Reviews in Aquaculture

The use of antibiotics in the aquafeeds to mitigate infectious diseases or to boost growth performance is commonly practiced. Recently, the prophylactic use of antibiotics and chemotherapies have been criticized which eventually led to their ban in aquaculture by law on many countries. In view of the current restrictions on antibiotic use, there is a critical need to evaluate other possible alternatives. One potential substitute for antibiotics in aquafeeds is the use of functional feed additives. The present review is a comprehensive and an updated compilation of the available works on different feed additives, their examples, modes of action and useful applications for improving aquafeeds. It highlights several functional feed additives including probiotics, prebiotics, synbiotics, immunostimulants, organic acids, nucleotides and medicinal herbs. Apart from boosting aquafeeds and safeguarding general health of aquatic animals, some of these substances have been found to possess beneficial immunostimulant and anti‐stress relieving properties. Furthermore, the use of these natural substances increases the consumer confidence of farmed fish. Due care has been taken to cover the on‐going trends and recent advances with a perspective vision and their holistic usages and beneficial applications in aquatic animals’ systems. The current study also shed light regarding the management and production aspects of aquatic animals which will enlighten farmers and producers for better economic growth.

Essential fatty acids (EFA) remain one of the least well‐understood and enigmatic nutrients in aquaculture nutrition. Of all dietary nutrients none has a greater direct impact on the composition of its consumer. Their importance stems not only to their impact on animal growth, but also to factors such as reproduction, immunity and product quality. Docosahexaenoic acid (DHA; 22:6n‐3) has consistently been shown to provide the greatest EFA value to most species. However, the nutritional value of eicosapentaenoic (EPA; 20:5n‐3) and arachidonic (ARA; 20:4n‐6) acids has also been significantly greater than that exhibited by linolenic (LNA; 18:3n‐3) and linoleic (LOA; 18:2n‐6) acids. All five fatty acids have been shown to provide EFA value to most aquaculture species, although the optimal dietary inclusion levels and balance among the fatty‐acid classes (n‐3 and n‐6) and fatty‐acid chain lengths (18‐C, 20‐C or 22‐C) vary among species. Environmental origin (freshwater, estuarine or marine) appears to be a primary factor influencing the difference in EFA requirements. The role that EFA play in osmoregulation clearly shows how these nutrients affect animals from different aquatic environments. The influence of EFA on growth also appears to be greatest in larval fish and crustaceans, possibly because of their reduced ability to digest and absorb lipids, but also because of a proportionally higher demand for EFA in the development of, in particular, neural tissues. Despite an abundance of research since the 1970s on the EFA requirements of aquaculture species there remains a need to better define the EFA requirements of most aquaculture species. Of all major aquaculture species only the penaeid shrimp has a comprehensively documented assessment of its nutritional requirements for EFA. The nutritional requirements for EFA in most fish species have not been comprehensively studied and those species that were fully examined in the 1970s and 1980s now need to be reassessed in light of recent changes to the use of high‐nutrient‐density diets that were not routinely used in either practice or research during that earlier period. In addition to changes in dietary specification strategies, declining dependence on marine‐origin lipid sources in recent years has placed an increased imperative on understanding the dietary need for long‐chain polyunsaturated fatty acids (lcPUFA). As aquaculture continues to grow there will be an increased use of alternative lipid resources, such as grain, algal and rendered oils, to provide dietary lipids. In addition to dietary dilution of natural EFA sources through the use of these raw materials, they will also bring new challenges, such as increased levels of n‐6 and 18‐C polyunsaturated fatty acids (PUFA). Introduction of these n‐6 and PUFA fatty acids to the diet of aquaculture species will not only influence the nutritional demands of these animals, but will also affect their flesh quality characteristics by reducing their level of n‐3 lcPUFA. This dilemma will demand an increased prioritisation on the value of lipid sources rich in n‐3 lcPUFA, but is also likely to stimulate the development of alternative sources of lcPUFA.

This review focuses on the current status of antibiotics use, effects on animal health and the environment, existing policies and regulatory mechanisms in the top 15 producing countries. Fifty papers were reviewed and represented the bulk of literature worldwide. We observed that 67 antibiotic compounds were used in 11 of the 15 countries between 2008 and 2018. Among these countries, 73% applied oxytetracycline, sulphadiazine and florfenicol. On average, countries used 15 antibiotics and the top users included Vietnam (39), China (33) and Bangladesh (21). On environmental and health risks, the review revealed sufficient evidence that directly links antibiotics use to food safety, occupational health hazards and antimicrobial resistance. Environmental risks included residue accumulation, aquatic biodiversity toxicity, microbial community selection for antibiotic resistance and the emergence of multi‐antibacterial resistant strains. Regarding policies, major players were the European Commission, the Food and Drug Administration, European Medicines Agency, Norwegian Veterinary Institute, the Norwegian Food Safety Authority, Codex and government ministries. In particular, the Norwegian Food Safety Authority and Norwegian Veterinary Institute demonstrated the most outstanding regulation mechanisms of antibiotics use in aquaculture that needs to be emulated. The future of aquaculture lies in guaranteeing the supply of safe aquatic products to the growing consumer market. Thus, international coordination of the policy and regulatory environment is needed, while increased investment in research for alternative aquatic health management strategies is essential. Future technologies should focus on the reduction of antibiotics use to safeguard the environment and ensure safety of consumers, feed industry and aquaculture workers.

Aquaculture, which constitutes one of the largest food production sectors in the world, is preferably practiced with natural organic products rather than with synthetic chemicals or antibiotics. In addition to the daunting challenge of providing food and livelihood to the exponentially increasing world population, the aquaculture industry is key to ensuring that development is based on environmentally sustainable practices, specifically in the production of aquafeeds. Terrestrial microorganisms that act as natural defence systems of cultured species have been identified as the main producer for the beneficial bacterial candidates. Probiotics have recently gained popularity as beneficial microbes candidates in cultured organisms to maintain the health condition and well‐being of different aquatic animals. This review aimed to understand the necessity of using probiotics as a sustainable alternative to regulate the growth performance, feed utilisation and general health condition for sustainable aquaculture. Also, explanatory discussion about the host microbiota and its ability to produce different probiotic strains and the probiotic functionality to ameliorate the host immunity to provide the interactive effects on the host‐derived probiotics. By presenting the results obtained from the previous studies about the ability of probiotics to sustain the aquatic animal's performances, this study condensed the current knowledge and information for future research and development of the probiotic application in aquaculture.

The presence of skeletal anomalies in farmed teleost fish is currently a major problem in aquaculture, entailing economical, biological and ethical issues. The common occurrence of skeletal abnormalities in farmed fish and the absence of effective solutions for avoiding their onset or definitely culling out the affected individuals as early as possible from the productive cycle, highlight the need to improve our knowledge on the basic processes regulating fish skeletogenesis and skeletal tissues differentiation, modelling and remodelling. Severe skeletal anomalies may actually occur throughout the entire life cycle of fish, but their development often begins with slight aberrations of the internal elements. Comprehensive investigation efforts conducted on reared larvae and juveniles could provide a great contribution in filling the gap in knowledge, as skeletogenesis and skeletal tissue differentiation occur during these early life stages. The aim of this review is to provide a synthetic but comprehensive picture of the actual knowledge on the ontogeny, typologies and occurrence of skeletal anomalies, and on the proposed causative factors for their onset in larvae and juveniles of European farmed fish. The state‐of‐art of knowledge of these issues is analysed critically intending to individualize the main gaps of knowledge that require to be filled, in order to optimize the morphological quality of farmed juveniles.

Meta‐analysis of literature data on mineral and trace element requirements of fish was performed with the major objectives of identifying appropriate response criteria and the factors affecting the minimal dietary inclusion levels. The primary data set included 25 studies on available P, 20 on Ca, 24 on Mg, 5 on K, 37 for Zn, 23 for Se, 19 for Mn, 16 for Fe and 13 for Cu. Broken line regression analysis with linear plateau model (P, Ca, Mg and K) or two‐linear line model (Zn, Cu, Fe, Mn and Se) was used for determining the minimal dietary inclusion levels. Vertebral mineral concentration (P, Ca, Zn and Mn), whole‐body mineral levels (Mg, K and Fe) and hepatic enzyme activity (Se) were found to be the most appropriate criteria for the respective minerals analysed. In general, weight gain as the criterion resulted in a lower estimate (by 18–42%) than those obtained using whole‐body or vertebral mineral concentrations as response criteria. The analysis also showed that different fish species do not show large variations in the mineral and trace element concentrations in the whole body and tissues. Factors such as species group and digestive physiology, type of experimental diet used and dietary interactions, type of mineral source and mineral concentration of water were found to affect the minimal dietary inclusion levels of certain minerals. Besides the meta‐analysis, research needs in mineral nutrition of fish with reference to growing changes in dietary strategies and rearing systems are discussed.

This study addressed the issue of the dietary supplements; probiotic, prebiotic and synbiotics considered as functional food ingredients in sturgeon aquaculture. Developments of this strategy have been suggested and practiced as an alternative to antibiotic administration in aquaculture. Sturgeons are commercially valuable species due to their meet and roe, which is made into caviar. Aquaculture of sturgeons up to marketable size has increased during the last decade while disquieting decreases have been noticed of the natural populations. In line with the expand evaluations of microbial dietary supplements and prebiotics in finfish and shellfish aquaculture, the scientific community has also increased their interest in approaches to these issues in sturgeon aquaculture. The present review summarizes and discusses the results on intestinal microbiota of sturgeon species and the effects of probiotic, prebiotic or synbiotic administration on growth performance, gut physiology, intestinal microbiota, immune response and health. Furthermore, this study tries to cover the gaps of existing knowledge and suggest issues that merits further investigations.

The growth of Chilean salmon production has not been free of important sanitary and environmental shortcomings. To ensure sustainability, it is necessary to understand the environmental impacts of salmon production on the Patagonian ecosystems. Currently, there is limited regulation or monitoring of impacts in the freshwater phase compared to the marine fattening stage, and there is some evidence of local eutrophication impact and diversity changes downstream the farms. Eutrophication of Patagonian channels and fjords from marine farms has been recognized as crucial environmental risk, although most scientific evidence comes from local effects below and around farms. So far, there are no regulations based on carrying capacity estimates to limit maximum fish biomass per area or water body. There is controversy regarding the potential role of nutrients derived from farming in triggering harmful algal blooms, yet current environmental monitoring and available information does not allow establishing or rejecting a cause–effect relationship. Pesticides used to control sea lice infestation have been shown to be deleterious to some non‐target species. There is evidence that the use of high quantities of antibiotics has allowed the development of antibiotic‐resistant bacteria in sediments and there is concern that salmon aquaculture has the potential to increase the proportion of antimicrobial‐resistant bacteria to antibiotics that are used in human medicine. There is an urgent need for more comprehensive ecosystem (beyond farm) studies on the impacts of antibiotics. Escapes of salmon (exotic species) from farms are a relevant environmental risk, although the most farmed species, Salmo salar, has shown little success in establishing wild populations. The review identifies critical knowledge gaps whose fulfilment is essential to advance towards an ecosystem approach to aquaculture and to protect Patagonian ecosystems.

Recirculation aquaculture systems (RASs) are increasingly considered as production systems of the future with a minimum ecological impact for the production of aquatic food. To maintain a good water quality and to produce quality and healthy fishery products, the systems depend on a diverse microbial community involved in different processes of water purification but also in unwanted effects. The article reviews the present knowledge on microorganisms in RAS, their relative distribution within the system compartments and their role in system performance. The system possesses different microniches in which microorganisms retrieve their preferential conditions for oxygen and nutrients. Ammonia and nitrite are oxidized under aerobic conditions into less toxic compounds. Anaerobic ammonium oxidizers in the biofilm oxidize anaerobically both ammonia and nitrites into less harmful dinitrogen gas. Heterotrophic bacteria mineralize organic matter derived from uneaten feeds, dead bodies and excreta of fish. Under conditions of high organic load and high C/N content, nitrifiers are overgrown by heterotrophs with negative effects to the nitrification process. For not yet understood reasons, the presence of off‐flavour‐producing microorganisms occurs also in RAS. Microbial management and management of inputs to the systems to prevent the proliferation of pathogens are discussed and possible management techniques of off‐flavours are also presented. Research orientations are given to explore further the potential of heterotrophic bacteria in microbial management and intensive aquaculture production in systems other than RAS.

The growing demand for fish products and the dwindling productivity of marine fish stocks due to the overexploitation of fisheries place the aquaculture industry as a key contributor to the global fish supply. The intensive development of aquaculture has raised a range of environmental concerns such as effluent discharge, excessive use of resources and dependence on commercial feed. In this context, the development of sustainable aquaculture systems is becoming the cornerstone for long‐term aquaculture expansion, and to achieve environmental sustainability. Integrated multitrophic aquaculture (IMTA) is regarded as a suitable approach to limit aquaculture nutrients and organic matter outputs through biomitigation. The cocultured species are used as biofilters, and each level has its own independent commercial value, providing both economic and environmental sustainability. Here, environmental issues of aquaculture and the current status of IMTA are reviewed and its future prospects discussed. Also, the opportunities to expand this systems’ complexity with increased added‐value and trophic levels are introduced.