Date Published: March 28, 2017
Publisher: Public Library of Science
Author(s): Heng Kong, Mathieu Chevalier, Pascal Laffaille, Sovan Lek, Judi Hewitt.
The Tonle Sap Lake (TSL) is a flood-pulse system. It is the largest natural lake in South-East Asia and constitutes one of the largest fisheries over the world, supporting the livelihood of million peoples. Nonetheless, the Mekong River Basin is changing rapidly due to accelerating water infrastructure development (hydropower, irrigation, flood control, and water supply) and climate change, bringing considerable modifications to the annual flood-pulse of the TSL. Such modifications are expected to have strong impacts on fish biodiversity and abundance. This paper aims to characterize the spatio-temporal variations of fish taxonomic composition and to highlights the underlying determinants of these variations. For this purpose, we used data collected from a community catch monitoring program conducted at six sites during 141 weeks, covering two full hydrological cycles. For each week, we estimated beta diversity as the total variance of the site-by-species community matrix and partitioned it into Local Contribution to Beta Diversity (LCBD) and Species Contribution to Beta Diversity (SCBD). We then performed multiple linear regressions to determine whether species richness, species abundances and water level explained the temporal variation in the contribution of site and species to beta diversity. Our results indicate strong temporal variation of beta diversity due to differential contributions of sites and species to the spatial variation of fish taxonomic composition. We further found that the direction, the shape and the relative effect of species richness, abundances and water level on temporal variation in LCBD and SCBD values greatly varied among sites, thus suggesting spatial variation in the processes leading to temporal variation in community composition. Overall, our results suggest that fish taxonomic composition is not homogeneously distributed over space and time and is likely to be impacted in the future if the flood-pulse dynamic of the system is altered by human activities.
Tropical freshwater systems, especially floodplain lakes and rivers, support productive fisheries, providing food and incomes for millions of people worldwide, particularly in the poorest countries [1,2]. In 1990, it was estimated that over 120 million people were involved in fisheries related activities, including capture, processing and sale of fish with 95% of them located in developing countries . In Malawi, fishing activities from Lake Chilwa support about US$18 million annually while Lake Naivasha support an export-oriented agriculture valued at US$ 613–640 million . Likewise, annual fish production from the Tonle Sap Great Lake (TSL) was estimated at 180,000 to 250,000 tons, representing approximately 60% of the total fish production of Cambodia . This fish resource provides food for 14 million people and represents approximately 16% of the Cambodia’s gross domestic product [6,7].
Among the six studied sites and the 141-week samples, 12,455,409 individuals, belonging to 242 species, 123 genera and 49 families were captured (S1 Table). The number of species captured ranged from 2 to 53 while the number of individuals ranged from 9 to 352,594. Species richness and total abundances were both higher in KT relative to the other sites whereas there was a trend toward lower values in KD (Fig 2). In Table 1 we show the means and standard errors of the number of individuals captured within the six sampling sites for the 20 most abundant species.
In this study, we aimed (1) to characterize the temporal variation in the spatial composition of fish communities (i.e. beta diversity) among six sites during 141 weeks, spanning two complete hydrological cycles and (2) to identify the determinants of the temporal variations in the contribution of site and species to spatial variation in community composition. We found that (1) some sites were more unique regarding fish community composition, (2) some species highly contributed to spatial differentiation of fish communities and (3) there is strong temporal variations regarding the contribution of site to beta diversity. The determinants involved in these temporal variations, their contribution and the shape (i.e. linear or quadratic) of the relationship greatly varied among sites, thus reflecting spatial variation in the processes structuring fish communities.
The TSL is the largest inland fisheries in South-east Asia and supports the livelihood of 2.5 million people around the lake . Its flood-pulse dynamic combined to the flow reversal of the TSR make it a unique system worldwide supporting high biodiversity by providing a large diversity of food and habitats for many birds and fishes. However, the growing demand for water for agricultural purposes and the construction of hydro-power dams along the Mekong river  combined to the effect of climate change is strongly threatening this system by altering and reducing flood intensity from 7% to 16% during the rainy season . Such changes in the water regime are likely to have strong impacts on fish community composition by modifying several phenological events  such as the timing of migration or spawning and also by reducing the amount of submerged habitats upon which fish depends for growing and spawning. This may ultimately lead to a decrease in fish productivity and biodiversity. For instance, in 2016, hundred tons of brood-stock fish died within the conservation zone of Boeung Chhmar (which is temporarily connected to the lake) due to a prolonged drought. The strong spatio-temporal variations highlighted regarding the uniqueness of fish communities are likely to be the result of both spatial variation in environmental conditions and the seasonal migration of particular species which occurrence depends on the connectivity to floodplain habitats critical for their reproduction and survival. Promoting the connectivity to floodplain habitats is therefore an important step toward the maintenance of fish biodiversity and productivity upon which millions of people depend for their livelihood.