Tilapia Farming in Bangladesh: Adaptation to Climate Change

: In Bangladesh, aquaculture is critically important in terms of providing food and nutrition, sustainable livelihoods, income, and export earnings. Nevertheless, aquaculture in Bangladesh has faced recent concerns due to climate change. Aquaculture is vulnerable to a combination of climatic factors, such as global warming, rainfall variation, ﬂood, drought, temperature ﬂuctuation, and salinity change. Considering the vulnerability of ﬁsh production to the impacts of climate change, tilapia farming is one of the possible strategies for adaptation to climate change. The positive culture attributes of tilapia are their tolerance to low water levels and poor water quality with rainfall variation, temperature ﬂuctuation, and salinity change. In fact, tilapia farming is possible in a wide range of water environments, including freshwater, brackish water, and saltwater conditions. We suggest that appropriate tilapia culture strategies with institutional support and collaboration with key stakeholders are needed for adaptation to environmental change.


Introduction
Aquaculture is the world's fastest growing food producing sector, with a mean annual growth rate of 5.3% during 2001-2018 [1]. Global aquaculture production achieved 82.1 million tons in 2018, of which inland freshwater aquaculture reached 51.3 million tons (62%), while coastal brackish water aquaculture and mariculture (aquaculture is generally practiced in three different types of locations and aquatic environments: (1) inland freshwater aquaculture; (2) coastal brackish water aquaculture; and (3) onshore and offshore marine aquaculture) yielded 30.8 million tons (38%) [1]. Asia led world fish production with a contribution of 89% in 2018. Globally, China is positioned first (58% of total production) among fish producing countries, followed by India, Indonesia, Vietnam, Bangladesh, Egypt, Norway, Chile, Myanmar, and Thailand [1].
Bangladesh is recognized one of the world's most suitable countries for aquaculture practices, because of its promising agroclimatic conditions and favorable biophysical resources. A vast area of shallow water with sub-tropical climate provides perfect situations for aquaculture production. In Bangladesh, both fish production and catch were estimated to be 4.38 million tons in the 2018-2019 fiscal year (Bangladeshi fiscal year: 1 July-30 June), of which 2.49 million tons (57%) were yielded from aquaculture, 1.23 million tons (28%) from inland capture fisheries, and 0.66 million tons (15%) from marine fisheries [2]. The major farming practices for aquaculture in Bangladesh are extensive, semi-intensive, and intensive polyculture and/or monoculture of Indian major carp, exotic carp, catfish, prawn, tilapia, and shrimp.
Aquaculture plays a critical role in the economy of Bangladesh, contributing to food, nutrition, sustainable livelihoods, the income of farming households and associated groups, and export earnings. Fish accounts for 60% of nationwide animal protein consumption.

History of Tilapia Farming
As a first exotic piscine, the Mozambique tilapia (Oreochromis mossambicus) was introduced to Bangladesh from Thailand in 1954 [33]. However, the acceptance level of this species was not satisfactory to the farmers because of its early maturation and frequent spawning behavior that leading to overcrowding and slow growth rates [34]. After two decades, in 1974, UNICEF (United Nations International Children's Emergency Fund) introduced the Chitralada strain of Nile tilapia (O. niloticus) to overcome these constrains [35], because of its hardy nature, faster growth, and high disease resistance [36]. Nonetheless, Nile tilapia also struggled as a culture species due to a lack of knowledge about management practices and biology, and thus, cannot attract farmers who have a strong affinity to carp culture, which is very popular in Bangladesh. A continued attempt was made to make tilapia popular through importing red tilapia (O. mossambicus × O. niloticus) from Thailand. Afterwards, the Bangladesh Fisheries Research Institute (BFRI) reintroduced Nile tilapia in 1987 and red tilapia in 1988 from Thailand [34]. The WorldFish Center, previously known as the International Center for Living Aquatic Resources Management (ICLARM) developed a synthetic strain of O. niloticus that is popularly known as Genetically Improved Farmed Tilapia (GIFT), which was further improved by the BFRI and introduced to Bangladesh in 1994 [37,38]. Moreover, the GIFT strain showed significantly better performance compared to other tilapia in numerous aspects [39,40]. Subsequent research was carried out to produce all male sex-reversed GIFT to avoid prolific breeding, commonly known as monosex tilapia [41].
After gradational introduction, tilapia has been becoming popular among fish farmers in Bangladesh. Tilapia aquaculture plays a vital role in fish production, distribution, marketing, and consumption, which contributes to the value adding process, and greater role in increasing food supply [42]. Tilapia is now the third most important fish species after pangas and rohu in Bangladesh (Figure 1). Due to its significant contribution to food After gradational introduction, tilapia has been becoming popular among fish farmers in Bangladesh. Tilapia aquaculture plays a vital role in fish production, distribution, marketing, and consumption, which contributes to the value adding process, and greater role in increasing food supply [42]. Tilapia is now the third most important fish species after pangas and rohu in Bangladesh (Figure 1). Due to its significant contribution to food production, tilapia is commonly known as "aquatic chicken" [43]. In fact, tilapia can play a major role as a source of low-priced animal protein in the same way as chicken. Tilapia has also been called as the "everyman's fish" [44].

Tilapia Culture in Freshwater Systems
Tilapia is well recognized as a suitable species for freshwater aquaculture systems in Bangladesh. The culture duration of tilapia is usually few months from April/May to November/December to allow more than one culture cycle [45]. Tilapia culture is mainly season based, starting prior to the summer and lasting until winter. As tilapia farming is completely dependent on hatchery-produced fry, culture season, stocking density, and culture duration are important factors. Tilapia can be cultured in small-scale, large-scale, low-input, or high-input operations in inland waterbodies. Tilapia culture can be classified into polyculture, monoculture, and integrated farming. Based on the level of input, tilapia culture can also be categorized as extensive, semi-intensive, and intensive.
Tilapia pond polyculture with other species (mostly carp) has been traditionally practiced in rural Bangladesh. Polyculture is usually practiced with a low or medium level of input under extensive or semi-intensive culture systems. Polyculture is based on the proper utilization of different ecological niches of a pond to obtain maximum yield per unit area [46]. Tilapia feed on organic particles available on the pond bottom, which are re-suspended by the sediment tabulation of carp species in the polyculture system. In this manner, tilapia can prevent the lifting of the organic load in the sediment, which in turn provides anaerobic conditions [47]. The stocking density of tilapia in polyculture system may vary depending on the culture species, natural food availability, and supplementary diet. For example, the inclusion of Nile tilapia at 2000/ha in a polyculture of bottom dweller common carp (Cyprinus carpio) and phytoplankton filter-feeding rohu (Labeo rohita), stocked at 5000 and 15,000/ha, increased the nutrient concentrations in ponds, reduced phytoplankton biomass and suspended solids, which resulted in additional fish production without affecting other cultured species [48]. Nile tilapia with a stocking density of 2200/ha in a polyculture system including filter-feeders 1000/ha of

Tilapia Culture in Freshwater Systems
Tilapia is well recognized as a suitable species for freshwater aquaculture systems in Bangladesh. The culture duration of tilapia is usually few months from April/May to November/December to allow more than one culture cycle [45]. Tilapia culture is mainly season based, starting prior to the summer and lasting until winter. As tilapia farming is completely dependent on hatchery-produced fry, culture season, stocking density, and culture duration are important factors. Tilapia can be cultured in small-scale, large-scale, low-input, or high-input operations in inland waterbodies. Tilapia culture can be classified into polyculture, monoculture, and integrated farming. Based on the level of input, tilapia culture can also be categorized as extensive, semi-intensive, and intensive.
Tilapia pond polyculture with other species (mostly carp) has been traditionally practiced in rural Bangladesh. Polyculture is usually practiced with a low or medium level of input under extensive or semi-intensive culture systems. Polyculture is based on the proper utilization of different ecological niches of a pond to obtain maximum yield per unit area [46]. Tilapia feed on organic particles available on the pond bottom, which are re-suspended by the sediment tabulation of carp species in the polyculture system. In this manner, tilapia can prevent the lifting of the organic load in the sediment, which in turn provides anaerobic conditions [47]. The stocking density of tilapia in polyculture system may vary depending on the culture species, natural food availability, and supplementary diet. For example, the inclusion of Nile tilapia at 2000/ha in a polyculture of bottom dweller common carp (Cyprinus carpio) and phytoplankton filter-feeding rohu (Labeo rohita), stocked at 5000 and 15,000/ha, increased the nutrient concentrations in ponds, reduced phytoplankton biomass and suspended solids, which resulted in additional fish production without affecting other cultured species [48]. Nile tilapia with a stocking density of 2200/ha in a polyculture system including filter-feeders 1000/ha of catla (Catla catla), 3000/ha of rohu (L. rohita), and 3000/ha of silver carp (Hypophthalmichthys molitrix) with 4000/ha of freshwater prawn (Macrobrachium rosenbergii) and 10,000/ha of mola (Amblypharyngodon mola) increased the production and economic benefits [49]. The stocking density of tilapia at 10,000/ha in polyculture with prawn was also found to be suitable for the growth of both tilapia and prawn [50,51]. In a periphyton-based polyculture system, a stocking ratio of 75% GIFT with 25% prawn at a total stocking of 20,000/ha resulted in suitable fish production and economic returns [52]. Tilapia with prawn in polyculture system supplemented with 30% of protein also showed better performance [53].
On the contrary, monoculture allows single species tilapia production with high stocking density with high level of feed inputs, and thus, higher yield than polyculture. Although tilapia is a suitable species for polyculture due to its uninterrupted parallel production with other species, in recent times in Bangladesh have preferred to farm tilapia in monoculture. Higher growth with high stocking density under intensive or semi-intensive farming is the main key point for tilapia monoculture. A stocking density (80-200/decimal or 20,000-50,000/ha) of tilapia in monoculture can be 10-20 times higher than polyculture [54][55][56]. Monoculture production is also dependent on the feeding frequencies, as four times feeding frequency shows a better growth of monosex male tilapia [57].
Based on biological criteria, tilapia has potential for cage culture. Tilapia is usually produced in cages under polyculture or monoculture. Tilapia cage culture in Bangladesh is considered a very popular and effective production system in open water. Tilapia cage culture can be a successful alternative way to eliminate poverty in poor rural communities in Bangladesh [58]. Growth performance and economic analysis show that the stocking density of 50 monosex tilapia per m 3 is the best for cage culture in Kaptai Lake in Bangladesh [58]. Moreover, tilapia production in cages could increase more than pond yield if they are fed with floating commercial diets supplemented with zymetin probiotics [59]. Tilapia culture is profitable if cages are designed in an inexpensive way with adaptive management techniques [60].
Tilapia culture under rice-fish farming is a potential system of fish production that would not hamper rice yield in Bangladesh. Rice-fish integration has potential to utilize land and water for the fulfillment of protein demand and increase the incomes of farmers [61]. In rice-fish coculture, tilapia is a promising species due to its higher adaptability and better growth compared to other species [62]. The stocking density of tilapia in rice-fish coculture can be varied due to extensive farming. Farm size can affect the stocking density, where a medium sized farm with 5000/ha monosex tilapia in a rainfed rice-fish ecosystem can offer better economic benefit [63]. Besides, 6 per m 2 tilapia combined with common carp in integrated rice-fish system is a suitable stocking density for better growth, survival, and maximum rice production [64]. The inclusion of tilapia with 2500/ha in freshwater prawn-mola polyculture in rotational rice-fish farming showed better production and profit [65]. Rice-tilapia coculture can also be more viable with the incorporation of basal fertilization in combination with compost fertilizer [66]. Integrated rice-tilapia culture, particularly seed production, could be one of the best approaches to reduce the tendency of farmers to use pesticides along with the increase in rice-fish production [67,68].
In Bangladesh, tilapia is also produced in pen culture. A pen is a fixed enclosure in which the bottom is the bed of the water body. In pen aquaculture, a 1.5-2.5 m depth of water in canals, oxbow lakes, and dead rivers with a high eutrophic status is considered as ideal and a soft muddy to hard sandy bottom is preferable for fish growth [69]. Tilapia pen culture under floodplain aquaculture is becoming popular in rural Bangladesh.

Coastal Brackish Water Tilapia Culture
Shrimp (Penaeus monodon) is considered a major species for brackish water aquaculture in coastal Bangladesh, which is one of the largest sectors for earning foreign currency [70]. Recently, shrimp production has been struggling due to environmental degradation with climate change and disease outbreaks [71]. To overcome this situation, farmers have been looking for an alternative method where crop diversification, such as polyculture instead of shrimp monoculture, can play an effective role in sustainable aquaculture. Tilapia can play a vital role in polyculture to compensate for partial economic loss due to unexpected mortality of shrimp [72]. Although tilapia is a very popular freshwater aquaculture species in Bangladesh, it can grow as a diversified crop in brackish water systems [37,73]. Tilapia can augment the production of shrimp because of cumulative environmentally friendly polyculture conditions as they utilize different ecological niches [74]. In fact, tilapia feed on Sustainability 2021, 13, 7657 6 of 20 the upper water layer, filtering phytoplankton and zooplankton [47], while shrimp mostly stay on the pond bottom for grazing substrate and detritus [75]. Moreover, tilapia can tolerate various environmental hazards [76], particularly salinity stress ranging from 0 to 25 ppt [73,77].
Three types of tilapia-shrimp polyculture systems are generally constructed, including simultaneous, sequential, and crop rotation systems [78]. Among different strains, GIFT and Nile tilapia would be most suitable candidate for brackish water aquaculture due to their 36-81% higher growth performance [79] and wide range of salinity tolerance compared to available salt tolerant species (grey mullet, silver barb, pangus) for shrimp polyculture. Shofiquzzoha and Alam [74] found that GIFT and shrimp coculture with 10,000/ha stocking density is more prosperous than the culture of silver barb and shrimp with the same density under polyculture. Studies also recommended that a stocking density for shrimp of not more than 2/m 2 [80,81], and for GIFT 1/m 2 [80] is feasible under extensive culture. In addition, monosex tilapia (10,000/ha) with prawn (20,000/ha) culture showed better performance even though water salinity varied greatly, from 3 to 10.3 ppt [82].

Productivity
Tilapia production was not initially satisfactory in Bangladesh. However, with the advancement of culture systems, converting extensive to semi-intensive or intensive culture following monoculture instead of polyculture, the productivity of tilapia has increased considerably. Total tilapia production in inland waterbodies has recently increased from 370,017 tons in 2016-2017 to 390,559 tons in 2018-2019 (Table 1). Tilapia productivity has gradually increased in most culture systems. However, tilapia production can vary depending on culture system and species. The highest average tilapia productivity was found in pond systems under freshwater aquaculture, followed by seasonal water body and cage-pen culture. Tilapia grows to a marketable size (100-150 g) within a short culture period (2-3 months), which facilitates farmers to produce more than one crop in a year [59]. The highest annual production of monosex tilapia under monoculture is around 10,000 kg/ha [83], and the production of tilapia is 8028 kg/ha with carp [84].

Low Water Quality
The positive culture characteristics of tilapia are their tolerance to poor water quality. Tilapia usually have lower oxygen requirements than other fish, such as carp. In turbid water, tilapia can easily survive up to 200 mg/L levels of turbidity with no significant effects on specific growth rate (SGR) and feed conversion ratio (FCR) [85].
Tilapia can also tolerate certain levels of water pollution. Flooding is common in Bangladesh during the monsoon leading to intense rainfall which can inundate canals and rivers in low-lying areas. Industrial and agricultural wastes are discharged into canals and rivers. Polluted waters are mixed with floodwaters during the monsoon and spread over unavoidable flood areas. Therefore, culturing pollution tolerant species can reduce Sustainability 2021, 13, 7657 7 of 20 the risk of production loss. In this sense, tilapia can play a substantial role in tackling these conditions. Nile tilapia (O. niloticus) can tolerate high levels of waterborne cadmium, which is a common toxicant of polluted water. The highest level of cadmium recorded for Nile tilapia was 96 h LC50 (14.8 mg/L Cd; hardness 50 mg/L CaCO 3 ) and branchial chloride cell numbers started to recover after 24 h of exposure [86].
Tilapia can survive under dissolved oxygen conditions as low as below 2.3 mg/L if other factors such as temperature and pH remain favorable. In the rainy season, dissolved oxygen levels may reduce due to the increase in turbidity, and oxygen levels may reach as low as 2 mg/L. Low dissolved oxygen levels ranging from 1-1.5 mg/L have no effect on tilapia survival [87]. In addition, algal bloom in a fertilized pond can also reduce oxygen levels to 0.3 mg/L, but no tilapia mortality was reported [88].

Drought Condition
As a consequence of climate change, farmers often suffer from low water volume in drought conditions because of rainfall variation in different regions of Bangladesh. Seasonal and metrological drought conditions cause inadequate access to water on fish farms. Severe droughts often cause short culture periods for fish. Prolonged droughts with low groundwater levels are also a concern for water shortage on fish farms. Year-round fish production is not feasible as a result of low water levels as well as water scarcity in ponds. Most fish farms including ponds dry up during the dry season, causing unfavorable conditions for fish, and thus fish become more stressed and crowded in low levels of water.
In the summer season, the water depth in fishponds can reduce up to 100 cm and sometimes lower than that. Surprisingly, tilapia can easily survive in water depths as shallow as 50 cm, though suitable growth was recorded at around 100 cm water depth [89], whereas carp need a minimum of 180 cm water volume in the pond for their suitable growth [90]. Thus, it can be presumed that tilapia can adapt to lower water volumes than other species due to drought condition and rainfall variation.
Pumping groundwater to irrigate ponds can help tilapia culture in the dry season. Rainwater harvesting with storage facilities in ponds and ditches may also help with tilapia culture in the dry season. Using rainwater for tilapia culture may increase water use efficiency. Adjusting tilapia culture in floodplain seasonal waterbodies during monsoon may also help to cope with drought.

Temperature Fluctuation
Temperature change-either seasonal or daily-is a common phenomenon in aquatic environments. Fish, which are critically important inhabitants of aquatic environments, cannot avoid temperature changes. Hence, adaptations, acclimatization, and physiological responses against temperature changes are the only survival techniques of fish. It is generally perceived for living organisms that "more adaptive capability-more distribution". The worldwide distribution of tilapia confirms this perception and different studies also support this notion. Table 2 summarizes the effects of temperature changes on the physiological alterations of tilapia. The thermal effects on tilapia showed that growth performance significantly decreased after at 34 • C, although this showed a decreasing tendency towards high temperature changes [91][92][93][94][95][96]. Before they reached 5 days old tilapia (O. mossambicus) showed significantly increased percentages of deformity, and a greater number of males was induced by high temperature (28 and 32 • C) after 10 days [97]. According to Desprez and Mélard [98], a higher proportion (98%) of male blue tilapia (Oreochromis aureus) are produced during sex determinism in high temperature regimes (34 • C). Non-specific immunity (lysozyme activity) of Nile tilapia can significantly decrease due to high temperature (33 • C) [99]. Recently, Islam et al. [96] found variable effects of high temperature (37 • C) on the hemato-biochemical parameters of tilapia. Here, the abundance of hemoglobin and red blood cells were significantly reduced, although the amount of white blood cells and blood glucose were increased. Higher temperature (31 • C) also showed positive effects on the metabolism of tilapia and greater growth was observed, which was supported by the increased digestive enzymes genetic expression [100].
On the other hand, at low temperatures, tilapia survivability suffers. Significantly decreased growth performance and mortality rate were perceived for tilapia below 21 • C [89,101]. It was also observed that low temperature significantly decreases hematocrit and hemoglobin parameters [101]. In addition, at temperatures below 11 • C, tilapia stop feeding, with high stress, fungal infection, and mortality due to decreases in plasma osmolarity, chloride, and sodium ion concentrations [89,102]. The exposure of tilapia larvae to low temperature (20 • C) of tilapia larvae before they were 10 days old induced a high ratio of females during sex differentiation [97]. A low temperature (13 • C) decreased serum glucose and cholesterol levels but increased cortisol levels of GIFT. Moreover, low temperatures also enhance catalase, glutathione, and superoxide dismutase levels in the liver with oxidative damage [103]. It was also found that reduced temperature decreases saturated fatty acid content, though it increases the polyunsaturated fatty acid content [104]. Gene expression analysis suggests that when tilapia are exposed to low-temperatures (10 • C), significant changes were viewed in the expression of genes associated nucleic acid and protein synthesis, amino acid metabolism, lipid and carbohydrate content, and immunity [105]. Acute lethal low temperature (8 • C) can disrupt kidney function and downregulate the immune-related pathway in the kidney of tilapia [106].
Tilapia is a thermophilic species. The ultimate upper lethal temperature for T. mossambica lies at 38 • C [107]. Recent studies showed that tilapia can tolerate temperatures of up to 34 • C without any significant effect on their growth rate [94,96]. In addition, the cortisol level, which is the stress indicator in tilapia, does not significantly increase even at 36 • C [108], which suggests that tilapia can tolerate high temperatures, whereas for L. rohita, another commonly cultured species in Bangladesh, a temperature of 36 • C may be hazardous for growth and can cause physiological damage [109][110][111]. The balanced sex ratio of O. niloticus does not deviate significantly even up to 36 • C [92]. A study on the haemato-biochemical parameters also revealed that temperatures up to 34 • C may not be hazardous for Nile tilapia [96]. A comparative study suggests that embryonic development and hatching of carp are significantly affected even at 33 • C [112]. However, Nile tilapia showed faster embryonic development and better survival rates (52.7%) after hatching at 34 • C than the control group (49.5%) at temperature 27 • C [113]. Tilapia may adopt some behavioral or physiological mechanisms for this high temperature tolerance. In fact, tilapia may adjust to extreme temperatures with their diurnal movement in open water. Tilapia can show other behaviors such as ventilation cessation behavior and aquatic surface respiration to prolong thermal tolerance limits both in freshwater and saltwater in response to thermal changes [114].
Different strains of Nile tilapia differ with respect to cold tolerance, but growth is usually hampered at temperatures under 16 • C. Although most fish species start to stop feeding at low temperatures (below 16 • C) in winter season [115], most strains of tilapia become severely stressed at 13 • C. Nile tilapia started to die when water temperature fell to 11 • C, and 100% death occurred by 7.4 • C. Low temperature ranges from 11 to 8.4 • C are lethal for the GIFT strain [116]. Cold acclimation can reduce triglyceride and total cholesterol in GIFT, which can help enhance cold tolerance [104]. Since the average low-temperature of Bangladesh in winter is 12.7 • C, tilapia can easily survive in winter even at low temperatures, and this low temperature tolerance can be improved through the use of 0.2% Astragalus membranaceus extract powder [117].  is produced with 60-81% survival [92] Bangladesh is called a country of six seasons, though three seasons (summer, autumn and winter) are mainly observed. Water temperature fluctuates throughout the country year round [123]. A survey report showed that monthwise average water temperature varied in the last five years by even more than 1 • C in some months (Table 3). Table 3. Average water temperature in Bangladesh (data were collected from Dhaka, Chittogong and Cox's bazar areas and adopted from sea water temperature info [123]

Salinity Change
Tilapia is considered as euryhaline fish which can grow comfortably both in freshwater and brackish water [124,125]. However, changes in water salinities can decrease growth performance and can alter the physiology of tilapia. For example, Nile tilapia can grow better when reared in water with a salinity of up to 7 ppt (salinity is usually measured in parts per thousand (ppt), which is equivalent to grams of salt per liter water (g/L)) [126,127], while T. rendalli can tolerate 10 ppt salinity with no significant effect on their growth [128]. The salinity tolerance of another strain Rufiji tilapia (Oreochromis urolepis urolepis) is higher than other tilapia strains; it is 25 ppt in which concentration SGR, daily growth rate, and total body weight showed higher rates even than in 5 ppt salinity [129]. In fact, the salinity tolerance of tilapia may be strain specific. Since Nile tilapia is a widely cultured species, most research has performed to evaluate the salinity tolerance of this species, the findings of which are summarized in Table 4.
Although, Nile tilapia grows well in low salinity (4-7 ppt), slightly higher salinity (8 ppt) significantly decreases average weight gain [130,131]. Other studies indicate conflicting results of salinity in the same concentration, where the highest survival (98%) and a linear growth in net biomass of Nile tilapia were observed at 8 ppt water salinity in recirculating systems [132], and this salinity is also recommended in biofloc technology [133]. These results suggest that rearing techniques may have the least effects on the salinity tolerance of Nile tilapia. Interestingly, the best survival and growth rate of Nile tilapia was obtained at a salinity of 16 ppt [134]. Similarly, the nutrient digestibility of Nile tilapia was increased at a salinity of 15 ppt and showed a large impact on the distal region of the intestine [135]. Moreover, Nile tilapia cultured at 16 ppt observed greater results than those cultured at 20 ppt due to lower death rates and higher expression of ion-regulated genes [136].
Besides growth parameters, salinity concentration also influences hematological parameters of Nile tilapia. According to de Azevedo et al. [127], hematocrit and the erythrocyte count significantly decrease at high water salinity (14 ppt). The authors also observed histological and histopathological alterations with the increased intensity of salinity. Polyunsaturated fatty acids significantly increased up to 16 ppt salinity [131]. The levels of Na + , Cl − and Ca 2+ and white blood cells increased significantly in Nile tilapia when exposed to 10 and 15 ppt salinity [137].
Ideally, tilapia is a potential candidate to cope with salinity changes. Endocrine studies revealed that tilapia can compensate for greater changes in external osmolality while cultured in a tidal salinity, maintaining osmoregulatory parameters similar to seawateracclimated fish [138,139]. Salt tolerance genes in Nile tilapia, which play vital role during adaptation in saltwater, were also identified [140]. For saltwater adaptation, Watanabe et al. [141] suggested that the progeny of red tilapia spawned and cultured through early ontogenetic development in brackish water are well adapted for growth in brackish and seawater, and may have greater resistance to cold stress in seawater than progeny spawned in freshwater.

Disease and Parasite Prevention
Climate change may affect water quality and disrupt host-pathogen interactions [144], sometimes creating beneficial conditions for pathogen amplification and spread [144][145][146], or for microbial and ecological dysbiosis [147]. Water quality disruptions stemming from climate change that may have consequences for the outbreak of marine parasites and diseases include changes in temperature, hypoxia, CO 2 accumulation (reduced pH), precipitation (leading to increased or decreased salinity), and cyclone frequencies and intensities [144,148]. Changes in water quality may result in immediate impacts on host-pathogen equilibriums or long-term impacts such as the expansion of hosts (primary and secondary) and pathogens into new regions [149]. Hence, these challenges for aquaculture need to be considered to be caused by climate change [150]. In fact, environmental change with pathogens and parasites outbreaks may be accelerated due to climate change.
Disease-resistant fish species can play critical role in mitigating these challenges, and tilapia is a suitable candidature in this context. It has been reported that tilapia are resistant to most diseases and parasites [151], and hybrid tilapia (male blue tilapia x female Nile tilapia) has higher disease resistance than blue tilapia and Nile tilapia [152]. Diseaseresistant tilapia can be developed following selective breeding program against viral [153] and bacterial diseases, as host resistance is highly heritable in a Nile tilapia breeding populations with GIFT origin [154]. In addition, selective breeding can be performed with high accuracy using Bayesian models followed by marker-based and pedigree-based genomic prediction against Streptococcosis diseases [155].
Disease avoidance or removal is not an easy task if soil and water of aquatic environment are infected with pathogens. Therefore, prevention is the best choice for maintaining sustainable aquaculture. However, collecting disease-free stock, avoiding overfeeding, minimizing unnecessary handling, reducing overcrowding, and removing dead fish will reduce the risks of disease outbreak [156,157]. Moreover, regular cleaning and disinfection of equipment and production units must be performed to reduce the transmission of pathogens. Maintaining water quality in farming systems is also essential [157]. In addition, different feed supplements such as probiotics [158], Aloe vera [159], assam tea extract [160], Sophora flavescens [161], corncob [162], vitamin C [163], orange peels [164], green tea [165], ginger [166], and Indian lotus leaf powder [167] can increase the disease resistance of tilapia. On the other hand, novel effective vaccines can also play an important role in disease prevention and control: vaccines have been developed against most common pathogens of tilapia-Streptococcosis [168,169], motile aeromonad septicemia [170], tilapia lake virus [171], and infectious hematopoietic necrosis virus [172]. Using a treatment and diagnosis system to alert farmers to the occurrence of bacterial infection in tilapia can be a very useful tool that can reduce economic losses and labor costs in aquaculture [173].

Conclusions
Aquaculture in Bangladesh is susceptible to a variety of climatic factors, including global warming, rainfall variation, salinity change, and temperature fluctuation. Considering susceptibility to the impacts of environmental change on aquaculture, tilapia farming is one of the potential strategies for adaptation to climate change. Compared to other fish, tilapia can adapt to low water quality caused by rainfall variation as well as flood and drought. Tilapia can also adapt to low water volume and drought conditions. Moreover, the positive culture characteristics of tilapia are their tolerance to temperature fluctuation and salinity change. Disease-resistant fish species can play a critical role in adapting to climate change, as tilapia is a suitable fish that can resist most diseases and parasites.
For adaptation to climate change, tilapia farming is possible in a wide range of water environments, including freshwater, brackish water, and saltwater conditions. Tilapia can be produced under polyculture, monoculture, and integrated farming in pond systems, cage culture, pen culture, floodplain aquaculture, and coastal ponds. Since tilapia can easily survive in low water volume with high stocking density and short culture duration, this species can be cultured in seasonal water bodies. In addition, tilapia farming in rice fields can also adapt to changes in water level and temperatures. In fact, integrated rice-tilapia farming could be another adaptation strategy to reduce further risks of climate change. Moreover, tilapia can be cultured with brackish water species in coastal ponds as tilapia can tolerate moderate level of salinity. Ultimately, tilapia is a suitable aquaculture fish for current and future adaptation to climate change.