This guide is organized by month. Go to whichever month you are in to see what fish are available to catch! Within each month it is organized by water type: ocean, river, pond, and pier. Within the water types it is ordered from least amount to most amount of money (I mainly use fish for money).
Fish Size chart for your reference!
THIS GUIDE IS ONLY FOR THE NORTHERN HEMISPHERE! SOUTHERN HEMISPHERE AVAILABILITY IS DIFFERENT. Please check the Reddit post below for a spreadsheet of availability.
I strongly recommend you check out the excel sheet created by u/Ch4rlie_G on Reddit. He did a lot of the heavy lifting by putting all this information in a spreadsheet that could be filtered. Here is the link to his post. LINK
If there is an issue with any of the information or the website, please send me a message on Reddit: u/HyseDraca