Vietnamese koi and monosex tilapia have potential for production in a polyculture as both fish species grow very fast and are adapted in same land. 30 decimals – several acres of land can be selected as pond for the polyculture of Vietnamese koi & monosex tilapia.1 The average size of fry is 2–3 gm and 300–350 pieces/ decimal mixed fry of koi and tilapia can be stocked in the pond for commercial fish culture.2 The protein content of the feed should be minimum 30% as koi fish needs higher protein content in their feed for the better growth.3 The average weights of Vietnamese koi and monosex tilapia are 300 gm and 400 gm within a four- month culture period.4,5 The productions of Vietnamese koi and monosex tilapia are about 12000 kg/hectare and 9000 kg/hectare after four months of the culture period.4,5 Although there are few drawbacks to culture of the fish species, but due to their fast growth rates, farmers should culture these fishes more commercially in polyculture system.

By- products that are not regarded as ordinary saleable products (fillet, round, eviscerated or beheaded fish), but which can be recirculated after treatment. Normally oil is produced from by-products of marine fatty fish species such as cod, anchovy, capelin, sardine, salmon, tuna, mackerel, Norway pout, Atlantic herring, sand eel etc. It is possible to extract oil by different process such as wet reduction, hydrolysis, silage, dry rendering and acid - alkali aid. Fish roe has high concentrations of lipids. Hydrolysis was better than wet rendering method as there was a chance for lipid oxidation in the wet rendering method. High quantity and quality of phospholipids were generated from the lipid fraction in the dry rendering process. To ensure high quality of produced products, it is important to process by-products immediately after production. It is also needed to increase the utilization of marine by- products by increasing knowledge on the chemical composition and stability of the by- products.

Atlantic salmon (Salmo salar L) is the major important species in salmonid families. But for the disease issue, farmers are losing a large number of fish and income. Melanin in fillet is one cause for the loss of the processing industries. The cause of melanin spot are related to the use of vaccines containing oil adjuvants (Fagerland et al., 2013), but other factors such as environmental conditions, genetics and diseases (Bjerkeng, 2000) also appear to play a role. The pigmentary and the immune systems are related each other. Larsen et al. (2012) reported that the melanogenesis occurs in muscle-located granulomas, which represents an association between the immune and pigmentary systems. Melanin spot in fish fillet or organs create many problems to farmers and processing industries. Consumers do not prefer spotted fillet so processing industries need to cut off the defected part of the fillet, as a result, they face some loss in their product production. Addition of Vitamin C, E and selenium in feed can contribute to reduce the number and size of the melanin spot (Wang, 2016).