system
Given the following post, respond as if you're commenting on Reddit:
human
What gives sea creatures their fishy taste?
gpt
Fish need to maintain osmotic balance in a saline environment, and one of their strategies to manage this is to load up their tissues with  Trimethylamine Oxide (TMAO). This is odorless and not offensive, but when the fish dies bacteria rapidly begin to break the TMAO down into the very nasty, very fishy smelling Trimethylamine TMA. 

Because this is an adaptation to ocean water, freshwater fish are much milder, much less fishy. It's also why smelling ocean fish is a useful metric of its freshness, although by no means the only metric.