Conservation Priorities for Threatened Fish to Withstand Climate Crisis: Sustainable Capture and Protection of Inland Hydrographic Ecosystems

Between the surface aquatic habitats, the total area of fresh waterbodies (0.77% of 1,386,000,000 km³ water) is much smaller than the vast marine water [1,2]. Freshwater bodies consist of an ‘open or lotic water body’ like rivers and canals and a ‘close or lentic water body’ like lakes and ponds. Although these small habitats accommodate a great diversity of species ranging from Protozoan to Chordates [3], they are facing a dramatic decrease in suitability, alteration, and fragmentation due to the rapid changes in climate and anthropogenic land use [4,5]. While habitats are unsuitable and disconnected, particular natural selection pressures significantly contribute to the decline of freshwater fish communities. For instance, the limiting factors of the habitat [6,7], species’ tolerance and adaptation mechanisms weakening [8], limitations in resource allocation [9], unstable population structures [10], pollution, and susceptibility to disease [8,11,12,13] are highly contributory selection pressures in declining species. In comparison to other Chordates, the decline in freshwater fish species often goes unnoticed and this causes severe consequences on threatened species. A comprehensive analysis of the status and distribution of suitable and accessible existing habitats, coupled with an examination of the population dynamics of these threatened taxa across a broad spectrum of climate and habitat variables, can provide the essential information needed by conservation policymakers.

During the last Red List assessment, there were five parameters employed to determine the conservation status of species; those parameters are (A) population size reduction, (B) the geographic range in the form of either the extent of occurrence or area of occupancy, (C) a small population size and decline, (D) a very small and restricted population, and (E) quantitative analysis [14]. Due to the absence of corresponding information from many countries, the assessment ended up mostly using the area of occupancy (AOO) and extent of occurrence (EOO) in parameter B and in the sub-category of parameter A, while other parameters were noted as ‘unknown’ or ‘not assessed’. Moreover, AOO and EOO referred to the aquatic areas where the species were recorded to occur, but did not indicate that the habitat is climatically and ecologically suitable for the species. It is noteworthy that the AOO and EOO of inland waterbodies, both lotic and lentic, are rapidly decreasing due to the anthropogenic land use activities in many developing countries [15]. Thus, the degree of changes in the existing area of occupancy should be analyzed thoroughly to understand the dynamics of the occupancy and distribution.

Climate change is considered a key process when determining the distribution of a suitable climate space for a species [16]. Since freshwater fishes are ectothermic, they are anticipated to be greatly affected by the lack of a climatically suitable habitat [17]. The lentic habitats, small lakes, and ponds in tropical countries are often perennial, and the drying out of those waterbodies is usually prolonged due to climate alteration [18]. In addition to the habitat connectivity, protected areas are often established to safeguard various Chordates from threats, but such conservation efforts for freshwater fishes are not extensive. Identifying climatically suitable areas for further establishing new protected areas and a ‘coldspot’ for those threatened fish species may increase the compatibility of conservation strategies [19]. Studying the shape, size, and geographical distribution of the suitable climate spaces over hydrographic networks (river, lakes, and floodplains) and protected areas may provide baseline information about the dynamics of the habitat suitability [20] and may help when designing conservation strategies.

In addition, non-sustainable fish harvesting practices from inland habitats have a great impact on rapid population decline in freshwater fish species [21]. In many countries like Bangladesh, freshwater fish is the main protein source for human consumers, though the capture of fishes from open waterbodies is poorly monitored [22]. Also, the rapid growth in fish culture industries is transforming the small-to-medium-sized inland freshwater bodies into controlled culture ponds for farming. This transformation is reducing the accessibility to those controlled habitats for native fish species [23]. Therefore, overharvesting, continuous reduction of the available habitat, and the impact of hybrid and exotic fish might have a harmful impact on the available habitat and stable population of the native threatened fish community. Analyzing the harvest from open and closed waterbodies may provide information about the population dynamics of freshwater fish for conservation ideas.

Bangladesh is a low-elevation country consisting of a massive network of more than 400 rivers accompanied by various lentic freshwater bodies like haors, baors, beels, and lakes [24]. Being situated in between Indo-Himalayan and Indo-Burmese hotspots, the country has diverse ecological structures with wide floodplains, 12 bio-ecological zones [25], and 7 climatic sub-regions [26]. This ecologically diversified country is home to about 253 freshwater fishes, of which more than 25% species were declared as threatened by IUCN Bangladesh [27] (Supplementary Table S1). This assessment has pointed out that habitat loss, overexploitation, and the culture of exotic fish are the key threats to the threatened fish species in Bangladesh [5,27]. In addition, this country is predicted to face a huge climate change impact in the future, where the existing AOO and EOO may lose their climate suitability [28]. While assessing the regional conservation status, the availability and climatically suitable habitat and population status of the threatened freshwater fish should be taken into consideration for the next assessment.

The threatened freshwater fish species in Bangladesh are expected to experience a significant reduction in climatically suitable habitats within the existing hydrographic networks of the country in the near future. Fish species at high risk like Awaous grammepomus, Barilius tileo, Labeo boga, Neolissochilus hexagonolepis, and Batasio tengana may face extinction due to the lack of suitable habitats in the current aquatic environments in Bangladesh. The species distribution model indicates a potential shift in suitable habitat distribution towards the hill tracts, suggesting a migration of suitable climate zones from the plains to higher elevations to accommodate temperature changes [8,38]. In the anticipated timeframe, it is suggested that existing high-altitude freshwater bodies, like the Kaptai Lake, the largest man-made lake in Bangladesh, could serve as alternative habitats to safeguard these threatened species in future climate scenarios. However, the physical barrier posed by hilly terrains may limit the migration of fish to these elevated suitable habitats [39,40,41]. Simultaneously, the quality of freshwater fish habitats in the plains is gradually declining due to rising surface water temperatures, shifts in precipitation patterns [42,43], and anthropogenic interference [44,45]. The proper conservation and management of high-elevated water bodies and freshwater bodies in plains may facilitate the future protection of threatened fish.

Most of the rivers in Bangladesh originate from the Indo-Himalayan mountain and flow through Bangladesh towards the Bay of Bengal [46]. Rivers constitute the primary elements of the hydrographic network, connecting through numerous small-to-large canals [47], and are suitable for 32 species at present, but may be suitable for only 15 species in future climate scenarios. These waterways undergo temporary disconnection during the dry season and reconnect during the wet season [48]. However, these lotic habitats are experiencing shallowing, narrowing, and fragmentation due to siltation [49] and unplanned development activities [18]. In addition, small ponds and lakes are rapidly transforming into controlled fish farming ponds. This transformation further diminishes the availability of freshwater habitats for native fish species. Therefore, it is imperative to maintain connections within both lotic and lentic components to ensure the protection of threatened freshwater fish in Bangladesh.

The heavy rainfall in the monsoon causes floods in low-elevated countries like Bangladesh, and that floodwater connects the lentic and lotic water bodies for a few months [50]. In the current climate scenario, floodwater appears to be one of the noteworthy habitats for freshwater fishes and has also been found to be highly suitable for threatened fishes in the future. Most of the native fishes depend on seasonal flooding for spawning cues and larval development [51]. Brood fishes move from deep to shallow waters for their reproduction during the monsoon, and flood-connected hydrographic networks provide travel ways for their breeding migration [52,53], allowing the population to mix up [50]. Floodplains and their ecological association with freshwater fishes have long been acknowledged [54] for their reinforcement in reproduction and food [55,56]. Around 80% of the surfaces of Bangladesh are considered floodplains [57,58] and 25–33% (maximum 57%) of the entire country [59] remains submerged for four-to-six months as a result of heavy monsoon rains and snow melt in the mountains [50,60]. Although inundation depth and duration vary spatially, but temporally, depending on the relief and soil type [61], flood flow increases the distribution of freshwater fish species around the floodplains to claim more suitable habitats [62,63]. To maintain the dispersal ways of threatened fish species over a suitable habitat, the establishment of inland water bodies and the management of seasonal flooded areas may play a vital role.

There are some small, seasonal, and temporary protected areas, known as fish sanctuaries, which are not providing support for long-term conservation [64]. Meanwhile, none of the permanent protected areas dedicated for wildlife conservation in Bangladesh are for fish [65]. Moreover, aquatic bodies in those protected areas provide a present suitable habitat for threatened fish species; however, there will be no suitable habitats in the future. This clearly indicates the necessity of permanent protected areas dedicated to freshwater fish conservation [66]. We strongly suggest the establishment and conservation of available aquatic habitats and the creation of new water bodies in predicted suitable climate spaces to interconnect the hydrographic networks and to expand and sustain their existence in the altered climate conditions.

Despite the habitat viability, the sustainable harvest of fishery resources depends on the management of fishing pressure to be maintained at the minimum level [67]. Our analyses revealed that uncontrolled and unsupervised fish capture from open water, particularly from rivers and floodwaters, exceeds fish harvest from any other sources by multiple times. The capture of broods, fries, and migrating fish during spawning season induce pressure on the communities of threatened fish [68,69]. Therefore, inadequate knowledge about threatened species among the native fishermen leads to an unsustainable harvest, driving threatened fish towards extinction [67]. In Bangladesh, in the recent past, the amount of fish captures has been increasing in most fish habitats in the open inland waters. Nevertheless, the conversion of ecologically complex aquatic ecosystems into monotypic fish farming ponds for fish farming ruins opportunities for habitat choice and invites inconceivable threats for threatened species [23]. Moreover, some exotic fish have recently become popular in the fish farming industry, with potential threats to them in the wild [70]. There should be a concise and outlined guideline for fishermen mentioning the fishing species, identity of threatened fish, harvesting areas, fishing time and duration, and types of fishing trawls to raise public consciousness.

Because the available climate niches in the rivers and floodplains are decreasing, they are not legally protected, and habitats in the hill tracts are not easily accessible, about half of the studied threatened fish species have been re-evaluated as belonging to the high-risk-of-extinction group. Among them, only 12 species (CR: Tila Shol, one variety of Mohashol, Bhangon Bata, Nandi, Baghair, and Chenua; EN: another type of Mohashol, Ghora Muikha, Rita, Bhool, Joya, and Rani) are declared as protected in Bangladesh under the Wildlife Act 2012, but no conservation policy has been developed for other high-risk species. Despite their economic importance for nutritional demands, ecological significance in the trophic equilibrium, and ornamental value in aquarium culture [71], freshwater fish are mostly overlooked by conservationists. Considering the potential impact of climate change, legal protection for threatened species, regular monitoring, a schedule awareness program, and the implementation of laws can be instrumental for the protection of threatened fish.