Spatial and temporal variation in fish community structure and diversity in the largest tropical flood‐pulse system of South‐East Asia

Abstract

The Tonle Sap River and Lake (TSRL) is South-East Asia's largest tropical flood pulse with a flow-reversal system that supports one of the world's largest freshwater fisheries. However, among the world's tropical floodplains, the resources of the TSRL have received little ecological research. Here, we described the spatiotemporal TSRL fish diversity and community variation using daily records from 2012 to 2015 on fish abundance from six sites covering the TSRL system. We found that high fish diversity occurred in sites located in the middle of Tonle Sap Lake, and the lowest diversity was observed in the southern section. The spatial abundance distribution patterns displayed a river-lake gradient, with three fish assemblages that were clustered based on their composition similarities and were characterised by 96 indicator species. In the southern section, fish assemblages were characterised by longitudinal migratory fishes; in contrast, in the middle system, fish assemblages were represented by species with combined ecological attributes (i.e. longitudinal and lateral migratory species and floodplain residents). Towards the northern section, fish assemblages were composed of lateral migratory and floodplain resident species. Species richness and abundance peaked at approximately 2-2.5 and 4 months, respectively, after the peak flow in early October, during which Tonle Sap River resumes its normal flow direction (outflow). This suggests that seasonal flood pulses (i.e. rising and falling water levels) play a pivotal role in structuring spatiotemporal variation in the TSRL fish assemblages. Our study has implications for fisheries monitoring and conservation initiatives.

Influence of Local Habitat and Climatic Factors on the Distribution of Fish Species in the Tonle Sap Lake

“…Similarly, climate change likely affected fish distributions, and as demonstrated here, temperature and precipitation likely affected different species in different manners. However, as indicated in our discussion, our results are still consistent with some recent studies examining the fish distribution patterns, diversity and assemblage structure in the TSL, e.g., [12,19] and elsewhere, such as in the floodplain-lake of the Amazon, where fisheries yields were significantly varied in accordance with the amount of flooded forest habitats [68]. The results presented in this paper could also serve as a reference point to quantify the changes in fish biomass and distributions in the lake.…”

“…were distributed from the central to the southern area of the lake. Our findings are consistent, and indeed strengthen the results of previous studies that found floodplain resident or non-migratory fishes were more abundant in the northern section of the lake [12,33,45], the area that is predominantly covered by flooded forest, swampy and shrub land, aquatic vegetation and rice fields [14]. Those floodplain residents spend most of their lifespans in the flooded forest, inhabiting the lake or swamps during the dry season and migrating to the flooded and open areas of the lake during the wet season for spawning and foraging [13].…”

“…[40,[53][54][55][56]. These species are usually intolerant to low oxygenated water, and need oxygenated water in the open water area of the lake as this area is exposed to wind mixing with water currents [12]. Moreover, these fishes are known to feed in the lake's open area on zooplankton, insect larvae, and crustaceans [37].…”

“…Seasonal mean catches of the study species per fishing lot (for all 33 fishing lots in the TSL) for the entire assessment period is given in the Supporting Information Table S2. These fish species are among the most commercially important species for the Cambodian inland fisheries [34,35] and among the most dominant species in the TSL [12,16,36]. Moreover, the species chosen for this study contained both riverine (longitudinal migratory) species (i.e., P. hypophthalmus, C. enoplos, Gymnostomus spp., B. gonionotus, and Phalacronotus spp.)…”

Mollusc fisheries and length–weight relationship in Tonle Sap flood pulse system, Cambodia

“…During the rainy season, water levels in the TS Lake is highly influenced by floods from the upstream Mekong. A large area of Kampong Chhnang province which is connected to the TS Lake is usually flooded, covering the belt of flooded forests, degraded forests and wetlands surrounding the lake and river (Lieng et al, 1995;Poulsen et al, 2002;Ngor et al, 2018a). These conditions enable the snail species to gain access to breeding and nursing grounds for reproduction and growth, and therefore drive their enormous production during the rainy season.…”