Fish sperm is usually inactive until it leaves the fish's body, where it then becomes mobile. The movement of fish sperm is influenced by factors like saltiness, ions, carbon dioxide, and acidity levels. Chemicals found in the environment can also impact the mobility of fish sperm. Researchers measure sperm movement by looking at how long they move, the percentage of mobile sperm, their speed, and how fast their tails beat. Changes in sperm activity within the fish's body are also considered. In short, the researchers discovered that fish sperm only starts moving when it's outside the fish, and many factors affect how fast and how well it can swim.