Abstract: Methods and compositions for enhancing weight gain and/or feed efficiency in fish and shellfish are disclosed. The methods include administering an agent, such as an antibody, which reduces the bioavailability in fish or shellfish of a prostaglandin or leukotriene lipid precursor. The methods also include administering an antibody which enhances weight gain by at least 10%, improves feeding efficiency by at least 5%, or both. Fish and shellfish food comprising antibodies as described herein are also disclosed.