The 50 Fathom line is a series of points or coordinates designated by CDFW to determine where the borderline is between legal and illegal fishing depths to put it simple. You cannot just go out past any specific depth of 300 and be safe to fish, so you better know where this line is or you will end up with a big ticket if caught. The link is below to the map and a screen shot of what the map looks like with the 50 fathom line in turquoise. The map is an interactive map with multiple data options in the map, but the 50 fathom is one you can select on.
UPDATE 9/12/23: Phone App FISH LEGAL offers an annual subscription for only $10/year that has an integrated 50 fathom line in the app that allows you to be on the water and see your position relative to the 50 fathom line. What I cannot guarantee, is that the app will work without cell signal, so make sure you have the CDFW provided coordinates also loaded in your chart plotter, so you have more than one method for verifying your fishing legally.
Here is a link to the MAP
CDFW INTERACTIVE WEBSITE WITH 50 FATHOM LINE
AVOIDING A TICKET
This is no guarantee, but there is a program that can help you potentially avoid getting a ticket from fishing an illegal area. It's a phone app called Navionics and what this allows you to do is load in the coordinates of the 50 Fathom line coordinates/points, so while you are fishing, you can be aware of where you are relative to the boundary lines of the 50 Fathom line seaward. The program doesn't show the actual lines, as it does in the picture above, unfortunately only points. You can however be creative in using a tool within the app for measuring distance between two points that would allow you to create a line and visually see a line between to points for reference in an area you might be fishing. I personally have not yet found out how to get a full overlay off the 50 Fathom line into Navionics as of yet.
Navionics has a very important function to allow the user to record a trip or "track" is the official term. The data it records is the total time of the trip, the path (coordinates), speed and the total distance traveled. With this data recorded, you have a time stamped "movie" as I like to call it that you can play, rewind and review, which is your "evidence" if you were to ever be given a ticket by law enforcement and you knew for a fact that you were wrongly ticketed. The track/trip recording would be your evidence and proof because it is time stamped and contains all the critical data that cannot be manipulated. If you were wrongly accused of being in an area you were not or fishing on a drift that you were not, your track/trip data would be solid evidence in court if you were to fight the ticket. Below is a screenshot of a track.
NAVIONICS APP SCREENSHOT
In the first picture, you can also see that the 50 Fathom line cuts right through an MPA. Another critical tool to have at your finger tips is an app called FISH LEGAL (see the screenshot below). It's a phone app that you can download for free. This app will allow you to see your live location while you are out on the water relative to the identified MPA's, so that you can make sure you don't drift into an MPA while fishing close to one. You really need to use multiple tools these days to be safe to monitor weather (Windy and Windfinder apps), Navionics to identify structure and also key land marks like the 50 Fathom lines and Fish Legal to avoid fishing in any MPA's that might end you up with a huge fine.
FISH LEGAL SCREENSHOT
I hope this information has been helpful to you. There may be better options, apps and updates out there, but I try and bring you the most current info I can to help as I identify it. Our fishery is a constantly changing and evolving system and we have to all help each other try to understand the fluid changes happening as best possible. Be safe out there in the deep waters friends! BASC - STEVE