader isotopic niche during the wet season, likely reflecting assimilation of resources from either a wider range of isotopically distinct prey items or a variety of habitats, or both. Furthermore, among-species isotopic niches tended to overlap and range more broadly during the wet season, suggesting that floodplain inundation promotes exploitation of more diverse and similar resources by different species in the fish community. Our study highlights that the flood-pulse dynamic that is typical of tropical aquatic ecosystems may be an essential element supporting freshwater fish community structure and the fish diversity that underpins the TSL food web. This flow regime is currently threatened by regional dam development, which may in turn impact the natural function and structure of the fishery food web.

SUMMARYThe Tonle Sap is the largest wetland in Southeast Asia and the heart of the largest inland fishery in the world. Its unique flood pulse system and annual flow reversal is a hotspot for biodiversity and productivity, as well as an essential habitat for many endangered fishes and birds. Despite predicted changes to the wetland's hydrology due to climate change and hydropower development in the Mekong, the consequent impacts on the fauna of the lake are poorly understood. A spatial modelling framework was developed to simulate the impact of potential scenarios of change using relationships between fauna and biophysical characteristics. Potential impacts on 61 animal species with documented nutritional, conservation or ecological value were examined. A large number of species rely on gallery forest to provide important habitats for their life history, yet this area is likely to be highly impacted by permanent inundation. There is a strong synchronicity between life histories and the flood pulse; consequently continued hydrological disruptions will have a significant impact on ecosystem dynamics, imposing further challenges to conservation. Protecting areas that may become suitable for gallery forests and shrublands under a modified flood regime will be crucial to management planning and the maintenance of a diverse and healthy ecosystem.

Inland fish provide food for billions and livelihoods for millions of people worldwide and are integral to effective freshwater ecosystem function, yet the recognition of these services is notably absent in development discussions and policies, such as the United Nations Sustainable Development Goals (SDGs). How might the SDGs be enhanced if inland fishery services were integrated into policies and development schemes? Here, we examine the relationships between inland fish, sustainable fisheries, and functioning freshwater systems and the targets of the SDGs. Our goal is to highlight synergies across the SDGs, particularly No Poverty (SDG 1), Zero Hunger (SDG 2), Clean Water and Sanitation (SDG 6), Responsible Consumption and Production (SDG 12), and Life on Land (SDG 15), that can be achieved with the inclusion of these overlooked inland fishery services. The United Nations 2030 Agenda for Sustainable Development establishes a unified set of global aspirations [i.e., the Sustainable Development Goals (SDGs); see Figure 1 inset] that provide a roadmap for future prosperity by addressing key challenges including world poverty, hunger, disease, and illiteracy 1. The 169 targets, across 17 SDGs, measured by 232 unique indicators, are ambitious and complex. Interdependencies among targets and the systems that contribute towards them mean that even the most effective efforts to address one global challenge may unintentionally exacerbate others if the efforts overlook potential wider impacts 2-4. Identifying and collecting official global statistics to track progress toward each indicator is an additional challenge; data exists for some indicators, while there are significant deficits for others (see https://ourworldindata.org/sdg-tracker-update). Achieving the holistic vision of the SDGs requires coordination at multiple scales and among sectors 5 , as well as inclusivity of services that * We use the general term "fish" in colloquial reference but, in most instances, "fishes" is more technically accurate as many inland fishery services involve multiple species.

who provided valuable feedback on elements of this document during a workshop (March 2018, The Dolomites World Heritage Site, Italy). IUCN also thanks UN Environment World Conservation Monitoring Centre for their contribution to this project.

The effects of environmental seasonality on food web structure have been notoriously understudied in empirical ecology. Here, we focus on seasonal changes in one key attribute of a food web, consumer trophic position. We ask whether fishes inhabiting tropical river–floodplain ecosystems behave as seasonal omnivores, by shifting their trophic positions in relation to the annual flood pulse, or whether they feed at the same trophic position all year, as much empirical work implicitly assumes. Using dietary data from the Tonle Sap Lake, Cambodia, and a literature review, we find evidence that some fishes, especially small piscivores, increased consumption of invertebrates and/or plant material during the wet season, as predicted. However, nitrogen stable isotope (δ15N) data for 26 Tonle Sap fishes, spanning a broader range of functional groups, uncovered high variation in seasonal trophic position responses among species (0 to ±0.52 trophic positions). Based on these findings, species respond to the flood pulse differently. Diverse behavioral responses to seasonality, underpinned by spatiotemporal variation at multiple scales, could be central for rerouting matter and energy flow in these dynamic ecosystems. Seasonally flexible foraging behaviors warrant further study given their potential influence on food web dynamics in a range of fluctuating environments.

The ecosystem services provided by freshwater biodiversity are threatened by development and environmental and climate change in the Anthropocene. Here, case studies are described to show that a focus on the shared dependence on freshwater ecosystem functioning can mutually benefit fisheries and conservation agendas in the Anthropocene. Meeting the threat to fish biodiversity and fisher livelihood is pertinent in developing regions where there is often a convergence between high biodiversity, high dependency on aquatic biota and rapid economic development (see Kafue River, Logone floodplain, Tonle Sap, and Rio Negro case studies). These case studies serve as evidence that biodiversity conservation goals can be achieved by emphasizing a sustainable fisheries agenda with partnerships, shared knowledge and innovation in fisheries management (see Kafue River and Kenai River case studies). In all case studies, aquatic biodiversity conservation and fisheries agendas are better served if efforts focused on creating synergies between fishing activities with ecosystem functioning yield long‐term livelihood and food security narratives. A unified voice from conservation and fisheries communities has more socio‐economic and political capital to advocate for biodiversity and social interests in freshwater governance decisions.

Tropical fisheries are among the most productive fisheries in the world, often providing the primary source of protein for the local population. Despite their importance, data on these systems are relatively limited, thus hampering management and policy development. Here, the implications of increasing fishing pressure are explored by critically evaluating the perceptions of the fishers who rely on these ecosystems to survive. A total of 169 fishers in 26 different fish‐dependent communities in the Tonlé Sap Lake, Cambodia, were surveyed to understand their perceptions of the impact that fishing has had on the ecosystem. The Tonlé Sap is one of the largest, yet poorest studied, freshwater fisheries in the world. Consistent with “fishing down the food web” theory of fisheries, survey data revealed that although fishers observed the total size of fish catch remaining consistent over recent years there has been a drastic decline in the size of individual fish, as well as a reduction in the diversity of species caught. These perceptions are examined with reference to food web theories that explore how fishing pressure leads to ecosystem change, including the more recent “indiscriminate fisheries” theory.