Freshwater aquaculture production in 2011 was 44.3 million tons or 29% of world fisheries production. Roughly 95% of this production was from Asia – China was the leading country. Ponds are the common culture system, but raceways, cages, net pens, and water re-use systems are important. Seed stock for growout is produced in hatcheries from farm-reared brood stock. Fertilizers, feeds and mechanical aeration increase production in ponds. Competition with agriculture for land and water, wetland destruction, water pollution, and lack of fish meal and oil for use in feeds could limit aquaculture . There is an effort to avoid these impacts.
Aquaculture, freshwater or marine, is dependent upon good water quality to sustain maximal fish growth. Ammonia and nitrate are toxic to fish. NH3 should be kept at levels below 0.05 mg l−1. Nitrite (NO2) should be kept below 0.5 mg l−1. Aerobic bacteria play a key role in detoxifying ammonia. Nitrosomonas bacteria convert NH3 to NO2, and Nitrobacter bacteria convert NO2 to NO3. For every milligram of ammonia converted about 5 mg of oxygen is consumed. An additional 5 mg of oxygen is required to satisfy the oxygen demand of the bacteria involved . Thus, reduced concentrations of dissolved oxygen may contribute to increased concentrations of ammonia, nitrate, and phosphate in the water column. Phosphate, which is an essential element for life, can be detrimental to the biosphere in high concentrations. Since the discovery in 1988 by L. Liebermann that yeast contain granules (volutin) composed of high-polymer polyphosphates, a diverse group of microorganisms have been reported to effectively take up inorganic phosphate from the medium and convert them into biopolymers. Polyphosphates have been associated with the capsule in Neisseria gonorrhoeae, outside the plasma membrane, or as long-chain cytoplasmic reserves. In some microorganisms, these granules can account for as much as 25% of the weight of the organism and the process can be highly efficient in phosphate removal . For example, in the A/O process of activated sludge treatment, involving alternate anaerobic and aerobic cycles, a bacterial population (largely composed of a typical soil bacterium – Acinetobacter lwoffi) that accumulates polyphosphate even under extremely low phosphate concentrations is enriched. Since the consortium in the A/O process denitrifies ammonium as well as removes phosphate from the environment, it provides attractive possibilities for aquaculture. A variation of this has already been incorporated into the sequencing batch reactor for recirculating aquaculture systems. Despite the disparities in size and volume of marine and freshwater realms, a strikingly similar number of species is found in each – with 15 150 Actinopterygian fishes in fresh water and 14 740 in the marine realm . Their ecological and societal values are widely recognized yet many marine and freshwater fishes increasingly risk local, regional or global extinction. The prevailing threats in aquatic systems are habitat loss and degradation, invasive species, pollution, over-exploitation and climate change. Unpredictable synergies with climate change greatly complicate the impacts of other stressors that threaten many marine and freshwater fishes. Isolated and fragmented habitats typically present the most challenging environments for small, specialized freshwater and marine fishes, whereas overfishing is by far the greatest threat to larger marine and freshwater species. Species that migrate within or between freshwater and marine realms may face high catchability in predictable migration bottlenecks, and degradation of breeding habitat, feeding habitat or the intervening migration corridors .
Conservation reserves are vital to protect species-rich habitats, important radiations, and threatened endemic species. Integration of processes that connect terrestrial, freshwater and marine protected areas promises more effective conservation outcomes than disconnected reserves. Diadromous species in particular require more attention in aquatic restoration and conservation planning across disparate government agencies .
Human activities and stressors that increasingly threaten freshwater and marine fishes must be curbed to avoid a wave of extinctions. Freshwater recovery programmes range from plans for individual species to recovery of entire basin faunas . Reducing risks to threatened marine species in coastal habitats also requires conservation actions at multiple scales. Most of the world's larger economically important fisheries are relatively well-monitored and well-managed but there are urgent needs to curb fishing mortality and minimize catch of the most endangered species in both realms.
Published Date: 2022-08-30; Received Date: 2022-08-02