Effect of replacing dietary menhaden oil with pollock or soybean oil on muscle fatty acid composition and growth performance of juvenile Pacific threadfin (Polydactylus sexfilis)

• Soybean oil can replace pollock oil in a feed containing 40% fishmeal.
• Total n-3 LC-PUFA decreases in muscle of Pacific threadfin fed with soybean oil.
• Pollock oil can replace menhaden oil & is a good lipid source for Pacific threadfin.

This study investigated effect of different dietary lipid sources on juvenile Pacific threadfin (Polydactylus sexfilis), a tropical marine fish, based on growth performance, proximate composition of whole fish and muscle fatty acid composition. The objectives were 1) to compare the nutritional value of menhaden oil to pollock oil, a product generated by the Alaskan fish processing industry; and 2) to evaluate the potential of replacing pollock oil with soybean oil in diets for juvenile Pacific threadfin. Six experimental diets were formulated to contain 42% crude protein and 12% crude lipid including 4% lipid from fishmeal and 8% added oil. The 8% added oil was provided by menhaden oil pollock oil (P) or soybean (S) oil mixed with pollock oil. Diets containing the mixed oil were named as P8S0, P6S2, P4S4, P2S6, and P0S8 to denote the level of pollock oil (decreasing from 8 to 0%) and soybean oil (increasing from 0 to 8%).Pacific threadfin (average body weight of 18.6 g) were fed each of the experimental diets for 8 weeks. Growth performance, proximate composition of whole body and muscle were similar among fish fed with different diets. Pollock oil and menhaden oil were shown to have a similar effect on the nutritional composition of fish or tissue. In contrast, the diet containing 8% soybean oil significantly increased liver lipid content compared to the other test diets. The fatty acid composition of muscle reflected the dietary fatty acids. Fish fed the soybean oil diets exhibited significantly lower levels of n-3 long chain polyunsaturated fatty acid (n-3 LC-PUFA) and higher levels of 18:2n-6 and 18:3n-3 than those fed the fish oil diets. Results of this study suggest that pollock oil can replace menhaden oil without any adverse effect on growth performance or nutritional quality of fish. Total replacement of pollock oil with soybean oil had no detrimental effect on growth performance of juvenile Pacific threadfin but altered the fatty acid composition of fish muscle tissue. These results are useful in formulating practical feeds at reduced cost for Pacific threadfin culture. However, a longer term study will be needed to investigate whether the changes in muscle fatty acid composition will influence the welfare as well as the market value of Pacific threadfin.