Fish excrete ammonia mostly from their _____.

Fish excrete ammonia primarily through their gills due to the high solubility of ammonia in water. As fish breathe, water passes over their gill membranes, where gases, including ammonia, are exchanged. Ammonia is a byproduct of protein metabolism and is toxic in high concentrations, so it must be removed efficiently. The gills provide a large surface area for this gas exchange, allowing ammonia to diffuse into the surrounding water, where it is diluted and rendered less harmful.

While fish do have several other anatomical structures that might facilitate the excretion of waste, such as the anus and vent, these primarily deal with solid wastes and not the gaseous ammonia produced during metabolic processes. The mouth can also be involved in the excretion of some waste materials, but it does not play a significant role in ammonia excretion. Thus, the gills are recognized as the main site for ammonia excretion in fish, making this the most relevant answer to the question.