there is no mention in the manga or anime of One Piece that humans can use Fishman Jujutsu. Fishman Karate is a martial art developed and practiced by Fishmen, who are a distinct race in the One Piece world. Fishmen possess physical traits and abilities that are different from regular humans, such as enhanced strength, agility, and underwater adaptability.
Koala is a notable character who was once a slave in the Fishman District and learned Fishman Karate during her captivity. However, it's important to note that while Koala is a human, she has been exposed to the Fishman culture and environment, which could explain her ability to learn and use Fishman Karate. But again, there is no indication in the series that humans, in general, can use Fishman Jujutsu.
Keep in mind that the One Piece manga and anime might have introduced new information or developments , so it's a good idea to check the latest chapters or episodes to see if there have been any changes or new revelations in the story.