Red seaweed ( <i>Gracilaria tenuistipitata</i> ) meal used as a partial substitute for fishmeal protein improves growth and feed efficiency in mudskipper ( <i>Pseudapocryptes elongatus</i> )

A 45-day feeding trial was conducted to evaluate the partial replacement of fishmeal protein with red seaweed ( Gracilaria tenuistipitata ) protein in the diets for mudskipper ( Pseudapocryptes elongatus ). Five isonitrogenous (30% crude protein) and isolipidic (7% crude lipid) diets were formulated, featuring fishmeal replacement levels of 0% (control), 10%, 20%, 30%, and 40%. Fish with an average initial weight of 0.54 ± 0.07 g were randomly distributed into triplicate tanks at a salinity of 10 ppt and fed to apparent satiation twice daily. Results showed that fish survival was unaffected by dietary treatments (94.4–96.7%). Fish fed the 20% replacement diet (GR20) exhibited significantly higher weight gain, improved feed conversion ratio, and enhanced protein efficiency compared with the control group, while growth declined at 40% replacement. Fillet proximate composition was influenced by diet: protein slightly increased in GR20, lipid decreased with higher seaweed inclusion, and ash content rose progressively. These results demonstrate that G. tenuistipitata protein can sustainably replace up to 20% of fishmeal protein, improving growth performance and feed efficiency in mudskippers.