Wetland, a natural capital, occupies 6% of world’s geographical land (Mitsch et al., 2013). It provides different provisional, regulating, habitat and cultural services of socio-ecological importance (Wang et al., 2020). Wetland serves as one of the major means of livelihood for developing nations like India, Bangladesh where a huge part of population is highly dependent on primary activities like fishing, agriculture, dairy farming, cattle ranching, etc. Since fish is the most demanding provisioning service found in wetlands, fishermen communities are dependent on these. However, wetland conversion at a fast rate is threatening for their livelihood. Among various factors, agricultural expansion in wetland areas is a major threat for wetland transformation (Saha and Pal, 2019). Hydrological modification through damming plays pivotal role in alteration of wetland characters (Dudgeon, 2000). Attenuation of water depth has occurred due to water diversion through canals, over lifting of water for various purposes. Reduction of flow in downstream is responsible for causing hydrological drought in rivers (Wen et al., 2011; Araujo et al., 2016). Gain and Giupponi (2014) has figured out flow reduction by 52% in Ganga River due to construction of Farakka Barrage, Talukdar and Pal (2018) has found reduction in Punarbhava river by 41%, Uday Kumar and Jayakumar (2019) has found out the reduction of flow by 30% in Krishna River. Flow reduction causes losing of tie channels for riparian wetlands which is responsible for depletion of water availability (Pal and Sarda, 2020). Eco-hydrological deficit is acute in some wetlands of Barind tract of Indo-Bangladesh (Saha and Pal, 2019); Talukdar and Pal, 2020).
A specific environmental gradient of some elements are necessary for fish survival; each component has different effects on the survival of fishes to which fish can grow at different levels of coziness. Among several essential hydro-ecological components, water depth is a major one which affects fish survival crucially (Hosen et al., 2019). Certain range of depth is required for economic growth for different fish species; this is mainly because of food availability and environmental suitability (Baras and Lucas, 2001; Mouton et al., 2007). MPEDA & NACA Manual (2015); Hosen et al (2019) have estimated the suitable depth of survival of some fishes. For instances, Hosen et al (2019) has identified depth of water between 1.2 to 2.8 m is suitable for carp fishes (Labeo Rohita, Gibelion Catla, Cirrhinus Mrigala), 1 to 2 m for Tilapia fish, 1 to 3 m for Shrimp fish, more than 2.14 m for catfishes (MPEDA & NACA Manual, 2015) etc. Hydrological modification is directly linked with water availability reduction and depletion of water depth which in turn affects fish storage and declining quality of fish habitability. As huge proportion of people is dependent on fishing, uncertainty for livelihood has been arising among fishermen due to hydro-ecological alteration. So this is a matter of great concern. Therefore, the present paper has tried to assess impact of flow reduction on hydrological elements and how their changes can influence fish habitat for some particular fishes in the studied region.
For the fulfillment of aforesaid objectives it is of immense necessity to model the alteration patterns of hydrological regimes for e.g. depth of water, wetland size, water presence consistency, hydro-duration in the concerned wetland. To find out suitable fish habitability these parameters are very important, but field data collection from such wider wetland tract is a major challenge. Consistency of water presence in seasonal wetlands of the Barind tract has been extracted through satellite imageries data by few previous researchers (Debanshi and Pal, 2020; Khatun et al., 2021). Water depth has been extracted by using indices like normalized difference water index (NDWI) by field depth calibration (Debanshi and Pal, 2020). Due to damming inconsistency of water presence has increased; water depth has reduced at a wide extent in this type of seasonal wetland. Beside these, hydro-period is an important hydrological determinant of fish habitability since patterns of wetlands are strongly connected with fish phenology. Hydro-duration is affected by damming in a negative way which affects availability of fish species and their productivity. It is quite cumbersome to mapping hydro-period by field investigation in such a wider area of wetland. So in this study a new approach has been adopted to model hydro-period by using multi-date image data in each year. By combining all these three hydrological parameters, fish habitat suitability can be assessed in a precise way.
Habitat suitability modeling has been applied by various scholars worldwide with different approaches. Among the several habitat suitability model, physical habitat simulation model (Boove, 1982) is worldwide applied model to modeling fish habitability by using water depth and flow velocity as parameters. Main function of this model is to link up different physical variables of flow regime to water richness by bi-variate or multi-variate statistics. Mouton et al. (2006) applied this model in Zwalm river in Belgium. Another popular method is Habitat Suitability Index which was developed in early 1980s. The index is calculated on the basis of several parameters like availability of foods, natural fish behavior like spawning, seasonal behavioral pattern, flood inundation areas. The index is calculated using HABITAT modeling in MATLAB toolbox by considering water depth and flow velocity in Abras de Mantiquilla wetland in Ecuador (Mieles et al, 2019). Similarly, HSI (Habitat Suitability Index) was calculated for Jinsha river basin in upper reaches of Yangtze river (Zhang et al, 2018). Apart from these, a widely used approach is fuzzy logic based habitat suitability index. By taking water depth, flow velocity and dominant substrate fuzzy logic was set and applied in Zwalm river, Belgium (Mouton et al, 2006) for various riverine fishes. But from various literatures it has been found that most of the fish habitat modeling is done on riverine ecosystems. Very few studies have been documented about fish habitat suitability measurement in wetland across the world. Novelty of the work lies in the fact that no works has been done yet on wetland so far based on only hydrological components like hydro-period, satellite driven WPF, calibrated water depth. In this work both individual parameter and combine parameter specific habitat suitability analysis has been done. However, it is very unique to apply rule based decision tree to integrate suitable fish habitat modeling by comprising only hydrological parameters. As Punarbhaba is highly modified hydrologically due to damming and large portions of it belongs to floodplain wetlands, it has been taken as a study area.