| Summary: | Olive flounder (<i>Paralichthys olivaceus</i>) is a commercially important and valuable species for aquaculture in Korea. Due to the unstable supply of fishmeal for farmed fish, an optimum fish-feed formulation should be researched to ensure the sustainability of <i>P. olivaceus</i> aquaculture. This study investigated the effect of three experimental diets: Con (basal diet); FM<sub>20</sub> (20% fishmeal replacement of CON); and FM<sub>30</sub> (30% fishmeal replacement of CON) on <i>P. olivaceus</i> over 20 weeks at a typical farm by monitoring the growth and factors relating to sexual maturation. The results showed that no differences in growth were observed between the CON and diet-replacement groups. Gonadal oocyte development was similar between the CON and diet-replacement groups. Moreover, sbGnRH and GH expression did not differ between the CON and diet-replacement groups. The levels of Erβ and Vtg expression were significantly higher in the FM<sub>20</sub> group than in the CON and FM<sub>30</sub> groups after the experimental period. The expression of PSS-I was significantly higher in the FM<sub>30</sub> group than in the CON and FM<sub>20</sub> groups. Therefore, although growth occurred when 30% of the fishmeal was replaced, such high dietary protein replacement may be ill-advised during the maturation of olive flounder at the commercial fish farm.
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