Modelling spatial and temporal dynamics of two small mud carp species in the Tonle Sap flood-pulse ecosystem

The main challenges for ecological studies are the complexity and non-stationarity of data that are difficult to handle using traditional modelling methods. Thus, to address these problems, modern modelling techniques have been developed and introduced for applications. In time-series data, the cross-wavelet transform (CWT) and cross-correlation function (CCF) serve as promising tools to analyze the dynamics of fish populations through time and space. Here, we employed these two well-known time-series modelling approaches to model the spatial and temporal dynamics of small mud carp species (Henicorhynchus lobatus and Henicorhynchus siamensis) and examine the influence of flow pulses on their dynamics using spatial and time-series data collected from the Tonle Sap Lake and River System. The phase angle given by the CWT provided a useful tool for statistically detecting and reconstructing the phase relationship between the two time series of fish data. Moreover, the correlation coefficients at each lag between the water level and fish abundance identified by the CCF provided a mechanism to understand how the flow pulse influenced the dynamics of fish. The results showed that H. lobatus moved out of the floodplain and the lake earlier than H. siamensis. Herein, H. lobatus retreats slower than H. siamensis in the Tonle Sap Lake, but they retreated at similar speeds when they reach the Tonle Sap River. This suggests that the two species respond differently to the out-flow pulse only when they are inside the TSL, indicating the main role of flow pulses in determining the occurrence and movement of fish in the Tonle Sap Ecosystem. We also discussed the role and applications of these modelling approaches for linking the changes in environmental parameters and fish. The CWT and CCF, based on our results, are helpful modelling approaches for analyzing time-series data to understand the phase relationship and the periodicity of synchronisation (joint periodicity), i.e. the role of environmental factors in shaping fish occurrence and movement in a flood-pulse ecosystem.