**1. Introduction**

Agriculture, livestock (bird, cattle, pig, etc.) farming and fish rearing are traditionally used to cater the nutritional requirement since ages. Evidence of agriculture, including meat farming, can be found as far back as the end of the Pleistocene Era, roughly around 12,000 years ago. In contrast, fish have only been farmed in aquaculture setting for over 2000 years [1]. Our world roughly comprises of 70% water and majority of them are unutilized due to inadequate knowledge and resources. Additionally, the availability of terrestrial space of agriculture and livestock farming are now on a decline. The lower FCR values of various aquaculturable species (e.g., cobia 0.96–1.50:1) than various terrestrial animals (e.g., cattle 5.15–6.95:1; poultry 2.13–2.61:1) [2, 3] are not only important for reducing the production cost but also have less environmental burden to bear. However, aquaculture, the farming of fish and aquatic plants, is the fastest growing food sector in the world, recently (since 1970s) growing exponentially to meet the increasing population and declining wild fish stock availability. The aquaculture industry's contribution to the total food supply has increased dramatically since 2000–2012 by 6.2% [4], and it is expected that by 2030, 60% of the total fish supply intended for direct human consumption will be produced by the aquaculture industry [5]. With the development of new and environment friendly Silvofisheries (fish integrated with mangroves), Aquaponics and IMTA

(integrated multitrophic aquaculture), etc., the probability of sustainable growth of aquaculture has been raised several folds. But, unlike agriculture and livestock farming, aquaculture has lot of geographical restriction, such as in North America and Europe, clams, oysters, and other shellfish are the main species being farmed, while in Japan, edible seaweed, marine shrimp, and yellowtail are the desired species for culture. Likewise, carps in India, freshwater prawns in Hawaii, and eels in Taiwan are the preferred culturable species [1]. Although numerous species (>694) have made their way into aquaculture and have international consumer market, only the Norwegian salmon has gained commercially popularity in recent years. If we analyze deeply, it is clear that it is neither the geographical restriction nor the consumer demand, but rather the huge industrial success of this specific salmon is related to meticulous research, better strain availability through years of selective breeding, authenticated and steady high quality seed availability and one stop consultancy [6].

Molecular biology and cloning set sailed its journey with the DNA molecule in 1950s and encountered several breakthrough including RNA and restriction endonucleases, however in reality, the recombinant DNA technology has made a revolution in modern molecular biology. Through this technique, large quantities of proteins present in trace amount, as well as other biologically active substances, could be generated through biotechnology and these genetically engineered macromolecules have very little side effects. Emerging technologies promise even greater possibilities, such as enabling researchers to seamlessly stitch together multiple DNA fragments and transform the resulting plasmids into bacteria in under 2 h, or the use of swappable gene cassettes, which can be easily moved between different constructs, to maximize speed and flexibility. During the past 2–3 decades, fish molecular biology has been intensively investigated in all aspects of fisheries, including diseases, genetics, nutrition, and ecology. Molecular tools are used to investigate changes in the DNA, RNA or proteins to detect certain genetic or biochemical changes that are associated with certain disease-causing pathogens [7–9]. Another advantage of molecular tools is that the analysis can be done on stored specimens and abundance of genetic information in the database. In recent years, great advances have been made to simplify the techniques and reduce the cost without compromising on the sensitivity. In this chapter we will discuss about the issues of aquaculture, and the potential of molecular cloning/ biology in fish.
