The dietary requirement of calcium (Ca) has been well established in terrestrial animals. However, investigations for dietary Ca requirement in fish are complicated and reports limited as fish can use waterborne Ca in addition to dietary Ca. Ca absorption in fishes from environmental water is species specific and depends on concentration of Ca in rearing environment and availability of dietary Ca. Ca and phosphorus are generally combined together in fish body therefore maintaining a dietary calcium: phosphorus ratio is important. Ca exists in a complex form in fish meal, which is not available to fishes; hence, although fish meal contains an appreciable amount of calcium, dietary Ca supplementation in fish meal‐based diets for some species is necessary. This review article summarizes and discusses pertinent information on absorption of Ca from water, essentiality of dietary Ca, bioavailability of Ca from different sources, optimum calcium:phosphorus ratio and dietary Ca requirement in fishes.

Aquatic fern Azolla pinnata comprises significant high food value with a good proportion of protein, vitamins, and minerals. This study was carried out to examine the effect of fresh A. pinnata as a substitution of commercial fish feed (CFF) for Thai silver barb Barbonymus gonionotus. Post fingerlings of B. gonionotus were reared in five treatments, labeled T₁ to T₅, by substituting 0%, 25%, 50%, 75%, and 100% protein of CFF with A. pinnata protein (dry matter basis) respectively for 56 days. The specific growth rate, net production rate, protein efficiency ratio, proximate composition, and overall conditions of fish were not significantly varied between the fish reared completely with CFF and 25% substitution with A. pinnata. However, a significantly higher profit rate (431.49 USD ha⁻¹ 56 day⁻¹) was calculated for fish reared in T₂ than other treatments. In contrast, there was a significant reduction of growth and other parameters of the fish that were observed in the case where more than 25% CFF was substituted with A. pinnata. The poorest performance was observed in fish fed completely with A. pinnata, at T₅. Based on the results, 25% of CFF of Thai silver barb could be substituted with fresh A. pinnata without significantly lowering their growth and product quality and could contribute significant to a higher profit margin.

Marine resources of Bangladesh are abundant, both in terms of huge sea area and its biological richness. The marine water area supports a large number of commercially important fin fish, mollusks, crustaceans, and seaweeds, offering significant economic potential due to their diversified use and high demand in the international market. Therefore, the current review comprehensively examines both the contribution and potentials of marine fisheries resources to the economy, as well as the challenges involved in marine fisheries sectors in attaining a sustainable blue economy in Bangladesh. The study revealed that, the marine fisheries plays an important role in national economy and has huge prospect in employment generation and ensuring food security of Bangladesh. The expansion of mariculture and effective utilization of marine resources will open a new window for the economic development of Bangladesh. Marine fisheries production of Bangladesh has increased over the last two decades, but its relative share in total fisheries production has declined. Despite being rich in marine living resources, commercial mariculture has been developed only for shrimp, prawn, and mud crab. Challenges hindering mariculture expansion includes limited research, technological deficiencies, inadequate skilled workforce, underdeveloped domestic seafood markets, and financial constraints. Concerted efforts including exploring new fishing grounds, conducting stock assessments, implementing scientific management strategies, enacting a national marine fisheries policy, enforcing legislation, and establishing robust surveillance and monitoring mechanisms are required to foster a sustainable blue economy in Bangladesh.