I used the change shell command chsh to change the default shell environment for my id from BASH to the FISH shell.

>_ chsh 
Changing shell for danesh.manoharan. 
New shell [/bin/bash]: /usr/bin/fish 
Password: 
Shell changed. 

>_ chsh -s /usr/bin/fish 
Changing shell for danesh.manoharan. 
chsh: Shell changed.